P3.2 Heating · KS3 Science

Internal energy and heat transfer

Internal energy is the total kinetic and potential energy of all the particles in a system.
Heating increases the internal energy: it either raises the temperature (the particles move faster) or changes the state (the forces between the particles are overcome).
Conduction: particles vibrate and pass energy to their neighbours. It is fastest in solids, especially metals.
Convection: in liquids and gases, the warmer part expands, becomes less dense and rises, and cooler fluid sinks, making a convection current.
Radiation: infrared waves transfer energy without particles, even through a vacuum. Dark, matt surfaces are the best emitters and absorbers; shiny, light surfaces are the worst, and reflect infrared.

Homes lose less energy with loft insulation, cavity wall insulation, double glazing and draught excluders. Insulating materials trap air, which reduces conduction and convection.
A vacuum flask keeps drinks hot because the vacuum stops conduction and convection, and the shiny walls reduce radiation.

Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Energy = mass × specific heat capacity × temperature change (E = m × c × Δθ).
Water has a high specific heat capacity (4200 J/kg°C), so it takes a lot of energy to heat up. Heating 2 kg of water by 10 °C needs 2 × 4200 × 10 = 84 000 J.
To measure it, heat a metal block of known mass with an electric heater, measure the energy supplied and the temperature rise, then calculate c = E ÷ (m × Δθ). Insulate the block to reduce energy losses.

Specific latent heat is the energy needed to change the state of 1 kg of a substance without changing its temperature: energy = mass × specific latent heat (E = m × L).
Latent heat of fusion is for melting; latent heat of vaporisation is for boiling.
While a substance changes state, its temperature stays the same, which shows as a flat section on a heating curve.

Internal energy: The total kinetic and potential energy of the particles in a system.
Conduction: Energy transfer through a material by particles passing on vibrations.
Convection: Energy transfer by the movement of a warm liquid or gas.
Radiation: Energy transfer by infrared waves, which can cross a vacuum.
Infrared: Invisible radiation given out by warm objects.
Insulation: A material that reduces energy transfer by heating.
Specific heat capacity: The energy needed to raise the temperature of 1 kg of a substance by 1 °C.
Specific latent heat: The energy needed to change the state of 1 kg of a substance.
Latent heat of fusion: The energy needed to melt 1 kg of a substance.
Latent heat of vaporisation: The energy needed to boil 1 kg of a substance.
Heating curve: A graph of temperature against time as a substance is heated.