Heat Energy & Temperature (OCR GCSE Physics A (Gateway))

Heat Energy & Temperature

• The molecules within a substance possess two forms of energy:

  • Kinetic energy (due to their random motion / vibration)

  • Potential energy (due to their position relative to each other)

• Together, these two form the total energy that makes up the internal energy of the system

• Internal energy is defined as:

  The total energy stored inside a system by the particles that make up the system due to their motion and positions

Heating and Temperature Change

• Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles

  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules

• The higher the temperature, the higher the kinetic energy of the molecules and vice versa

  • This means they move around faster

• This increase in kinetic energy (and therefore internal energy) can:

  • Cause the temperature of the system to increase

  • Or, produce a change of state (solid to liquid or liquid to gas)

As the container is heated up, the gas molecules move faster with higher kinetic energy and therefore higher internal energy

Heating and Changes of State

• When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead

• This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state

  • For example, liquid to gas

• When a substance changes its state:

  • The potential energy of the molecules increases, breaking the bonds between them and becoming further apart

  • The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated

Heating Curve

• This graph shows how the temperature of a substance changes with time as it is heated

• The substance is heated until it has melted to become a liquid, and then boiled to become a gas

Heating curve of a substance showing the energy changes as temperature is increased

The different sections of the graph show:

• ORIGIN to A: Added heat energy is being used to increase the kinetic energy of the particles while it is a solid

• A to B: Added heat energy is being used to break the bonds between the solid molecules, increasing the potential energy and melting the substance

• B to C: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a liquid

• C to D: Added heat energy is being used to break the bonds between the liquid molecules, further increasing the potential energy and boiling the substance

• D to E: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a gas

Specific Heat Capacity vs Specific Latent Heat

Specific Heat Capacity

• If the temperature of the system increases, the increase in temperature of this system depends on:

  • The mass of the substance heated

  • The type of material

  • The energy input to the system

• The specific heat capacity of a substance is defined as:

The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C

• Different substances have different specific heat capacities

  • If a substance has a low specific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature)

  • If a substance has a high specific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature)

Low vs 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 a substance

  • This energy is known as latent heat

• The specific latent heat of a substance is defined as:

The amount of thermal energy required to change the state of 1 kg of a substance with no change in temperature

• There are two types of specific latent heat:

  • Specific latent heat of fusion (solid to liquid and vice versa)

  • Specific latent heat of vaporisation (liquid to gas and vice versa)

• Latent heat is represented by the symbol L with units joules per kilogram (J/kg)

Heat supplied against temperature graph, showing the changes in state for a substance

• The specific latent heat of fusion is defined as:

The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

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

  • When a solid substance melts, its temperature stays constant until all of the substance has become a liquid

  • The latent heat of fusion is the energy needed to break the bonds between the molecules

• The specific latent heat of vaporisation is defined as:

The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

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

  • When a liquid substance is heated up to its boiling point, the substance boils and turns into vapour

  • The latent heat of vaporisation is the energy needed by the particles to break away from their neighbouring particles in the liquid

• Specific heat capacity and specific latent heat are slightly different

  • Specific heat capacity is used for a change in temperature in the same state

  • Specific latent heat is used for a change in state but no change in temperature

The difference between specific heat capacity and specific latent heat