# Heat for OCR A-level Physics

1. Thermal energy
2. Latent heat

When the temperature of a substance changes, energy also changes. When the temperature increases, the substance stores energy, whereas when the temperature decreases, it releases energy. The amount of energy depends on the mass of the substance that is being cooled or heated and can be calculated using the given equation.

The specific heat capacity is the energy needed to raise the temperature of 1 kg by 1 K and is different for different substances.

The specific latent heat of a substance is the energy required for 1 kg of it to change from a solid to liquid or liquid to gas without a change in temperature and is given in J/kg. The formula for it is given below.

The specific latent heat of fusion is the energy required to change 1 kg of solid into liquid without a change in temperature, whereas the specific latent heat of vapourisation is the energy required to change 1 kg of liquid into gas without a change in temperature. Since many bonds need to be broken for boiling to occur, it requires more energy than melting, and thus takes longer, therefore the specific latent heat of vapourisation is always greater than that of fusion.

# 1

Find the mass of a sample of ammonia given that its temperature drops 13 ºC when it releases 73320 J. The specific heat capacity of ammonia is 4700 J/kgºC.

Using E = mcΔθ, 73320 J = mass × 4700 J/kgºC × 13 ºC, thus mass = 1.2 kg.

1.2 kg

# 2

Calculate the temperature change in temperature, when 650 g of water releases 9555 J of energy. Use that the specific heat capacity of water is 4200 J/kgºC.

Using E = mcΔθ, 9555 J = 0.65 kg × 4200 J/kgºC × Δθ, thus Δθ = 3.5 ºC.

3.5 ºC

# 3

Zinc has a specific heat capacity of 380 J/kg°C and iron has a specific heat capacity of 450 J/kg°C. For the same mass of the two elements, which one will require more energy to change its temperature? Explain your answer.

The equation E = mcΔθ must be used to calculate the energy required to change the temperature of a substance. Since the mass and change in temperature is constant for both substances and the specific heat capacity of iron is greater, iron requires more energy.

iron

# 4

A kettle is filled up with 500 g of water and is turned on for two minutes. Given that the specific latent heat of vapourisation is 2260000 J/kg and that the mass of the water after the kettle switches off is 383g, calculate the power of the kettle.

Mass of evaporated water = 500 g − 383 g = 117 g = 0.117 kg.
Using Q = ml, Q = 0.117 kg × 2260000 J/kg = 264420 J.
Using P = E/t, Power = 264420 J/120 s = 2203.5 W.

2203.5 W

# 5

Given that the specific heat capacity of nitrogen is 1040 J/kgºC, explain what this means.

The fact that nitrogen has a specific heat capacity of 1040 J/kgºC means that the energy needed to raise the temperature of 1 kg of nitrogen by 1 °C is 1040 J.

energy needed to raise the temperature of 1 kg of nitrogen by 1 °C is 1040 J

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