Course: Combined Science + Separate Physics | Tier: Foundation + Higher | Linked equations: E = mL, ΔE = mcΔθ
Notes
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- 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. This either raises its temperature or changes its state.
- During a change of state, the temperature stays the same: the energy is used to break bonds between particles, increasing their potential energy, not their kinetic energy.
- Specific latent heat is the energy needed to change the state of 1 kg of a substance with no change in temperature.
- Latent heat of fusion: solid ↔ liquid. Latent heat of vaporisation: liquid ↔ vapour.
Heating and cooling curves
On a graph of temperature against time while heating ice:
- Sloping sections: the temperature rises (the particles gain kinetic energy).
- Flat sections: the substance is changing state (melting at 0 °C, boiling at 100 °C for water). Energy is still going in, but the temperature doesn’t change.
- A cooling curve has the same shape upside down, with flat sections at condensing and freezing.
Don’t confuse the two equations: ΔE = mcΔθ is for a temperature change; E = mL is for a change of state.
Questions
Q1 (F) What is internal energy?
Show answer
The total kinetic energy and potential energy of all the particles in a system.
Q2 (F) Give the two things that can happen when a substance is heated.
Show answer
Its temperature can rise, or it can change state.
Q3 (F) What is happening during the flat part of a heating curve?
Show answer
The substance is changing state. The temperature stays constant even though energy is still being supplied.
Q4 (F/H) Define specific latent heat.
Show answer
The amount of energy needed to change the state of 1 kg of a substance with no change in temperature.
Q5 (F/H) Calculate the energy needed to melt 0.50 kg of ice at 0 °C. (Specific latent heat of fusion of ice = 334 000 J/kg)
Show answer
F: E = m L
I: E = 0.50 × 334 000
A: 167 000 J
Q6 (H) Explain why the temperature of boiling water stays at 100 °C even though the kettle is still supplying energy.
Show answer
The energy is used to break the bonds between the water molecules, increasing their potential energy (internal energy) rather than their kinetic energy. So the temperature doesn’t rise while the water changes state.
Q7 (H) 0.20 kg of water at 100 °C is turned into steam using 452 000 J. Calculate the specific latent heat of vaporisation of water.
Show answer
F: E = m L
I: 452 000 = 0.20 × L
F: L = 452 000 ÷ 0.20
A: 2 260 000 J/kg
