Lesson overview
Specific heat capacity: Why does water take so long to heat up?
Weigh a block of aluminium, heat it with an immersion heater through a joulemeter and plot its temperature against the energy supplied, with a magnifier showing its particles. Work out its specific heat capacity, improve the measurement with an insulating jacket, then measure the specific heat capacity of a kilogram of water.
Suggested sequence
- Lesson 1 · Heating a block of aluminium. Internal energy and temperature; specific heat capacity, its definition and c = ΔE ÷ (m Δθ); measuring it for a solid, and why an insulating jacket improves the result. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 2 · Measuring the specific heat capacity of water. Measuring the specific heat capacity of water; why water warms and cools slowly, and where that matters. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- End of module · final check. 2 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 2 questions, 15 min, lesson closed.
Learning goals
Part 1: Heating a block of aluminium
- State that a rise in the temperature of an object increases its internal energy (2.2.2.1)
- Describe an increase in the temperature of an object in terms of an increase in the average kinetic energy of all of its particles (2.2.2.2)
- Define specific heat capacity as the energy required per unit mass per unit temperature increase, and recall and use c = ΔE/(mΔθ) (2.2.2.3)
- Describe an experiment to measure the specific heat capacity of a solid, and explain why the measured value comes out too high (2.2.2.4)
Part 2: Measuring the specific heat capacity of water
- Describe an experiment to measure the specific heat capacity of a liquid, and summarise the method for a solid and a liquid (2.2.2.4)
- Use c = ΔE/(mΔθ), with masses in grams or kilograms, to compare materials and to calculate energies and temperature rises (2.2.2.3)
- Use specific heat capacity to explain why water warms and cools slowly, and to choose a material for a job (2.2.2.3)
Misconceptions to listen for
- “They have the same internal energy, because they have the same mass.” — The mass tells you how many particles there are, not how fast they move. Warming block B to 60 °C took energy, and that energy is stored in it: same mass, higher temperature, more internal energy.
- “Neither block has any internal energy until it is heated.” — Every object has internal energy: its particles are always moving, even at 20 °C. Heating adds to it.
- “(2) expand” — Each particle stays the same size when the block is heated. What changes is how fast each particle vibrates.
- “(3) potential” — The energy something has because it is moving is kinetic energy. A rise in temperature is a rise in the average kinetic energy of the particles.
- “c = 5850 ÷ (0.500 × 50.0) = 234 J/(kg °C).” — 50.0 °C is the final temperature. Use the rise in temperature: 50.0 °C − 20.0 °C = 30.0 °C.
- “c = 5.85 ÷ (0.500 × 30.0) = 0.39 J/(kg °C).” — The energy must be in joules: 5.85 kJ = 5850 J.
- “Read the starting temperature after switching on.” — Once the heater is on, the block starts to warm, so a late starting reading is too high and the rise you work out is too small. Read it before any energy goes in.
- “Set the joulemeter to zero at the end.” — The joulemeter counts the energy from the moment it is set to zero, so zero it before you switch on; its final reading is then the energy supplied.
- “ΔE = 250 × 390 × 100 = 9 750 000 J.” — Specific heat capacity is the energy per kilogram, so the mass must be in kilograms: 250 g = 0.250 kg.
- “ΔE = 0.250 × 390 × 120 = 11 700 J.” — 120 °C is the final temperature. Use the rise: 120 °C − 20 °C = 100 °C.
Scope
A rise in the temperature of an object increases its internal energy (2.2.2.1); the rise in temperature in terms of an increase in the average kinetic energy of its particles (2.2.2.2); specific heat capacity as the energy needed per unit mass per unit temperature increase, with c = ΔE ÷ (m Δθ) (2.2.2.3); experiments to measure the specific heat capacity of a solid and of a liquid (2.2.2.4). Changes of state, melting and boiling, are in MELT01; conduction, convection and radiation are in TRANS01.