AQA Physics (8464) 6.3.2.1 · Model
Internal energy
Heat boiling water harder. Where does the extra energy go, and why is energy not the same thing as temperature?
Combined Science · Foundation tier
Start here
Turn the gas up under boiling water.
A pan of water is boiling on a hob. A thermometer in the water reads 100 °C. You turn the flame up to full and keep it there.
If you had to guess, while the water keeps boiling, does the thermometer reading go up, or stay at 100 °C?
The particles that make up a substance store energy in two ways. They move, so they have kinetic energy. There are forces between them, so they also have potential energy, which depends on how far apart they are. Internal energy is the total kinetic energy and potential energy of all the particles that make up a system. Heating a system increases its internal energy, and that does one of two things: it raises the temperature, or it changes the state. Heat the ice on the bench one stage at a time, and call which before it happens.
The heating bench · stage 1 of 4
Ice at −10 °C. The heater goes on.
You keep heating. What happens next?
The temperature of a gas is related to the average kinetic energy of its particles: how energetic a typical particle is. Internal energy is the total over every particle, kinetic and potential. The two effects of heating each have an equation, both on the equation sheet, which the next lessons use: raising the temperature, ΔE = m c Δθ; changing state, E = m L.
Think again · hotter is not more
“The coffee is hotter, so it has more internal energy.”
Spot the flaw
ExplainAn iceberg at −5 °C and a cup of coffee at 80 °C. Lily says the coffee has more internal energy because it is hotter. What is wrong with her reasoning?
Compare
Which has more internal energy?
Each card names two systems. Decide which one holds more.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Cooling runs the heating curve backwards. Energy leaves, so the internal energy falls the whole way down, and the flat sections are where the substance condenses or freezes.
Read the cooling curve
A gas cools until it is a solid at room temperature.
Section by section
What is happening to the particles?
Place each section of the graph.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Determine
- Read it from the graph: a flat section gives a melting or boiling point.
- Explain
- Link each step: energy in or out, which energy of the particles changes, what the temperature does.
- Describe
- Say what happens at each stage, in order, using the particles.
Key note · AQA 6.3.2.1 (8464)
Internal energy
- Internal energy = KE + PE of all particles.
- Heating → increases internal energy → either raises temperature (KE) or causes change of state (PE).
- Temperature = average KE per particle.
- During change of state: temperature constant, PE increasing.
- Heating curve: slopes = temperature rise; flat sections = changes of state.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Mixing up temperature and internal energy?
The bench follows the GCSE model: warming raises only the particles' kinetic energy, and a change of state raises only their potential energy. Bar lengths and the time axis are not to scale, particles are drawn as spheres, and the water is at normal atmospheric pressure, so it boils at 100 °C. Both graphs are sketches, not measured data.