Hey Tara! Welcome to Topic 2 — Thermal Physics. This topic is all about heat, temperature, and how particles behave. You already experience these concepts every day — from your morning chai getting cold to the pressure cooker whistling in the kitchen. We will connect every idea to things you already know. Let us go!
2.1.1 States of Matter
Everything around you - the chair you are sitting on, the water you drink, the air you breathe - is made up of matter. All matter exists in one of three states: solid, liquid, or gas. Let us look at what makes each one different.
Solids
Think about a cricket bat, an ice cube, or your wooden study desk. These are all solids. A solid has these properties:
- Fixed shape - a solid keeps its own shape. Your phone does not change shape when you put it on a table versus in your bag.
- Fixed volume - the amount of space a solid takes up stays the same. A brick is always the same size, no matter where you place it.
- Cannot be compressed - you cannot squeeze a solid into a smaller space. Try squeezing a stone - it will not get smaller.
- High density - solids are generally dense because their particles are packed tightly together.
Liquids
Think about the water in your bottle, the coconut oil in your kitchen, or the chai your family makes every morning. These are all liquids. A liquid has these properties:
- No fixed shape - a liquid takes the shape of whatever container you pour it into. Pour water into a round glass and it becomes round. Pour it into a square box and it becomes square.
- Fixed volume - even though a liquid changes shape, its volume stays the same. 500 mL of water is still 500 mL whether it is in a bottle, a cup, or a bucket.
- Cannot be easily compressed - you can squeeze a liquid a tiny, tiny bit, but for your exam, treat liquids as incompressible.
- Moderate density - liquids are less dense than most solids but much denser than gases.
Gases
Think about the air around you right now, the steam rising from hot sambar, or the smell of agarbatti (incense stick) spreading through a room. These involve gases. A gas has these properties:
- No fixed shape - a gas spreads out to fill whatever container it is in. Open a bottle of perfume in one corner of your room, and soon you can smell it everywhere.
- No fixed volume - a gas expands to fill all available space. A small amount of gas can fill a huge room.
- Easily compressed - you can squeeze a gas into a much smaller space. That is how LPG (cooking gas) cylinders work - the gas is compressed into the cylinder.
- Very low density - gases are much, much less dense than solids or liquids.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed shape | Takes shape of container | Fills entire container |
| Volume | Fixed volume | Fixed volume | No fixed volume |
| Compressibility | Cannot be compressed | Cannot be easily compressed | Easily compressed |
| Particle arrangement | Regular, closely packed pattern | Irregular, close together | Random, very far apart |
| Particle motion | Vibrate about fixed positions | Move around each other | Move quickly in all directions |
| Particle separation | Very close (touching) | Close (almost touching) | Very far apart |
| Forces between particles | Very strong | Moderate | Very weak (almost none) |
| Density | High | Moderate | Very low |
Think of the particles in each state like people: Solids = students sitting in fixed seats in a classroom (vibrating but not moving around). Liquids = people walking around in a crowded market (moving past each other but close together). Gases = birds flying freely in a huge open sky (zooming around with lots of space between them).
Changes of State
Matter can change from one state to another when you heat it or cool it. Here are the key changes of state you need to know:
| Change of State | From | To | Energy Change | Indian Example |
|---|---|---|---|---|
| Melting | Solid | Liquid | Heat energy absorbed | Ice cream melting in Bangalore's 35°C summer heat |
| Freezing | Liquid | Solid | Heat energy released | Coconut oil becoming solid in a Bangalore winter (below 24°C) |
| Boiling / Evaporation | Liquid | Gas | Heat energy absorbed | Water boiling when your amma makes chai on the stove |
| Condensation | Gas | Liquid | Heat energy released | Morning dew forming on cars in Bangalore - water vapour in air cools overnight |
Important note: Your syllabus says you do NOT need to know about gas-to-solid (deposition) or solid-to-gas (sublimation) changes. Just focus on the four changes above.
Evaporation vs Boiling - Both change liquid to gas, but they are different! Evaporation happens at ANY temperature, only at the surface of the liquid, and is a slow process. Boiling happens at a specific temperature (the boiling point), throughout the entire liquid, and is a fast process with bubbles forming inside the liquid. Examiners love testing this difference!
To remember the changes: Melting and Boiling both go "up" in energy (solid→liquid→gas) and both absorb heat. Freezing and Condensation both go "down" (gas→liquid→solid) and both release heat. Think: "MB goes up, FC comes down" - like texting your friend!
2.1.2 The Particle Model
Particle Arrangement in Solids, Liquids, and Gases
All matter is made up of tiny particles (these can be atoms, molecules, or ions - but for now, just think of them as tiny balls). The way these particles are arranged and how they move is what gives each state of matter its properties.
Let us summarise what the particles are doing in each state:
- Solid: Particles are packed tightly in a regular pattern. They do not move from place to place - they only vibrate (jiggle) about their fixed positions. The forces between particles are very strong, which is why solids hold their shape.
- Liquid: Particles are still close together, but they are arranged irregularly (no neat pattern). They can slide and move past each other, which is why liquids can flow and take the shape of their container. The forces between particles are weaker than in solids.
- Gas: Particles are very far apart with lots of empty space between them. They move quickly and randomly in all directions, bouncing off each other and the walls of the container. The forces between particles are almost zero.
Temperature and Particle Motion
Here is a really important idea: temperature is a measure of the average kinetic energy of the particles. In simple words, the hotter something is, the faster its particles move.
- When you heat a substance, you give its particles more energy. They move faster (or vibrate more vigorously).
- When you cool a substance, you take energy away from its particles. They slow down.
Indian example: Think about hot sambar on your dining table. The particles in the hot sambar are moving very quickly - that is why steam rises from it. As the sambar cools down over time, its particles slow down and the steam stops.
Absolute Zero
If you keep cooling something down, the particles move slower and slower. There is a temperature at which the particles have the least possible kinetic energy - they can barely move at all. This temperature is called absolute zero.
Absolute zero = −273 °C = 0 K (zero kelvin)
You cannot get any colder than this. There is no such thing as a temperature below absolute zero, because the particles cannot move any less than "almost not at all."
At absolute zero, particles do NOT completely stop moving. They have the least possible kinetic energy, but quantum mechanics tells us they still have a tiny amount of energy. For your exam, say "least kinetic energy" rather than "zero kinetic energy" or "no movement."
Gas Pressure - Why Gases Push on Surfaces
Gas particles are constantly zooming around at high speed. When these particles hit the walls of their container, they exert a force on the wall. This force spread over the area of the wall is what we call pressure.
Indian example: Think about a balloon. The air particles inside the balloon are constantly bouncing off the inside surface. Each time a particle hits the rubber, it pushes on it a tiny bit. Billions and billions of these tiny pushes per second create enough force to keep the balloon inflated.
Gas pressure increases when:
- Particles move faster (which happens when you heat the gas) - faster particles hit the walls harder and more often.
- There are more particles in the same space - more particles means more collisions with the walls.
- The volume decreases (container gets smaller) - particles hit the walls more often because they do not have to travel as far between collisions.
Indian example: Your family's pressure cooker is a perfect example! When you heat it on the stove, the water boils and creates steam (water vapour). The steam particles move very fast and collide with the inside walls of the sealed cooker. As more steam forms and the temperature rises, the pressure builds and builds until the weight on top lifts to release the excess pressure with that familiar "whistle" sound.
Evidence for the Particle Model: Brownian Motion
How do we know that particles are really moving around randomly? We cannot see individual atoms or molecules - they are far too small. But we CAN see evidence of their movement through something called Brownian motion.
In 1827, a Scottish botanist named Robert Brown was looking at tiny pollen grains floating in water through a microscope. He noticed that the pollen grains were jiggling around randomly - they would suddenly move one way, then another, in a completely unpredictable zigzag path. He could not explain why.
The explanation came later: the pollen grains were being constantly bombarded (hit) by invisible water molecules moving at high speed. Each time a water molecule hit a pollen grain, it gave it a tiny push. Because the water molecules hit from all sides randomly, the pollen grain moves in a random, zigzag path.
You can also see Brownian motion by looking at smoke particles in air through a microscope. The smoke particles (which are much bigger than air molecules, but still tiny) jiggle around randomly because air molecules keep hitting them from all sides.
Indian example: Light an agarbatti (incense stick) or a mosquito coil in a still room. Watch the smoke carefully near the glowing end. You will see the tiny smoke particles dancing and swirling randomly - this is because invisible air molecules are hitting them from all directions.
A very common exam mistake: students say that Brownian motion shows "molecules moving." No! You CANNOT see molecules through a microscope. What you see is the larger particle (smoke or pollen) moving. The zigzag path of the larger particle is evidence that invisible molecules are colliding with it. Be very precise with your wording in the exam.
Think of Brownian motion like a football (the big particle) being kicked randomly by invisible players (the molecules). You can see the football bouncing around unpredictably, even though you cannot see who is kicking it. The random movement of the football is proof that invisible kickers exist!
Forces and Distances Between Particles
The properties of solids, liquids, and gases depend on three things about their particles:
- The forces between particles - in solids, the attractive forces are very strong, keeping particles locked in place. In liquids, the forces are weaker, allowing particles to slide past each other. In gases, the forces are negligible (almost zero), so particles move freely.
- The distances between particles - in solids, particles are very close together (touching). In liquids, they are slightly further apart. In gases, the distances between particles are huge compared to the size of the particles themselves.
- The motion of particles - in solids, particles vibrate. In liquids, they move slowly past each other. In gases, they move very quickly in random directions.
These three factors together explain why solids are rigid, liquids flow, and gases expand to fill their containers.
Pressure as Force per Unit Area
For the extended syllabus, you need to understand pressure more precisely. When gas particles collide with a surface, each collision exerts a tiny force. Pressure is the total force from all these collisions divided by the area of the surface:
The unit of pressure is the pascal (Pa), which is the same as N/m² (newtons per square metre).
When gas particles hit the walls of a container:
- Each particle exerts a tiny force on the wall when it bounces off.
- Billions of particles hitting the wall every second create a total force.
- This total force divided by the area of the wall gives the pressure.
Atoms, Molecules, and Microscopic Particles
For the extended syllabus, you need to use precise language:
- Atoms are the smallest particles of an element. Example: a single gold atom, a single oxygen atom.
- Molecules are groups of atoms bonded together. Example: a water molecule (H₂O) is two hydrogen atoms bonded to one oxygen atom. An oxygen molecule (O₂) is two oxygen atoms bonded together.
- Microscopic particles are much bigger than atoms or molecules - things like pollen grains, smoke particles, or dust. They are "microscopic" because you need a microscope to see them, but they are HUGE compared to individual molecules.
In Brownian motion: the microscopic particles (smoke, pollen) are moved by collisions with light, fast-moving molecules (air molecules, water molecules). The molecules are invisible; the microscopic particles are visible through a microscope.
2.1.3 Gases and the Absolute Scale of Temperature
Effect of Temperature on Gas Pressure (Constant Volume)
Imagine you have a sealed container of gas - the gas cannot escape, and the container cannot expand. The volume stays constant. What happens when you heat this gas?
- The particles gain kinetic energy and move faster.
- Faster particles hit the walls of the container harder (with more force per collision).
- Faster particles also hit the walls more often (more collisions per second).
- More force + more collisions = higher pressure.
So: heating a gas at constant volume increases its pressure.
Indian example: This is exactly what happens inside a pressure cooker! The cooker is sealed (constant volume). As the stove heats the gas and steam inside, the particles move faster, hit the walls harder and more often, and the pressure builds up. Eventually, the pressure gets so high that it lifts the weight on top, causing the "whistle."
And the reverse: cooling a gas at constant volume decreases its pressure. The particles slow down, hit the walls less often and with less force.
Effect of Volume on Gas Pressure (Constant Temperature)
Now imagine you have a gas at constant temperature, but you change the volume of the container (like pushing a piston in a syringe).
If you decrease the volume (make the container smaller):
- The same number of particles are now in a smaller space.
- The particles do not have to travel as far before hitting a wall.
- So they hit the walls more often (more collisions per second).
- More collisions = higher pressure.
If you increase the volume (make the container bigger):
- The particles have more space to move around in.
- They have to travel further between wall collisions.
- So they hit the walls less often.
- Fewer collisions = lower pressure.
Indian example: Try this with a syringe (without a needle!). Block the end with your finger and push the plunger in. You are decreasing the volume. You will feel it getting harder to push because the pressure inside is increasing. Now pull the plunger out (increasing volume) and the air inside expands - the pressure drops.
When explaining pressure changes, always mention THREE things: (1) what the particles do (move faster / slower, or same speed), (2) how often they hit the walls (more / fewer collisions), and (3) how hard they hit (more / less force per collision). This is what examiners are looking for in a full-mark answer.
The Kelvin Scale
In everyday life, we use degrees Celsius (°C) to measure temperature. But in physics, we often use the kelvin (K) scale instead.
The kelvin scale starts at absolute zero (the coldest possible temperature). On the Celsius scale, absolute zero is −273 °C. On the kelvin scale, it is simply 0 K.
To convert between the two scales:
Notice: a change of 1 °C is exactly the same as a change of 1 K. The two scales have the same "step size" - the kelvin scale is just shifted by 273.
To go from Celsius to Kelvin, just add 273. To go from Kelvin to Celsius, just subtract 273. Think of it as "Celsius is 273 less than Kelvin" or remember: "C + 273 = K". Easy!
Important: The unit is "kelvin" (lowercase k in the word), but the symbol is a capital K. We write "300 K" (not "300 °K" - there is no degree symbol with kelvin).
Common exam mistakes with temperature conversion: (1) Forgetting which way to add or subtract 273. Remember: Kelvin is always the BIGGER number (since it adds 273). (2) Writing "°K" instead of just "K". There is NO degree symbol with kelvin! (3) Getting negative values wrong: −50°C = −50 + 273 = 223 K (not −323 K!).
Boyle's Law (pV = constant)
For a fixed mass of gas at constant temperature, there is a beautiful mathematical relationship between pressure and volume:
This means that if you increase the pressure, the volume decreases by the same proportion, and vice versa. We call this an inverse relationship - as one goes up, the other goes down.
Another very useful way to write this is:
This is called Boyle's Law. It only works when:
- The temperature stays constant (does not change).
- The mass of gas stays the same (no gas escapes or is added).
Why does this happen? If you squash a gas into half the volume (at constant temperature), the particles have the same speed (same temperature) but now they are in half the space. They hit the walls twice as often, so the pressure doubles. That is why pV stays constant!
When using Boyle's Law, the units of pressure and volume do NOT have to be in SI units - they just have to be consistent. If p₁ is in kPa, then p₂ must also be in kPa. If V₁ is in cm³, then V₂ will also be in cm³. But if the question asks for the answer in a specific unit, convert at the end.
Boyle's Law says "pressure and volume are inverse friends." When one goes up, the other comes down by the same proportion. If you halve the volume, pressure doubles. If you triple the volume, pressure goes to one-third. Just think: "squeeze the box, pressure goes up!"
Diffusion: How Smells Spread
Have you ever walked into the kitchen and immediately smelled Mysore Sandal soap from the bathroom, even though the bathroom is far away? Or noticed how the fragrance of agarbatti (incense stick) slowly spreads through the entire room? This happens because of diffusion.
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration. In simple words, particles naturally spread out from where there are lots of them to where there are fewer of them.
Indian example: When you make chai and add milk, watch what happens. The milk starts spreading into the tea all by itself, without you stirring. This is diffusion - milk particles move from where they are concentrated (the drop of milk) to where there are fewer milk particles (the rest of the tea). Eventually, the whole cup becomes uniformly milky.
Diffusion happens because particles are in constant random motion. They naturally spread out and mix over time. Diffusion is faster when:
- The temperature is higher (particles move faster)
- The particles are smaller and lighter (they move faster)
- The concentration difference is larger
Diffusion is fastest in gases, slower in liquids, and almost impossible in solids (because solid particles cannot move from place to place).
Think of diffusion like gossip spreading in school. A piece of news starts in one corner (high concentration of people who know it) and spreads to everyone (low concentration). It spreads faster in an open playground (gas) than in a crowded corridor (liquid), and almost not at all through a brick wall (solid)!
Using the pressure law (P₁/T₁ = P₂/T₂), converting to Kelvin:
T₁ = 22 + 273 = 295 K
T₂ = 42 + 273 = 315 K
P₂ = P₁ × T₂ / T₁ = 200 × 315 / 295 = 213.6 kPa
Using Boyle's law (constant temperature):
P₁V₁ = P₂V₂
20,000 × 12 = 100 × V₂
V₂ = 240,000 / 100 = 2,400 litres
Inside a sealed pressure cooker, steam cannot escape. The steam particles above the water create extra pressure (~200 kPa above atmospheric = ~300 kPa total). This means water particles need even MORE kinetic energy to escape as steam. So the boiling point rises to about 120–130°C.
Food cooks at 120°C instead of 100°C — that's 20–30% hotter. Since chemical reactions (cooking) roughly double in speed for every 10°C rise, food cooks about 3–4 times faster.
Hey Tara! This guide covers two really important topics: Thermal Properties & Temperature (2.2) and Transfer of Thermal Energy (2.3). These topics are everywhere in your daily life in Bangalore -- from your morning chai heating up in a steel tumbler, to wet clothes drying on the terrace, to why you feel cooler wearing a white salwar on a summer day. We will go step by step, and by the end of this you will feel confident tackling any thermal physics question the exam throws at you. Let us get started!
2.2.1 Thermal Expansion of Solids, Liquids and Gases
You know how the metal lid on a glass pickle jar gets stuck? Your amma probably runs it under hot water to open it. That works because the metal lid expands when heated -- it becomes slightly bigger, loosening its grip on the glass. This is thermal expansion, and it happens to solids, liquids, and gases.
What is Thermal Expansion?
When you heat a substance, its particles gain more kinetic energy and vibrate (or move) more vigorously. As they vibrate harder, they push their neighbours away slightly, so the substance takes up more space. This increase in size when temperature rises is called thermal expansion.
The key idea:
- Solids expand the least -- their particles are tightly packed in a fixed arrangement and can only vibrate a little more.
- Liquids expand more than solids -- their particles are close but can slide past each other, so they spread out more when heated.
- Gases expand the most -- their particles are already far apart and moving freely, so when they gain energy they spread out enormously.
Think S-L-G for expansion order: Solids expand Slightly, Liquids expand Larger, Gases expand Gigantically. The less tightly held the particles, the more they can spread.
Explaining Expansion Using Particle Theory
In a solid, particles are held in a regular lattice by strong forces. When heated, they vibrate with greater amplitude about their fixed positions, pushing neighbouring particles slightly further apart. Because the forces are strong, the expansion is small.
In a liquid, particles are close together but can move around each other. The forces between them are weaker than in a solid, so when the particles gain kinetic energy and move faster, they push apart more easily. Liquids therefore expand more than solids.
In a gas, particles are far apart with very weak forces between them. They move freely at high speeds. When heated, they move even faster and collide more energetically with each other and the container walls. At constant pressure, the gas must occupy a much larger volume to accommodate this increased motion. Gases expand far more than liquids or solids.
Everyday Applications and Consequences
| Application / Consequence | How Thermal Expansion is Involved |
|---|---|
| Gaps in railway tracks | Steel rails expand in the Indian summer heat. Small gaps are left between rail sections so they have room to expand without buckling. (Modern continuously welded rail uses pre-stressed steel and concrete sleepers to handle expansion.) |
| Expansion joints in bridges | Long bridges like the ones on Bangalore's Outer Ring Road have special joints that allow the concrete/steel deck to expand and contract with temperature changes without cracking. |
| Bimetallic strip in an iron box (press) | Two metals (e.g., brass and iron) bonded together expand by different amounts. When heated, the strip bends because one side expands more. This is used in thermostats to switch off the heating element when the iron reaches the set temperature. |
| Overhead electrical wires sagging | On a hot Bangalore afternoon, copper wires expand and become slightly longer, causing them to sag. Utility companies leave slack in the wires to prevent snapping in summer. |
| Mercury / alcohol in thermometers | The liquid inside a thermometer expands as temperature rises and moves up the narrow tube. The expansion of the liquid is used to measure temperature. |
| Hot water in a glass tumbler | Pouring boiling water into a thick glass tumbler can crack it because the inside expands before the outside. This is why lab beakers are made of borosilicate glass (low expansion). Your steel tumbler does not crack because metal expands more uniformly. |
| Tight jar lids | Running hot water over a metal lid makes it expand more than the glass jar, loosening it. |
When describing an application of thermal expansion, always explain what expands, why it expands (particles gain energy and vibrate/move more), and what effect this has. A common mistake is just saying "it gets bigger" without linking to particle theory or the practical consequence.
2.2.2 Specific Heat Capacity
Internal Energy and Temperature
Every object has internal energy. This is the total kinetic energy and potential energy of all the particles inside it. When you heat an object -- say, you put a steel tumbler of water on the gas stove -- you are transferring energy to it. This increases its internal energy, and its temperature rises.
Key fact: A rise in the temperature of an object increases its internal energy.
Temperature and Average Kinetic Energy
Temperature is a measure of the average kinetic energy of the particles in a substance. When you heat water on the stove, the water molecules move faster -- they have more kinetic energy. The temperature reading on your thermometer goes up because the average speed of the molecules has increased.
Important: temperature is about the average kinetic energy, not total energy. A cup of chai at 80 degrees C has particles with the same average kinetic energy as a bucketful of water at 80 degrees C -- but the bucket has more total internal energy because it has far more particles.
What is Specific Heat Capacity?
Have you ever noticed that when you pour chai into a steel tumbler and a clay kulhar, the steel tumbler gets burning hot almost instantly, but the clay cup stays warm enough to hold? That is because different materials need different amounts of energy to heat up. This property is called specific heat capacity.
Definition
Specific heat capacity (c) is the energy required per unit mass per unit temperature increase. In simpler words, it is the amount of energy (in joules) needed to raise the temperature of 1 kg of a substance by 1 degree C.
| Substance | Specific Heat Capacity, c (J/(kg degC)) |
|---|---|
| Water | 4200 |
| Aluminium | 900 |
| Iron / Steel | 450 |
| Copper | 390 |
Look at that table. Water has a much higher specific heat capacity than metals. This means water needs a lot more energy to heat up -- but it also holds onto that energy for longer. That is why coastal cities like Mumbai are cooler in summer and warmer in winter than inland cities: the sea absorbs and releases heat slowly.
And that is why your steel tumbler burns your hand -- steel has a low specific heat capacity (450), so it heats up quickly with only a small amount of energy transferred to it.
The Equation
You can rearrange this formula:
- To find specific heat capacity: c = E / (m × ΔΘ)
- To find mass: m = E / (c × ΔΘ)
- To find temperature change: ΔΘ = E / (m × c)
Think of the equation as "E = mc-delta-theta" -- say it out loud a few times! It sounds a bit like Einstein's famous E = mc squared, which might help you remember it. Just remember: Energy equals mass times capacity times change in temperature.
ΔΘ is always the CHANGE in temperature, not the final temperature. Calculate it as: final temperature minus initial temperature. Many students lose marks by plugging in just the final temperature instead of the change.
Experiments to Measure Specific Heat Capacity
Experiment 1: Specific Heat Capacity of a Solid (Metal Block)
Setup: Take a metal block (e.g., aluminium) with two holes drilled in it -- one for a thermometer, one for an electric heater. Weigh the block on a balance. Wrap the block in insulation (cotton wool or bubble wrap) to reduce heat loss. Connect the heater to a power supply, ammeter, and voltmeter (or use a joulemeter).
Method:
- Record the mass of the block (m).
- Record the initial temperature (Θ1).
- Switch on the heater and start a stopwatch. If using an ammeter and voltmeter, note the current (I) and voltage (V).
- Heat for a set time (t), say 5 minutes (300 s).
- Record the final temperature (Θ2).
- Calculate energy supplied: E = V × I × t (or read directly from joulemeter).
- Calculate: c = E / (m × ΔΘ) where ΔΘ = Θ2 - Θ1.
Sources of error: Heat loss to surroundings (makes your answer too high for c, because not all energy goes into heating the block). Using insulation helps. Also, the thermometer might not be in perfect thermal contact with the block -- use a drop of oil in the thermometer hole for better contact.
Experiment 2: Specific Heat Capacity of a Liquid (Water)
Setup: Pour a known mass of water into an insulated container (like a polystyrene cup). Place a thermometer and an immersion heater into the water. Connect the heater to a power supply with ammeter and voltmeter (or joulemeter).
Method:
- Measure the mass of water (m) by weighing the cup empty and then with water, and subtracting.
- Record the initial temperature (Θ1).
- Switch on the heater, stir the water gently to distribute heat evenly.
- After a set time (t), switch off and record the final temperature (Θ2).
- Calculate energy: E = V × I × t.
- Calculate: c = E / (m × ΔΘ).
Key precaution: Stir the water while heating so the temperature is uniform throughout. Use an insulated container to minimise heat loss.
2.2.3 Melting, Boiling and Evaporation
Changes of State
Matter can change between solid, liquid, and gas. You see this every day:
- Melting = solid → liquid (ice from the freezer melts into water)
- Boiling = liquid → gas (water in a pressure cooker turns to steam)
- Condensation = gas → liquid (water droplets on the outside of a cold steel glass of buttermilk)
- Solidification (freezing) = liquid → solid (water freezing into ice cubes in your fridge)
- Evaporation = liquid → gas (but only from the surface -- wet clothes drying on your terrace)
Key Temperatures for Water
At standard atmospheric pressure (1 atm):
- Melting point of water = 0 degC (ice turns to water)
- Boiling point of water = 100 degC (water turns to steam)
You MUST know these two numbers: water melts at 0 degC and boils at 100 degC at standard atmospheric pressure. The phrase "at standard atmospheric pressure" is important because these temperatures change with pressure -- a pressure cooker increases pressure above 1 atm, which raises the boiling point above 100 degC, cooking food faster. On top of a mountain, pressure is lower, so water boils below 100 degC.
Energy Input Without Temperature Change
Here is something that surprises many students: during melting and boiling, energy is being supplied but the temperature does NOT change.
Imagine you take ice out of the freezer at -10 degC and heat it steadily:
- The ice warms up from -10 degC to 0 degC -- temperature rises.
- At 0 degC, the ice starts melting. You keep supplying energy, but the temperature stays at 0 degC until ALL the ice has melted. The energy is being used to break the bonds between particles, not to make them move faster.
- Once all the ice has melted, the water starts warming from 0 degC to 100 degC -- temperature rises again.
- At 100 degC, the water starts boiling. Again, the temperature stays at 100 degC until all the water has turned to steam. The energy goes into separating the particles completely.
- After all the water has boiled, the steam temperature can rise above 100 degC.
During melting, the energy goes into breaking the regular arrangement of particles in the solid so they can move more freely (becoming a liquid). During boiling, the energy goes into completely separating the particles so they can move independently as a gas.
Condensation and Solidification
These are the reverse processes:
- Condensation: Gas particles lose energy, slow down, and come closer together. The attractive forces pull them into a liquid. Think of water droplets forming on the outside of a cold Coke bottle on a hot Bangalore day -- the water vapour in the warm air touches the cold surface, loses energy, and condenses into liquid droplets.
- Solidification: Liquid particles lose energy, slow down further, and lock into fixed positions in a regular arrangement, forming a solid. This is how ice forms in your freezer -- the water molecules slow down enough for the forces between them to hold them in place.
Evaporation
Evaporation is different from boiling, and this distinction is a favourite exam question!
Evaporation happens when the more energetic particles at the surface of a liquid escape into the air as gas. Think about wet clothes drying on your terrace in Bangalore. The water does not need to reach 100 degC to disappear -- it evaporates at any temperature. Here is why:
In any liquid, particles have a range of kinetic energies. Some are moving slowly, some are moving fast. The fastest-moving particles near the surface have enough energy to overcome the attractive forces of their neighbours and escape into the air. When these high-energy particles leave, the average kinetic energy of the remaining particles decreases. Since temperature depends on average kinetic energy, the liquid cools down.
Key fact: Evaporation causes cooling of a liquid.
This is why you feel cool when you step out of a swimming pool -- the water on your skin evaporates, taking energy from your body. It is also the principle behind the matka (clay pot) that keeps water cool: the porous clay allows water to seep to the outer surface, where it evaporates and cools the remaining water inside.
Think of evaporation like this: the "sporty" (fast) particles at the surface "jump out" of the liquid. The remaining "lazy" (slow) particles have less average energy, so the liquid gets cooler. Fast ones leave, slow ones stay, temperature drops.
Differences Between Boiling and Evaporation
| Feature | Boiling | Evaporation |
|---|---|---|
| Temperature | Occurs at a fixed temperature (boiling point) -- 100 degC for water at 1 atm | Occurs at ANY temperature below the boiling point |
| Where it happens | Throughout the entire liquid (bubbles form inside the liquid and rise) | Only at the surface of the liquid |
| Bubbles | Yes -- vigorous bubbling | No bubbles |
| Energy source | Requires continuous heating (external energy source like a stove) | Energy comes from the liquid itself (internal energy) |
| Speed | Fast, vigorous process | Slow, gentle process |
| Effect on liquid temp | Temperature stays constant at boiling point | Liquid cools down (average KE decreases) |
Factors Affecting Evaporation Rate
Three main factors speed up evaporation:
- Higher temperature: More particles have enough energy to escape from the surface. Your clothes dry faster on a hot summer day than on a cool Bangalore winter morning.
- Larger surface area: More surface means more particles are at the surface and can escape. That is why you spread clothes out flat on the terrace instead of leaving them in a lump -- more surface area exposed to air.
- Air movement (wind/breeze): Moving air carries away the escaped particles, preventing them from returning to the liquid. A breeze or a fan speeds up drying. Without wind, escaped particles can be pushed back into the liquid by surrounding air molecules.
Cooling by Evaporation -- Explained
When an evaporating liquid is in contact with an object, it cools that object. Here is why:
The most energetic particles in the liquid escape from the surface. To overcome the attractive forces and leave, they need energy. They take this energy from the liquid itself and from the object in contact with it. The remaining liquid has a lower average kinetic energy (lower temperature), and the object in contact also loses energy, so it cools down.
Matka example: A clay pot (matka) has tiny pores. Water seeps through to the outer surface and evaporates. The evaporating water takes energy from the water inside the pot, cooling it. This is why matka water tastes cooler than bottled water on a hot day -- it can be 5-10 degC cooler!
Sweating: When you play sports, your body sweats. The sweat evaporates from your skin, taking energy from your body and cooling you down. A ceiling fan helps by increasing air movement over your skin, speeding up evaporation.
A very common exam question asks you to "explain why evaporation causes cooling." The perfect answer has THREE parts: (1) particles in a liquid have a range of kinetic energies, (2) the most energetic particles at the surface escape, (3) this reduces the average kinetic energy of the remaining particles, so the temperature decreases. Do not just write "energy is lost" -- explain WHO has the energy and WHERE it goes.
E = 3 × 450 × (200 − 25)
E = 3 × 450 × 175
E = 236,250 J (about 236 kJ)
The vendor prefers cast iron because its high mass and reasonable specific heat capacity mean it stores a large amount of thermal energy. When cold dosa batter hits the tawa, the temperature drops only slightly because the tawa has so much stored energy to give. A thin aluminium pan would cool down sharply when batter is added, giving uneven cooking.
Your skin loses thermal energy → your skin temperature drops → temperature receptors in your skin detect the drop → you feel cold.
The key point: the spirit didn't need to be cold to make you feel cold. The cooling effect comes from the evaporation process stealing energy from your skin.
Efusion = mLf = 5 × 400,000 = 2,000,000 J = 2 MJ
Energy absorbed during vaporisation:
Evap = mLv = 5 × 3,000,000 = 15,000,000 J = 15 MJ
Total energy absorbed = 2 + 15 = 17 MJ
Vaporisation absorbs 15 MJ vs only 2 MJ for melting — that's 7.5 times more energy! This is because vaporisation requires completely breaking all intermolecular bonds (particles fly apart), while melting only loosens them (particles still touch).
E₁ = mcΔθ = 0.2 × 3,200 × 30 = 19,200 J
Stage 2 — Freezing at 0°C:
E₂ = mL = 0.2 × 250,000 = 50,000 J
Total energy removed = 19,200 + 50,000 = 69,200 J
The temperature stays at 0°C during freezing because the energy being removed is used to form bonds between particles as the liquid becomes solid. The particles slow down and lock into fixed positions. No energy goes into changing temperature — all of it goes into the change of state.
miron × ciron × (800 − T) = mwater × cwater × (T − 20)
0.5 × 450 × (800 − T) = 10 × 4,200 × (T − 20)
225 × (800 − T) = 42,000 × (T − 20)
180,000 − 225T = 42,000T − 840,000
180,000 + 840,000 = 42,000T + 225T
1,020,000 = 42,225T
T = 1,020,000 / 42,225
T ≈ 24.2°C
There are three ways thermal energy can move from a hot place to a cold place: conduction, convection, and radiation. Let us look at each one carefully.
2.3.1 Conduction
Conduction is the transfer of thermal energy through a material without the material itself moving. Energy is passed from particle to particle through vibrations and (in metals) through free electrons.
Think about a dosa tawa on the gas stove. The gas flame heats the bottom of the tawa. The particles at the bottom vibrate faster and bump into their neighbours, passing on energy. Gradually, the whole tawa gets hot -- even the parts not directly over the flame. That is conduction at work.
Good Conductors and Bad Conductors (Insulators)
Good conductors transfer thermal energy quickly. All metals are good conductors: copper, aluminium, iron, steel, silver. That is why cooking utensils are made of metal -- they conduct heat from the stove to the food efficiently.
Bad conductors (insulators) transfer thermal energy slowly. Examples: wood, plastic, rubber, glass, air, cotton, wool. That is why saucepan handles are made of wood or plastic (like your pressure cooker handle) -- so you can hold them without burning your hand.
Experiment: Demonstrating Good and Bad Conductors
A classic experiment uses rods of different materials (copper, aluminium, iron, glass, wood) of the same length and width. One end of each rod is coated with a small blob of wax. The other ends are all placed in boiling water (or heated equally with a Bunsen burner).
The wax melts first on the best conductor (copper) and last on the worst conductor (wood). Glass falls somewhere in between metals and true insulators.
Observation: The order of melting is typically: copper > aluminium > iron/steel > glass > wood. This shows that metals are good conductors, while non-metals like glass and wood are poor conductors.
When describing a conduction experiment, always mention: (1) the rods must be the same dimensions (fair test), (2) they are all heated equally, (3) the wax acts as a temperature indicator. If the question asks you to describe the results, state which rod's wax melts first and explain this means it conducts heat the best.
How Conduction Works at the Particle Level
In all solids (including non-metals): Conduction works through lattice vibrations. When one end of a solid is heated, the particles there vibrate more vigorously. They bump into their neighbours, making those particles vibrate more too. This vibration is passed along from particle to particle through the solid. This is slow because energy is transferred one particle at a time.
In metals (additionally): Metals have a special advantage -- they contain free (delocalised) electrons. These electrons are not attached to any particular atom and can move freely through the metal. When one end is heated, these free electrons gain kinetic energy, move quickly through the metal, and collide with atoms further along, transferring energy to them. This is much faster than lattice vibrations alone, which is why metals are much better conductors than non-metals.
Think of it like this: In a non-metal, energy moves like a chain of people passing a bucket of water -- slow, one by one. In a metal, it is like having motorcyclists (free electrons) zooming through the crowd delivering buckets -- much faster!
Why Gases and Liquids Are Bad Conductors
In gases, particles are far apart with large gaps between them. They rarely collide, so vibrations are not easily passed from one particle to the next. The large spaces between particles mean conduction is very poor.
In most liquids, particles are close together but not in a fixed arrangement. They can slide past each other rather than transmitting vibrations efficiently. Liquids lack the rigid lattice structure of solids and lack free electrons (unless they are liquid metals like mercury). So conduction in liquids is generally poor, though better than in gases.
Intermediate Conductors
Many non-metallic solids (like glass, concrete, brick) conduct thermal energy better than insulators like air or cotton, but much less well than metals. They have a rigid lattice so vibrations can pass through, but they lack free electrons. These materials are sometimes called intermediate conductors.
2.3.2 Convection
Convection is the transfer of thermal energy by the movement of a fluid (liquid or gas) itself. It is an important method of thermal energy transfer in liquids and gases. Convection does NOT happen in solids because the particles in a solid cannot move from place to place.
How Convection Works
Here is the step-by-step process -- use this structure in your exam answers:
- A region of fluid is heated (e.g., water near the bottom of a pan on the stove).
- The heated fluid expands (particles move faster, spread out, take up more volume).
- Because it expands, it becomes less dense (same mass, larger volume = lower density).
- The less dense warm fluid rises (it floats upward because it is lighter than the cooler fluid above it).
- Cooler, denser fluid sinks to take its place at the bottom.
- This cooler fluid is now heated, expands, becomes less dense, and rises -- and the cycle repeats.
- This creates a continuous loop called a convection current.
Hot rises, cold sinks. Think of a hot air balloon -- the hot air inside is less dense than the surrounding cool air, so the balloon rises. Same principle! For the exam, remember the chain: Heated → Expands → Less dense → Rises → Replaced by cooler fluid → Convection current.
Experiment to Illustrate Convection
In water: Fill a glass beaker with water. Add a small crystal of potassium permanganate (KMnO4) at the bottom. Gently heat the water directly below the crystal using a small flame. You will see purple streaks of coloured water rising from the crystal, moving up, spreading across the top, cooling, and then sinking back down the sides. This shows the convection current clearly.
In air: Light a candle and hold a smoking joss stick (agarbatti) above it. The smoke rises straight up with the convection current of hot air from the candle. If you hold the agarbatti to the side at the same height, the smoke drifts towards the candle -- showing that cooler air is being drawn in to replace the rising hot air.
Sea breeze example (Goa/Kerala): During the day, land heats up faster than the sea. Hot air rises over the land, and cooler air from over the sea moves in to replace it -- creating a refreshing sea breeze. At night, the land cools faster, so the reverse happens (land breeze blows from land to sea).
Ceiling fans: Your ceiling fan does not cool the air itself. It pushes air downward, which increases air movement over your skin, speeding up evaporation of sweat. But it also helps distribute warm air that rises and collects near the ceiling, mixing it with cooler air below.
A common mistake is writing "hot air rises because heat rises." This is WRONG. The correct answer is: hot air rises because it is less dense than the surrounding cooler air. Always link the rising to a density difference caused by expansion.
2.3.3 Radiation
Thermal radiation is infrared radiation -- a type of electromagnetic wave. Every object emits (gives out) infrared radiation. Yes, even you! Even a cold ice cube emits some infrared radiation (just much less than a hot object).
Key Facts About Radiation
- Thermal radiation is infrared radiation (part of the electromagnetic spectrum).
- All objects emit thermal radiation -- not just hot ones.
- Radiation does NOT require a medium (material) to travel through. It can travel through a vacuum. This is how the Sun's energy reaches Earth -- through 150 million km of empty space!
- Radiation travels at the speed of light.
Effect of Surface Colour and Texture
This is one of the most-tested topics in IGCSE Physics. The surface of an object affects how well it emits, absorbs, and reflects infrared radiation.
| Surface Type | Emission | Absorption | Reflection |
|---|---|---|---|
| Matt black / dark, rough | Good emitter | Good absorber | Poor reflector |
| Shiny silver / light, smooth | Poor emitter | Poor absorber | Good reflector |
Indian examples:
- White clothes in summer: You wear light-coloured clothes on a hot Bangalore summer day because white/light surfaces are poor absorbers of infrared radiation -- they reflect most of the Sun's radiation away from your body, keeping you cooler.
- Dark clothes in winter: On a chilly December morning in Bangalore, wearing dark clothes helps because dark surfaces are good absorbers of infrared radiation from the Sun.
- Solar water heaters on Indian rooftops: The absorber plate is painted matt black to absorb maximum solar radiation. The storage tank may be shiny/silvery to reduce heat loss by radiation.
Remember: "Black grabs, white throws back." Matt black surfaces are like sponges for radiation -- they absorb and emit well. Shiny silver surfaces are like mirrors -- they reflect radiation away and are bad at absorbing and emitting.
Thermal Equilibrium -- Energy Balance
For an object to stay at a constant temperature, the rate at which it receives energy must equal the rate at which it transfers energy away. This is thermal equilibrium.
- If an object receives energy faster than it transfers it away, its temperature increases.
- If an object transfers energy away faster than it receives energy, its temperature decreases.
- If the rates are equal, the temperature stays constant.
Earth's Temperature Balance
The Earth's average temperature is determined by the balance between:
- Incoming radiation from the Sun (short-wavelength visible and ultraviolet radiation)
- Outgoing radiation emitted by the Earth (long-wavelength infrared radiation)
If more energy comes in than goes out, the Earth warms up (global warming). Factors that affect this balance include:
- Greenhouse gases (CO2, methane, water vapour) in the atmosphere absorb some of the outgoing infrared radiation and re-emit it back towards Earth, reducing the rate of energy loss. More greenhouse gases = Earth warms up.
- Reflectivity of Earth's surface: Ice and snow reflect solar radiation back into space. If ice melts (due to warming), less radiation is reflected, more is absorbed, and Earth warms further.
- Cloud cover: Clouds can both reflect incoming solar radiation (cooling effect) and trap outgoing infrared radiation (warming effect).
Experiments: Good and Bad Emitters
Leslie's cube is a metal container with four different surfaces: matt black, shiny black, matt white, and shiny silver. It is filled with hot water. An infrared detector (or thermometer) is placed at equal distances from each face.
Result: The matt black surface gives the highest reading (best emitter). The shiny silver surface gives the lowest reading (worst emitter). This shows that dark, rough surfaces emit more infrared radiation than light, shiny surfaces.
Experiments: Good and Bad Absorbers
Place two identical metal plates -- one painted matt black, one painted shiny silver -- at equal distances from a radiant heater (like a bar fire). Attach a thermometer to the back of each plate.
Result: The matt black plate heats up faster -- its thermometer reading rises more quickly. This shows matt black surfaces are better absorbers of infrared radiation than shiny silver surfaces.
Rate of Emission Depends On:
- Surface temperature: Hotter objects emit infrared radiation at a greater rate. A cup of chai at 80 degC emits much more radiation than the same cup at 40 degC.
- Surface area: Larger surface areas emit more radiation. A car radiator has fins to increase its surface area, so it can emit (and transfer) heat more quickly.
Students often confuse the three methods of heat transfer. Remember: Conduction = through a material (solids mainly), Convection = by movement of a fluid (liquids/gases), Radiation = electromagnetic waves (no medium needed, works through vacuum). The Sun's energy reaches Earth by radiation ONLY -- there is no air in space for conduction or convection.
2.3.4 Consequences of Thermal Energy Transfer
Simple Applications (One Transfer Method Dominates)
(a) Heating a kitchen pan:
When you heat a steel pan on the gas stove, the flame transfers energy to the base of the pan mainly by conduction. The metal base is a good conductor -- the particles in the metal vibrate more and pass energy through the pan via lattice vibrations and free electrons. The food or water inside the pan is then heated by conduction from the pan walls, and also by convection within the liquid (hot water at the bottom rises, cooler water sinks). The pan handle is made of wood or plastic (insulator) so you can hold it -- these materials are bad conductors and slow down thermal energy transfer to your hand.
(b) Heating a room by convection:
A heater placed at the bottom of a room heats the air directly around it. This air expands, becomes less dense, and rises. Cooler, denser air from elsewhere in the room moves in to replace it. This creates a convection current that circulates warm air throughout the room. This is why heaters are placed at ground level -- so the warm air can rise and spread naturally. (Air conditioners, by contrast, are placed high up because they blow cold air which is denser and sinks.)
Complex Applications (Multiple Transfer Methods)
(a) A fire burning wood or coal:
A campfire or a coal sigri demonstrates all three methods of thermal energy transfer:
- Conduction: If you poke a metal rod into the fire, the end in the fire gets hot first, and heat conducts along the rod towards your hand. The ground beneath the fire also gets warm through conduction.
- Convection: The fire heats the air above it. This hot air rises (it becomes less dense), carrying heat energy upward. That is why smoke and sparks rise. Cooler air is drawn in from the sides to replace the rising hot air, feeding the fire with oxygen.
- Radiation: You can feel the heat from a campfire even if you stand to the side with no air flowing towards you. Infrared radiation is emitted by the hot flames and coals and travels through the air (and could even travel through a vacuum) to warm your skin. This is why you feel warm facing a fire even from several metres away.
(b) A radiator in a car:
A car engine generates a lot of heat. The cooling system uses all three transfer methods to keep it from overheating:
- Conduction: Heat is conducted from the hot engine block to the coolant liquid (water mixed with antifreeze) flowing through channels in the engine. The metal engine and pipe walls are good conductors.
- Convection: The coolant liquid circulates through the engine and to the radiator (usually by a pump, so this is "forced convection"). In the radiator, hot coolant transfers energy to the metal fins. Hot air also rises away from the radiator naturally.
- Radiation: The radiator has a large surface area (many metal fins) and emits infrared radiation to the surroundings. The fins are often painted black to maximise radiation emission.
A fan behind the radiator blows air across the fins, increasing convection and speeding up cooling when the car is stationary (like in Bangalore traffic!).
• The black car has a dark surface that absorbs most of the infrared radiation. The absorbed energy increases the kinetic energy of the particles in the car body, raising its temperature. This heat is then conducted and re-radiated into the car interior.
• The white car reflects most of the infrared radiation, so much less energy is absorbed. Its surface stays cooler, and less heat enters the interior.
The temperature difference (65°C vs 48°C = 17°C difference) shows how significant this effect is. Auto-rickshaw drivers use white cloth as a cheap radiation shield — it reflects sunlight and infrared radiation, keeping the vehicle cooler for passengers.
Wooden spoon: Wood has no free electrons. Its particles are loosely bonded in a complex structure with air pockets. Vibrations are passed along very slowly. After 5 minutes, the heat has barely travelled a few centimetres up the handle.
This is why saucepan handles are made of insulators — they prevent thermal energy from conducting to your hand, even though the pan itself must be metal (a good conductor) to efficiently transfer heat to the food.
• The land heats up faster than the sea (land has lower specific heat capacity)
• Air above the land gets heated, expands, becomes less dense, and rises
• Cooler air from above the sea flows in to replace it
• This creates a breeze from sea → land
Evening (land breeze):
• The land cools down faster than the sea
• The sea is now warmer than the land
• Air above the sea is heated, expands, becomes less dense, and rises
• Cooler air from above the land flows towards the sea to replace it
• This creates a breeze from land → sea
The driving force is always: hot air rises, cold air moves in to replace it.
Conduction needs particles to pass vibrations along. Convection needs a fluid to circulate. A vacuum has no particles at all, so neither can occur across the gap.
2. Silvered (shiny) inner surfaces → Reduces radiation
The shiny, silver coating reflects infrared radiation back into the flask instead of letting it escape. Shiny surfaces are poor emitters AND poor absorbers of radiation.
3. Plastic/cork stopper → Prevents conduction and convection at the top
Plastic is a poor conductor (insulator), so little heat is conducted out through the stopper. The stopper also prevents hot air/steam from escaping by convection.
4. Glass walls (not metal) → Reduces conduction
Glass is a much poorer conductor than metal, so less heat is conducted through the walls themselves.
Polystyrene foam contains millions of tiny trapped air pockets. Air is a very poor conductor, and the pockets prevent convection currents from forming within the foam. This creates an excellent insulating barrier between the hot outside and cold inside.
2. White reflective roof paint → Reduces radiation
The roof receives the most direct sunlight. White paint reflects most infrared radiation and visible light from the sun, preventing the roof from absorbing heat. A dark roof would absorb radiation and conduct heat into the warehouse.
3. Plastic strip curtains at loading docks → Reduces convection
When doors open, warm outside air would rush in (convection — warm air displaces cold air). The plastic strips create a physical barrier that blocks air movement while still allowing forklifts and workers to pass through.
4. Double concrete walls → Reduces conduction
Two layers with insulation between them conduct less heat than a single thick wall, because the insulation layer has much lower thermal conductivity than concrete.
Quick Reference: Everything You Need to Know
2.2 Thermal Properties
- Thermal expansion: Substances expand when heated because particles gain energy and vibrate/move more, pushing apart. Order: solids < liquids < gases.
- Internal energy increases when temperature rises.
- Temperature = measure of average kinetic energy of particles.
| Substance | c (J/(kg degC)) |
|---|---|
| Water | 4200 |
| Aluminium | 900 |
| Iron/Steel | 450 |
| Copper | 390 |
- Water: melts at 0 degC, boils at 100 degC (at 1 atm).
- During melting/boiling: energy is supplied but temperature stays constant (energy breaks bonds, not increases KE).
- Evaporation: surface only, any temperature, fastest particles escape, liquid cools.
- Boiling: throughout liquid, at boiling point only, needs heating, temperature constant.
- Factors increasing evaporation: higher temperature, larger surface area, more air movement.
2.3 Transfer of Thermal Energy
| Property | Conduction | Convection | Radiation |
|---|---|---|---|
| How it works | Particle vibrations + free electrons (metals) | Movement of fluid due to density changes | Electromagnetic (infrared) waves |
| Requires medium? | Yes (solids mainly) | Yes (fluids only) | No (works through vacuum) |
| Best in | Metals (worst in gases) | Liquids and gases | Any -- even empty space |
| Example | Dosa tawa, metal spoon | Sea breeze, boiling water | Sun to Earth, campfire warmth |
- Matt black: best absorber AND emitter of infrared radiation.
- Shiny silver: best reflector, worst absorber and emitter.
- Thermal equilibrium: rate in = rate out means constant temperature.
- Rate of emission depends on: surface temperature and surface area.
- Earth's temperature: balance between incoming solar radiation and outgoing infrared radiation, affected by greenhouse gases.
In the exam, always think about WHICH method of heat transfer is relevant. Ask yourself: Is it going through a solid (conduction)? Is a fluid moving (convection)? Is it through empty space or between surfaces (radiation)? Many questions ask you to identify and explain which methods are at work. Use the key words: "vibrations," "free electrons," "less dense," "rises," "infrared," and "vacuum."
You have made it through two big topics, Tara! These concepts show up again and again in IGCSE Physics -- in questions about energy, the environment, and even electricity. Go through the MCQs, review any questions you got wrong, and come back to this guide before your exam. You have got this!