Specific heat capacity
What is specific heat capacity?
The of a material is the energy needed to raise the temperature of 1 kg of it by 1 °C. Water's is high, at 4200 J/kg °C, so it takes a lot of energy to heat up and stays warm for a long time. Metals have much lower values.
This equation is on the equation sheet:
change in thermal energy = mass × specific heat capacity × temperature change, or ΔE = mcΔθ
| Quantity | Symbol | Unit |
|---|---|---|
| Change in thermal energy | ΔE | joules (J) |
| Mass | m | kilograms (kg) |
| Specific heat capacity | c | joules per kilogram per degree Celsius (J/kg °C) |
| Temperature change | Δθ | degrees Celsius (°C) |
Key terms
Using ΔE = mcΔθ
Example 1: how much energy does it take to heat 2 kg of water from 20 °C to 80 °C? (c = 4200 J/kg °C)
- Δθ = 80 − 20 = 60 °C
- ΔE = mcΔθ = 2 × 4200 × 60
- ΔE = 504 000 J (504 kJ)
Example 2: 9000 J raises the temperature of a 0.5 kg aluminium block by 20 °C. Find its specific heat capacity.
- Rearrange: c = ΔE ÷ (mΔθ)
- c = 9000 ÷ (0.5 × 20)
- c = 900 J/kg °C
Common mistake
Δθ is the temperature change, not the final temperature. Subtract the starting temperature first.
Key terms
Required practical 1: method
This practical finds the specific heat capacity of a metal block.
- Measure the of the block (often 1 kg) on a balance.
- Wrap the block in insulation, such as cotton wool.
- Put an electric heater in one hole and a thermometer in the other.
- Connect the heater to a , or to an ammeter and voltmeter.
- Record the starting temperature, then switch on the heater.
- After 10 minutes, record the temperature and the energy transferred. Without a joulemeter, work out the power (P = VI) and use E = P × t, with t in seconds.
- Calculate c = ΔE ÷ (mΔθ).
You can also record the temperature every minute and plot temperature against energy. The line is straight, and its equals 1 ÷ (mc).
Key terms
Required practical 1: variables, safety and accuracy
- the energy transferred to the block (or the heating time).
- the temperature of the block.
- the mass and material of the block, the heater's power and the insulation.
Hazard: the heater and block get hot enough to burn you, so let them cool before you touch them.
Improving accuracy: some energy heats the surroundings instead of the block, so the value you get for c comes out too high. Insulating the block reduces this. A few drops of water in the thermometer hole give better thermal contact, so the reading is more accurate.