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Heat for Edexcel GCSE Physics

Heat

This page covers the following topics:

1. Thermal energy
2. Latent heat
3. Thermal insulation

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.

Thermal energy

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.

Latent heat

Objects lose thermal energy until they reach room temperature. This loss of thermal energy can be reduced through the process of insulation. Insulation aims to reduce the rate at which the thermal energy of an object is lost.

Conduction and convection can be reduced through the use of good insulator materials. These materials consist of air, and since air is a poor conductor, insulation is achieved as heat transfer is reduced. Good insulating materials tend to have low densities, since there will be bigger gaps between the particles of the material and thus more packets of air within it. The air is also trapped in the insulating object, therefore reducing unwanted convection.

Radiation can be reduced through the use of shiny materials, since these are poor emitters of radiation.

When energy is travelling through an object, the more conductive an object is, the more energy will be lost across the material, since energy will be transferred through conduction. Furthermore, the thicker an object is, the more reduced the energy transfer across it is, due to the increased insulation at the walls of the object.

Thermal insulation

1

Describe what insulation does.

Insulation aims to reduce the rate at which the thermal energy of an object is lost.

reduces rate of thermal energy loss

Describe what insulation does.

2

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

Using E = mcฮ”ฮธ, 73320 J = mass ร— 4700 J/kgยบC ร— 13 ยบC, thus mass = 1.2 kg.

1.2 kg

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

3

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/kgยบC.

Using E = mcฮ”ฮธ, 9555 J = 0.65 kg ร— 4200 J/kgยบC ร— ฮ”ฮธ, thus ฮ”ฮธ = 3.5 ยบC.

3.5 ยบC

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/kgยบC.

4

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

The equation E = mcฮ”ฮธ 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

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

5

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 โˆ’ 383 g = 117 g = 0.117 kg.
Using Q = ml, Q = 0.117 kg ร— 2260000 J/kg = 264420 J.
Using P = E/t, Power = 264420 J/120 s = 2203.5 W.

2203.5 W

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.

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