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Science

Heating houses

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Energy calculations

You should be able to state, and use, the equations relating energy to specific heat capacity and to specific latent heat. You may need to rearrange the equations in the exam.

Specific heat capacity

Here is the equation relating energy to specific heat capacity:

energy (J) = mass (kg) × specific heat capacity (J/kg/ºC) × temperature change (ºC)

Question

How much energy is needed to increase the temperature of 500 g of lead from 20 ºC to 45 ºC? The specific heat capacity of lead is 128 J/kg/ºC.

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Answer

Mass of lead = 500 ÷ 1000 = 0.5 kg

Temperature change = 45 – 20 = 25 ºC

energy needed = 0.5 × 128 × 25 = 1600 J (1.6 kJ)

Specific latent heat

Here is the equation relating energy to specific latent heat:

energy (J) = mass (kg) × specific latent heat (J/kg)

Question

How much energy is needed to melt 10 g of ice? The specific latent heat of melting for water is 334,000 J/kg.

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Answer

mass of ice = 10 ÷ 1000 = 0.01 kg

energy needed = 0.01 × 334000 = 3340 J (3.34 kJ)

Remember that the temperature does not alter during melting and boiling. This is because the energy is used to break the bonds between the particles in the substance.

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