Page 352 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
BOX 21.2: Determining specific latent heat L (continued)
Summary
■■ The kinetic model of matter allows us to explain behaviour (e.g. changes of state) and macroscopic properties (e.g. specific heat capacity and specific latent heat) in terms of the behaviour of molecules.
■■ The internal energy of a system is the sum of the random distribution of kinetic and potential energies associated with the atoms or molecules that make up the system.
■■ If the temperature of an object increases, there is an increase in its internal energy.
■■ Internal energy also increases during a change of state, but there is no change in temperature.
■■ The first law of thermodynamics expresses the conservation of energy:
increase in internal energy = energy supplied by heating + work done on the system
■■ Temperatures on the thermodynamic (Kelvin) and Celsius scales of temperature are related by:
T (K) = θ (°C) + 273.15 θ (°C) = T (K) − 273.15
■■ At absolute zero, all substances have a minimum internal energy.
■■ A thermometer makes use of a physical property of a material that varies with temperature.
■■ The word equation for the specific heat capacity of a substance is:
energy supplied specific heat capacity = mass × temperature change
The specific heat capacity of a substance is the energy required per unit mass of the substance to raise the temperature by 1 K (or 1 °C).
■■ The energy transferred in raising the temperature of a substance is given by E = mcΔθ.
■■ The specific latent heat of a substance is the energy required per kilogram of the substance to change its state without any change in temperature: E = mL.
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A similar approach can be used to determine the specific latent heat of fusion of ice. In this case, the ice is heated electrically in a funnel; water runs out of the funnel and is collected in a beaker on a balance.
As with any experiment, we should consider sources of error in measuring L and their effects on the final result. When water is heated to produce steam, some
energy may escape to the surroundings so that the measured energy is greater than that supplied to the water. This systematic error gives a value of L which is greater than the true value. When ice is melted, energy from the surroundings will conduct into the ice, so that the measured value of L will be an underestimate.