Specific Heat Capacity & Latent Heat (Edexcel International A Level Physics)

Specific Heat Capacity

• Specific heat capacity is defined as:

  The energy required to raise the temperature of one kilogram of a substance by one kelvin

• The increase in temperature of an object depends on:

  • The amount of heat energy transferred

  • The mass of the object

  • The specific heat capacity of the material from which the object is made

Explanation of Heat Energy

• The energy input is in the form of heat energy

• The amount of heat energy needed is given by the equation:

ΔE = mcΔθ

• Where:

  • ΔE = change in heat energy, in joules (J)

  • m = mass, in kilograms (kg)

  • c = specific heat capacity, in joules per kilogram per degree Kelvin (J/kg K or J/kg °C)

  • Δθ = change in temperature, in Kelvin or Celsius

• (The symbol Δ in Maths is used to denote a change in value)

Examples of heat energy

• The thermodynamic Kelvin temperature scale is defined as:

  An absolute temperature scale where each degree is the same size as those on the Celsius scale

• Different materials:

  • Have different specific heat capacities because of their molecular structure

  • Have different rises in temperature for the same change in heat energy

• Specific heat capacity is mainly used for liquids and solids

• Good electrical conductors, such as copper and lead, are excellent conductors of heat due to their low specific heat capacity

  • On the other hand, water has a very high specific heat capacity, making it ideal for heating homes as the water remains hot in a radiator for a long time

• The specific heat capacity of different substances determines how useful they would be for a specific purpose eg. choosing the best material for kitchen appliances

Low v high specific heat capacity

• The specific heat capacity of some substances are given in the table below as examples:

Table of values of specific heat capacity for various substances

Specific Latent Heat

• Energy is required to change the state of substance

• Examples of changes of state are:

  • Melting = solid to liquid

  • Evaporation/vaporisation/boiling = liquid to gas

  • Sublimation = solid to gas

  • Freezing = liquid to solid

  • Condensation = gas to liquid

The example of changes of state between solids, liquids and gases

• When a substance changes state, there is no temperature change

• The energy supplied to change the state is called the latent heat and is defined as:

  The thermal energy required to change the state of one kilogram of a substance without any change of temperature

• There are two types of latent heat:

  • Specific latent heat of fusion (melting)

  • Specific latent heat of vaporisation (boiling)

The changes of state with heat supplied against temperature. There is no change in temperature during changes of state

• The specific latent heat of fusion is defined as:

   The thermal energy required to convert one kilogram of solid to liquid with no change in temperature

  • This is used when melting a solid or freezing a liquid

• The specific latent heat of vaporisation is defined as:

   The thermal energy required to convert one kilogram of liquid to gas with no change in temperature

  • This is used when vaporising a liquid or condensing a gas

Calculating Specific Latent Heat

• The amount of energy ΔE required to melt or vaporise a mass of m with latent heat L is:

ΔE = LΔm

• Where:

  • ΔE =  amount of heat energy to change the state (J)

  • L = latent heat of fusion or vaporisation (J kg^-1)

  • Δm = change in mass of the substance changing state (kg)

• The values of latent heat for water are:

  • Specific latent heat of fusion = 330 kJ kg^-1

  • Specific latent heat of vaporisation = 2.26 MJ kg^-1

• Therefore, evaporating 1 kg of water requires roughly seven times more energy than melting the same amount of ice to form water

• The reason for this is to do with intermolecular forces:

  • When ice melts: energy is required to just increase the molecular separation until molecules can flow freely over each other

  • When water boils: energy is required to completely separate the molecules until there are no longer forces of attraction between them, hence this requires much more energy