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All revision notes Revision notes · ThermodynamicsThermodynamics revision notes
Temperature and internal energy, specific heat capacity, expansion, conduction, convection and radiation, latent heat, gas pressure, heat engines and energy resources. Each section follows one lesson: what you need to know, key terms, the equations with units, a worked example, common mistakes and an exam tip.
1 Heat & Temperature
What you need to know
Temperature
- Temperature measures the average kinetic energy of the particles. Heating makes particles move faster; it never makes them bigger.
- Internal energy is the total kinetic and potential energy of all the particles in an object. A bath and a mug at the same temperature have the same average particle speed, but the bath holds far more internal energy.
- Heating a system raises its internal energy: it either raises the temperature or changes the state.
Specific Heat Capacity
- Specific heat capacity c is the energy needed to raise the temperature of 1 kg of a substance by 1 °C. Water's is high, about 4200 J/kg °C; copper's is about 385 J/kg °C.
- The energy needed depends on the mass, the material and the temperature rise: ΔE = mcΔθ. A 1 °C step is the same size as a 1 K step.
- In the practical, an electric heater warms a lagged block; a joulemeter (or power × time) gives the energy and a thermometer the rise. Energy lost to the air makes the measured c too big, so insulate the block.
Heat Stores
- A good heat store has a high specific heat capacity: for the same mass and the same temperature drop, it gives out the most energy. That is why radiators and coolants use water.
- The thermal capacity (heat capacity) of a whole object is the energy needed to raise its temperature by 1 °C; it equals mc.
- Rearrange ΔE = mcΔθ to find the mass of a store, m = ΔE ÷ (cΔθ), or how far it cools, Δθ = ΔE ÷ (mc).
Key terms
- Temperature
- a measure of the average kinetic energy of the particles, in °C or K
- Internal energy
- the total kinetic and potential energy of all the particles in a system, in J
- Specific heat capacity
- the energy needed to raise 1 kg of a substance by 1 °C, in J/kg °C
- Thermal capacity
- the energy needed to raise a whole object by 1 °C, in J/°C
Equations
| Equation | What it says, symbols and units |
|---|---|
ΔE = mcΔθ | Energy needed to change the temperature of a mass; c is the energy to warm 1 kg by 1 °C.
|
c = ΔEmΔθ | Specific heat capacity is the energy needed to raise 1 kg of a material by 1 °C.
|
P = Et | Power is the energy transferred each second; 1 W is 1 J/s.
|
Diagram to know
The specific heat capacity practical: a metal block with two holes, an immersion heater in one and a thermometer in the other, the block wrapped in insulation, the heater wired to a power supply through a joulemeter (or an ammeter and a voltmeter).
Worked example
A 50 W heater warms a 0.80 kg aluminium block for 4.0 minutes. The block's temperature rises from 21 °C to 36 °C. Find the specific heat capacity of aluminium from these results, and explain why it is higher than the book value of 900 J/kg °C.
- Time = 4.0 × 60 = 240 s, so the energy supplied is E = Pt = 50 × 240 = 12 000 J.
- Temperature rise Δθ = 36 − 21 = 15 °C.
- c = ΔE ÷ (mΔθ) = 12 000 ÷ (0.80 × 15) = 1000 J/kg °C.
- Some of the heater's energy warmed the air instead of the block, so fewer degrees were gained per joule and the calculated c came out too big.
Answer: 1000 J/kg °C; energy lost to the surroundings makes the measured value too high
Common mistakes
- Using a thermometer reading instead of the temperature change for Δθ.
- Saying temperature and internal energy are the same thing. A large cool object can hold more internal energy than a small hot one.
- Leaving the time in minutes when working out E = Pt.
Exam tip
When asked to improve the practical, name the error and its fix together: "insulate the block, because energy lost to the air makes c too large". A fix with no reason rarely gets the mark.
AP Physics and NGSS: Thermal physics is not part of AP Physics 1; AP Physics 2 (Unit 9) writes the same law as Q = mcΔT. NGSS MS-PS3-4 asks students to plan an investigation linking the energy transferred, the type and mass of matter and the temperature change.
2 Expansion & Changes of State
What you need to know
Thermal Expansion
- Solids, liquids and gases all expand when heated. For the same temperature rise a gas expands most and a solid least (gases are compared at constant pressure).
- The particles do not get bigger. They move or vibrate faster, so on average they sit further apart.
- The order follows the forces between particles: very strong in a solid, strong in a liquid, almost none in a gas, so nothing holds a gas's particles together.
Expansion at Work
- Where expansion could cause damage, leave room for it: gaps in railways and bridges, expansion joints, power lines hung with some sag.
- Where it is useful, let it move something: the liquid in a liquid-in-glass thermometer expands more than the glass and climbs the tube.
- A bimetallic strip is two metals riveted together. Heated, it bends with the metal that expands more on the outside of the curve; thermostats use this to switch a circuit off.
Weigh the Change
- Melting, freezing, boiling, evaporating, condensing and subliming are physical changes: reverse them and the same substance comes back.
- Changes of state conserve mass. In a sealed container the reading on a balance does not change while ice melts and the water boils.
- When a gas condenses or a liquid freezes, its particles lose energy, slow down and pull closer together.
Key terms
- Thermal expansion
- the increase in size of a material when its temperature rises
- Bimetallic strip
- two metals joined together that bend when heated because they expand by different amounts
- Condensation
- the change of state from gas to liquid
- Sublimation
- the change of state from solid straight to gas, as with dry ice
Equations
| Equation | What it says, symbols and units |
|---|---|
growth = length × growth per metre per °C × temperature rise | How much longer a solid gets when it warms; the growth per metre per °C is given in the question.
A handy rule for solids, not one the syllabuses ask you to learn: the figure per metre (steel about 0.012 mm per °C) is always given. |
Diagram to know
A bimetallic strip before and after heating: brass and steel joined side by side, straight when cold, curved when hot with the brass on the outside of the curve, and the contact it pulls away from in a thermostat.
Worked example
A 30 m steel rail is laid on a day when it is at 12 °C. In full sun it can reach 47 °C. Steel grows by 0.012 mm per metre for each °C rise. What is the smallest gap to leave at each joint?
- Temperature rise = 47 − 12 = 35 °C.
- Growth = 30 × 0.012 × 35 = 12.6 mm.
- The gap must be at least as big as the growth, so that the rail does not push against the next one and buckle.
Answer: At least 12.6 mm (about 13 mm)
Common mistakes
- Saying the particles expand. The particles stay the same size; the gaps between them grow.
- Saying a substance gets lighter when it boils. In a sealed container the mass stays the same; in an open one the vapour leaves.
- Putting the metal that expands more on the inside of a bimetallic strip's curve.
Exam tip
In an "explain using particles" question, always say three things: the particles gain kinetic energy, they move or vibrate faster, and so they move further apart. Each idea is usually one mark.
AP Physics and NGSS: Thermal expansion is not in AP Physics 1 or GCSE specifications; NGSS MS-PS1-4 links adding thermal energy to particle motion, temperature and state.
3 Conduction, Convection & Radiation
What you need to know
Conduction
- Conduction passes energy from particle to particle through a material, mainly in solids. Hotter particles vibrate more and pass energy to their neighbours.
- Metals are good conductors because their free electrons also carry energy. Wood, plastic, glass and still air are poor conductors (insulators).
- A metal at room temperature feels colder than wood because it conducts energy away from your hand faster, not because it is colder.
Convection
- Convection happens only in liquids and gases. Warmed fluid expands, its density falls and it rises; cooler, denser fluid sinks to take its place, making a convection current.
- So heaters are placed low in a room and cooling units high up.
Radiation
- Every object emits infrared radiation, and a hotter object emits more. Radiation needs no particles, so it crosses a vacuum.
- Matt black surfaces are the best emitters and the best absorbers of infrared; shiny silver surfaces are the worst, because they reflect it.
Insulation
- Energy escapes by conduction, convection and radiation, and faster across a bigger temperature difference, so a cooling curve starts steep and flattens out.
- Insulation slows all three. Trapped air is a poor conductor and cannot form convection currents; shiny foil cuts radiation; a lid stops evaporation and convection from a liquid's surface.
- A thicker layer, a material with a lower thermal conductivity and a smaller area all make the rate of energy loss smaller.
Key terms
- Conduction
- energy transfer through a material by particles passing it to their neighbours
- Convection current
- the circulation of a fluid in which warm fluid rises and cool fluid sinks
- Infrared radiation
- electromagnetic waves emitted by every object, more strongly when hotter
- Thermal conductivity
- how well a material conducts energy; a low value means a good insulator
Equations
| Equation | What it says, symbols and units |
|---|---|
P = kAΔTd | Rate of energy transfer by conduction through a layer.
AP Physics 2 only. IGCSE and GCSE ask only that a higher conductivity, a bigger area or a thinner layer means a faster transfer. |
Diagram to know
A vacuum flask in cross-section: a double-walled glass bottle with a vacuum between the walls, silvered surfaces facing the vacuum, a plastic stopper at the top and a protective case with insulating supports.
Worked example
Explain how a vacuum flask keeps a hot drink hot, naming the way of energy transfer each feature reduces.
- The vacuum between the two glass walls has no particles, so energy cannot cross it by conduction or convection.
- The silvered surfaces are poor emitters of infrared, and reflect infrared from the drink back towards it, so little energy crosses the vacuum by radiation.
- The stopper is made of a poor conductor and stops warm air rising out of the flask, reducing conduction and convection (and evaporation) at the top.
- The thin glass walls and plastic supports conduct only a little energy to the outer case.
Answer: Vacuum: no conduction or convection; silvering: less radiation; stopper: less conduction, convection and evaporation
Common mistakes
- Saying "heat rises". It is the warmer, less dense fluid that rises.
- Saying convection happens in solids. Particles in a solid cannot flow.
- Saying a white surface is a poor emitter of infrared. Colour in visible light says little; it is matt black against shiny silver that matters.
Exam tip
Name the process and the reason together: "the trapped air is a poor conductor and cannot move, so it reduces conduction and convection". Marks are given for linking each feature to the transfer it reduces.
AP Physics and NGSS: AP Physics 2 (Unit 9) uses the conduction equation above; AP Physics 1 does not cover thermal transfer. NGSS MS-PS3-3 asks students to design a device that slows or speeds energy transfer, such as an insulated cup.
4 Radiation Balance
What you need to know
Good Absorbers
- Dark, matt surfaces are good absorbers of infrared; light, shiny surfaces are poor absorbers because they reflect it. Good absorbers are also good emitters.
- To compare absorbers, shine the same heater on two identical cans of water, one matt black and one shiny, and compare their temperature rises in the same time.
Steady Temperature
- An object stays at a constant temperature when it transfers energy away as fast as it receives it. If it receives more than it loses, it warms; if less, it cools.
- As an object warms it emits faster, so its temperature rises until the power out matches the power in, then levels off.
- The rate of emission increases with the surface temperature and with the surface area.
Earth's Thermostat
- Earth's temperature is steady when it emits infrared to space as fast as it absorbs radiation from the Sun.
- Ice and cloud reflect sunlight, so less is absorbed. Greenhouse gases such as carbon dioxide and water vapour absorb infrared leaving the surface and send some back down, so Earth must be warmer to emit enough.
- More greenhouse gas or less ice upsets the balance and Earth warms until the outgoing power matches the incoming power again.
All Three at Once
- Real devices use all three transfers together. A car radiator moves energy by conduction through its metal fins and then by convection into the moving air; only a small share leaves by radiation.
- Very hot objects, such as a coal fire, transfer much more of their energy by radiation.
Key terms
- Absorber
- a surface that takes in radiation that falls on it rather than reflecting it
- Emitter
- a surface that gives out radiation
- Thermal equilibrium with the surroundings
- a steady temperature where the power received equals the power lost
- Greenhouse gas
- a gas such as carbon dioxide or water vapour that absorbs infrared emitted by Earth's surface
Equations
| Equation | What it says, symbols and units |
|---|---|
P = mcΔTt | The power absorbed by a mass of water from its temperature rise in a time, ignoring losses.
|
P = (1 − r)S | Sunlight absorbed per square metre when a fraction r is reflected; at balance Earth emits the same power as infrared.
A way to put numbers on the balance; the syllabuses ask for the idea, not this equation. |
Diagram to know
Earth's energy balance: arrows of incoming sunlight, some reflected by cloud and ice, the rest absorbed by the surface; infrared arrows leaving the surface, some absorbed by greenhouse gases and re-emitted back down, the rest escaping to space.
Worked example
A matt black can holds 0.25 kg of water. Under a heater it warms by 12 °C in 8.0 minutes. At what rate does it absorb energy, ignoring losses? (c of water = 4200 J/kg °C)
- Energy absorbed = mcΔT = 0.25 × 4200 × 12 = 12 600 J.
- Time = 8.0 × 60 = 480 s.
- Power = 12 600 ÷ 480 = 26 W (26.25 W).
Answer: About 26 W
Common mistakes
- Saying greenhouse gases let more sunlight in. They change how much infrared gets out, not how much sunlight arrives.
- Saying an object at a steady temperature has stopped absorbing or emitting. Both continue, at equal rates.
- Saying a car radiator works mainly by radiation. Almost all of its energy leaves by convection.
Exam tip
For a balance question, use the words "rate of absorption" and "rate of emission" and compare them: "the rate of emission is less than the rate of absorption, so the temperature rises". Comparing rates is what the mark scheme looks for.
AP Physics and NGSS: Earth's energy balance is NGSS HS-ESS2-4 and HS-PS3-1; AP Physics 1 does not cover thermal radiation.
5 The Laws of Thermodynamics
What you need to know
Thermal Equilibrium
- Energy flows from a hotter object to a colder one until both reach the same temperature: thermal equilibrium. Then there is no net flow.
- The zeroth law: two objects each in thermal equilibrium with a third (such as a thermometer) are in equilibrium with each other.
- With no energy escaping, energy lost by the hot object = energy gained by the cold one: m₁c₁(T₁ − Tf) = m₂c₂(Tf − T₂).
The First Law
- The first law is conservation of energy: a gas's internal energy rises by the energy heated into it minus the work it does, ΔU = Q − W.
- Doing work on a gas warms it, with no heating at all: a bicycle pump gets warm, and a diesel engine ignites its fuel by squeezing the air quickly.
- Pushed slowly, the energy leaks out through the walls as it is added; pushed fast, it stays in the gas and the temperature rises sharply.
The Second Law
- The second law: by itself, energy flows from hot to cold, never the other way. Moving it from cold to hot, as a fridge does, needs work.
- A fridge gives out at the back the energy it takes from inside plus the work done by its compressor: Qhot = Qcold + W.
- In every real process some energy is dissipated and becomes less useful, so no machine is 100% efficient.
Key terms
- Thermal equilibrium
- the state in which two objects are at the same temperature and no net energy flows between them
- Zeroth law
- two objects in thermal equilibrium with a third are in thermal equilibrium with each other
- First law of thermodynamics
- energy is conserved: ΔU = Q − W
- Second law of thermodynamics
- energy flows by itself only from hotter to colder
Equations
| Equation | What it says, symbols and units |
|---|---|
m1c1(T1 − Tf) = m2c2(Tf − T2) | Energy lost by the hot object equals energy gained by the cold one, if none escapes.
|
ΔU = Q − W | The change in internal energy is the energy heated in minus the work the gas does.
AP Physics 2 writes ΔU = Q + W, with W the work done on the gas: the same physics, the other sign. |
Qhot = Qcold + W | A fridge or heat pump dumps the energy taken from the cold side plus the work done on it.
|
Diagram to know
Two temperature–time lines on one graph, one for hot milk falling and one for cold water rising, meeting at the shared final temperature where both flatten.
Worked example
0.20 kg of water at 80 °C is mixed with 0.30 kg of water at 20 °C in an insulated cup. Find the final temperature, and show that the energy lost equals the energy gained. (c = 4200 J/kg °C)
- Energy lost = energy gained: 0.20 × 4200 × (80 − T) = 0.30 × 4200 × (T − 20). The c cancels.
- 16 − 0.20T = 0.30T − 6, so 22 = 0.50T and T = 44 °C.
- Hot water: 0.20 × 4200 × 36 = 30 240 J lost (80 − 44 = 36 °C).
- Cold water: 0.30 × 4200 × 24 = 30 240 J gained (44 − 20 = 24 °C). The two match.
Answer: 44 °C; 30 240 J lost and 30 240 J gained
Common mistakes
- Giving a final temperature outside the two starting temperatures. It must lie between them.
- Saying a fridge "makes cold". It moves energy out of the cold space, and needs work to do it.
- Mixing up the sign of W: in ΔU = Q − W, W is the work done by the gas.
Exam tip
Check every mixing answer: the final temperature is nearer the starting temperature of the object with the bigger mc. If it is not, a subtraction is the wrong way round.
AP Physics and NGSS: The laws of thermodynamics, ΔU = Q + W and entropy belong to AP Physics 2 (Unit 9), not AP Physics 1. NGSS HS-PS3-4 asks for an investigation showing that energy flows until two components reach thermal equilibrium.
6 Latent Heat & Phase Changes
What you need to know
Heating Curves
- A heating curve goes flat while a substance melts or boils: energy goes in but the temperature stays steady.
- On a flat part the energy breaks bonds between particles, raising their potential energy, not their kinetic energy. So ΔE = mcΔθ does not apply there.
- Pure water melts at 0 °C and boils at 100 °C at normal atmospheric pressure. Freezing and condensing give the energy back at the same temperatures.
Latent Heat
- Specific latent heat L is the energy needed to change the state of 1 kg of a substance with no change in temperature: E = mL.
- Latent heat of fusion is for melting (water: 334 000 J/kg); latent heat of vaporisation is for boiling (water: 2 260 000 J/kg). Boiling takes far more energy, because the particles are separated completely.
Evaporation
- Evaporation takes the fastest particles from a liquid's surface. The average kinetic energy of those left falls, so the liquid cools, and so does anything touching it.
- Evaporation is faster at a higher temperature, with a larger surface area and with air moving over the surface.
- Evaporation happens at any temperature and only at the surface; boiling happens at one temperature, all through the liquid, with bubbles.
Ice to Steam
- To take ice to steam, add every stage: mcΔθ to warm each state (ice c ≈ 2100 J/kg °C, water c = 4200 J/kg °C) and mL for each change of state.
- The boiling stage needs by far the most energy.
Key terms
- Specific latent heat
- the energy needed to change the state of 1 kg of a substance at constant temperature, in J/kg
- Latent heat of fusion
- the energy to melt 1 kg of a solid at its melting point
- Latent heat of vaporisation
- the energy to boil 1 kg of a liquid at its boiling point
- Evaporation
- the escape of the most energetic particles from the surface of a liquid, below its boiling point
Equations
| Equation | What it says, symbols and units |
|---|---|
E = mL | Energy needed to melt or boil a mass, with no change in temperature.
|
ΔE = mcΔθ | Energy needed to change the temperature of a mass; c is the energy to warm 1 kg by 1 °C.
|
Diagram to know
A heating curve for water: temperature against time, rising for ice, flat at 0 °C (melting), rising for water, flat at 100 °C (boiling, a longer flat part), then rising for steam. Label each flat part with its change of state.
Worked example
How much energy is needed to turn 0.20 kg of ice at −10 °C into water at 20 °C? (c of ice = 2100 J/kg °C, c of water = 4200 J/kg °C, L of fusion = 334 000 J/kg)
- Warm the ice to 0 °C: 0.20 × 2100 × 10 = 4200 J.
- Melt it at 0 °C: 0.20 × 334 000 = 66 800 J.
- Warm the water to 20 °C: 0.20 × 4200 × 20 = 16 800 J.
- Total = 4200 + 66 800 + 16 800 = 87 800 J.
Answer: 87 800 J (about 88 kJ)
Common mistakes
- Using ΔE = mcΔθ across a change of state. The flat part needs E = mL.
- Saying the temperature rises while ice melts. It stays at 0 °C until all the ice has melted.
- Saying evaporation only happens at 100 °C. It happens at any temperature.
Exam tip
In a multi-stage question, set out one line per stage with its own equation and answer, then add. Method marks are given stage by stage even if the final total is wrong.
AP Physics and NGSS: Latent heat is not in AP Physics 1. NGSS MS-PS1-4 and HS-PS3-1 cover the particle model of changes of state and the energy calculations.
7 Gas Particles & Pressure
What you need to know
Brownian Motion
- Brownian motion: tiny specks of smoke or pollen seen under a microscope move in random, jerky paths because fast, invisible molecules hit them unevenly from different sides.
- It is evidence that matter is made of particles in constant random motion. A molecule is far smaller than a speck; heavier, slower specks are hit by light, fast molecules.
- Warmer air or water means faster molecules and a more vigorous jiggle.
Pressure from Collisions
- Gas particles move fast in random directions and hit every surface. Each hit exerts a force, and the force per unit area is the gas pressure, acting at right angles to every surface.
- More particles in the same space, or faster particles (a higher temperature), mean more hits each second and a higher pressure.
- At constant volume, heating a gas raises its pressure, because the particles hit the walls harder and more often.
Squeeze a Gas
- For a fixed mass of gas at constant temperature, pressure × volume is constant: p₁V₁ = p₂V₂. Halve the volume and the pressure doubles.
- Less room means the particles hit the walls more often. A graph of p against V is a curve; p against 1/V is a straight line through the origin.
- The law holds only if the temperature stays the same: squeeze a gas quickly and it warms.
Key terms
- Brownian motion
- the random movement of microscopic specks caused by uneven collisions with fast-moving molecules
- Gas pressure
- the force per unit area exerted by gas particles colliding with a surface, in Pa
- Random motion
- movement with no pattern, in every direction
- Boyle's law
- for a fixed mass of gas at constant temperature, pV is constant
Equations
| Equation | What it says, symbols and units |
|---|---|
p = FA | Pressure is force per unit area: the same force on a bigger area gives a smaller pressure.
|
pV = constant | For a fixed mass of gas at constant temperature, halve the volume and the pressure doubles (p₁V₁ = p₂V₂).
Use the same units on both sides of p₁V₁ = p₂V₂ (kPa and cm³ are fine). |
Diagram to know
A gas in a sealed box with particles drawn as dots with arrows in random directions, one hitting a wall and bouncing back; and a graph of p against 1/V as a straight line through the origin.
Worked example
A bicycle pump holds 180 cm³ of air at 100 kPa. The plunger is pushed in slowly, so the temperature stays the same, until the pressure is 300 kPa. Find the new volume, and the distance the plunger moved if the cylinder's cross-section is 6.0 cm².
- p₁V₁ = 100 × 180 = 18 000 kPa cm³.
- V₂ = 18 000 ÷ 300 = 60 cm³.
- Volume swept out = 180 − 60 = 120 cm³, so the distance = 120 ÷ 6.0 = 20 cm.
Answer: 60 cm³; 20 cm
Common mistakes
- Saying gas pressure is caused by particles hitting each other. It is caused by particles hitting the walls.
- Using p₁V₁ = p₂V₂ when the temperature changes.
- Saying Brownian motion is the movement of the molecules themselves. The visible specks move; the molecules are too small to see.
Exam tip
To explain a pressure change, use the chain "more collisions with the walls per second, so a bigger force on each square metre, so a bigger pressure". Mentioning only "more collisions" usually earns one mark of two.
AP Physics and NGSS: AP Physics 2 (Unit 9) adds the ideal gas law, pV = nRT, and links temperature to the average kinetic energy of the molecules; AP Physics 1 does not cover gases.
8 Heat Engines & Thermodynamics
What you need to know
Absolute Zero
- Cooling a gas in a sealed flask lowers its pressure along a straight line. Extended, the line reaches zero pressure at about −273 °C: absolute zero, 0 K.
- At absolute zero the particles have the least energy they can have; nothing can be colder.
- Convert with T (K) = θ (°C) + 273. A 1 K step is the same size as a 1 °C step; the kelvin scale only moves the zero.
Heat Engines
- A heat engine takes energy from something hot, turns part of it into useful work and must transfer the rest to something colder, so it can never be 100% efficient.
- Efficiency = useful energy (or power) out ÷ total energy (or power) in, × 100%.
- Reducing friction (lubrication) and insulating hot parts cut the waste, but the energy dumped to the cold surroundings remains.
Key terms
- Absolute zero
- the lowest possible temperature, −273 °C or 0 K
- Kelvin
- the unit of absolute temperature; a change of 1 K equals a change of 1 °C
- Heat engine
- a machine that turns part of the energy from a hot source into work and transfers the rest to a cold sink
- Efficiency
- the fraction of the energy input that is usefully transferred
Equations
| Equation | What it says, symbols and units |
|---|---|
T (in K) = θ (in °C) + 273 | Absolute zero, −273 °C, is 0 K; a change of 1 °C is a change of 1 K.
|
efficiency = useful energy outputtotal energy input (× 100%) | The share of the input that ends up useful; the same with useful power output over total power input. Never more than 100%.
|
Qin = W + Qout | A heat engine splits the energy taken in into useful work and energy dumped to the cold side.
AP Physics 2 writes it this way; IGCSE and GCSE use efficiency. |
Diagram to know
A graph of gas pressure against temperature in °C for a fixed volume: a straight line of points, extended with a dashed line to meet the temperature axis at −273 °C.
Worked example
(a) Liquid nitrogen boils at −196 °C. What is this in kelvin? (b) A steam engine takes in 5.0 MJ of energy from its fuel and does 0.60 MJ of useful work. Find its efficiency and the energy wasted.
- (a) T = −196 + 273 = 77 K.
- (b) Efficiency = 0.60 ÷ 5.0 × 100% = 12%.
- Energy wasted = 5.0 − 0.60 = 4.4 MJ, mostly as heat to the surroundings.
Answer: (a) 77 K (b) 12%; 4.4 MJ wasted
Common mistakes
- Adding 273 to a temperature change. A change of 30 °C is a change of 30 K.
- Giving an efficiency above 100%, or as a number like 12 with no % or as 0.12%.
- Saying better design could make a heat engine 100% efficient. Some energy must always go to the cold side.
Exam tip
Always write efficiency as a decimal or a percentage, with the % sign, and check it is less than 1 (or 100%). If it is not, the useful and total values are the wrong way up.
AP Physics and NGSS: AP Physics 2 (Unit 9) covers absolute temperature, the pressure–temperature law (p/T constant at fixed volume) and heat engines with Qin = W + Qout. NGSS HS-PS3-2 links temperature to particle energy.
9 Energy Resources
What you need to know
Boil, Spin, Generate
- Most power stations boil water: the steam spins a turbine, the turbine spins a generator. Coal, oil, gas, wood (biomass), nuclear fuel and geothermal heat all work this way.
- Dams, waves and tides spin the turbine with moving water instead; wind turbines with moving air.
- Steam power stations waste most of their input energy as heat: efficiency = useful power out ÷ total power in.
Back to the Sun
- Radiation from the Sun is the original source of almost all our energy resources: fossil fuels and biomass store energy from photosynthesis, and the Sun drives wind, waves and the water cycle that fills dams.
- The exceptions are geothermal, nuclear and tidal energy (tides come mainly from the Moon's gravity).
- The Sun releases energy by nuclear fusion of hydrogen into helium. Solar cells turn light directly into electricity; solar panels use infrared to heat water.
Keep the Lights On
- Renewable resources (wind, solar, hydroelectric, tidal, waves, geothermal, biomass) are not used up; non-renewable ones (coal, oil, gas, nuclear fuel) will run out.
- Compare resources by reliability, cost, scale and environmental impact. Wind and solar vary with the weather, so a grid needs a mix and some supply that can be switched on when demand rises.
- Burning fossil fuels releases carbon dioxide, which adds to the greenhouse effect; this is why many countries are moving to wind and solar power.
Key terms
- Renewable resource
- an energy resource that is replaced as fast as it is used
- Non-renewable resource
- an energy resource that will run out because it is not replaced
- Turbine
- a wheel of blades spun by steam, water or air, which turns a generator
- Nuclear fusion
- the joining of light nuclei, such as hydrogen into helium, releasing energy
Equations
| Equation | What it says, symbols and units |
|---|---|
efficiency = useful energy outputtotal energy input (× 100%) | The share of the input that ends up useful; the same with useful power output over total power input. Never more than 100%.
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P = Et | Power is the energy transferred each second; 1 W is 1 J/s.
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Diagram to know
A block diagram of a thermal power station: fuel → boiler (water to steam) → turbine → generator → transformer and grid, with the condenser and cooling tower returning water to the boiler and carrying waste heat away.
Worked example
A coal-fired power station takes in 2000 MW of power from its fuel. Its efficiency is 38%. Find the electrical power it delivers and the power wasted, and say where most of the waste goes.
- Useful power out = 0.38 × 2000 = 760 MW.
- Power wasted = 2000 − 760 = 1240 MW.
- Most of the waste is heat carried away by the cooling water and cooling towers, and in the hot exhaust gases.
Answer: 760 MW delivered; 1240 MW wasted, mostly as heat to the surroundings
Common mistakes
- Saying renewable resources produce no environmental impact. Dams flood valleys, wind farms change landscapes; they simply do not release carbon dioxide while running.
- Saying nuclear power is renewable. Uranium is mined and will run out.
- Calling a solar panel that heats water a solar cell.
Exam tip
In an "evaluate" question on energy resources, give at least one advantage and one disadvantage of each resource named, then end with a judgement linked to the question ("so wind alone cannot meet demand on a still evening").
AP Physics and NGSS: NGSS HS-PS3-3 and HS-ESS3-2 ask students to design energy-conversion devices and to evaluate energy solutions by cost, benefit and risk; AP Physics 1 does not cover energy resources.
Written in Physiworld's own words. The course tags come from the same mapping as the syllabus maps; equations are on the IGCSE, GCSE, AP Physics 1 and NGSS formula sheets. Values use g = 9.8 N/kg.
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