Internal Energy (OCR A Level Physics) : Revision Note

Internal Energy

• The energy stored in a substance can be classified into two forms:

  • kinetic energy

  • electrostatic potential energy

• The kinetic energy stored in the molecules of a substance is:

  • due to the speed and mass of the molecules

  • related to the temperature of the substance

• The electrostatic potential energy stored in the molecules of a substance is:

  • due to the separation and forces of attraction (bonds) between the molecules

  • related to the phase of state of the substance

• The amount of kinetic and electrostatic potential energy a substance contains depends on its phase of matter (solid, liquid, or gas)

  • This is known as internal energy

• The internal energy of a substance is defined as:

The sum of the randomly distributed kinetic and potential energies of atoms or molecules within a substance 

When a substance is heated, both the kinetic energy and potential energy of the particles increase

• The symbol for internal energy is U, with units of Joules (J)

• Particles are randomly distributed, meaning they all have different speeds and the separation between each molecule varies

• The internal energy of a system is determined by:

  • Temperature

    • Higher temperature means greater kinetic energy

    • Lower temperature means less kinetic energy

  • The random motion of molecules

  • The phase of matter: gases have the highest internal energy, solids have the lowest

  • Intermolecular forces between the particles

    • Stronger intermolecular forces mean higher potential energy

    • Weaker intermolecular forces mean lower potential energy

    • The strength of the intermolecular forces is linked to the phase (solid, liquid, gas) of the substance

• The internal energy of a system increases by:

  • Doing work on it

  • Adding heat to it

• The internal energy of a system decreases by:

  • Losing heat to its surroundings

  • Changing phase from a gas to liquid or liquid to solid 

Absolute Zero

• On the thermodynamic (Kelvin) temperature scale, absolute zero is defined as:

The lowest temperature possible. Equal to 0 K or -273.15 °C 

• It is not possible to have a temperature lower than 0 K

  • This means a temperature in Kelvin will never be a negative value

• Absolute zero is defined as:

The temperature at which the molecules in a substance have zero kinetic energy 

• This means that for a system at 0 K, it is not possible to remove any more energy from it

• Even in space, the temperature is roughly 2.7 K, just above absolute zero

Internal Energy and Temperature

• When a substance is heated, its internal energy (sometimes referred to as thermal energy or heat) increases

• As a substance’s internal energy increases, so will its temperature

  • The higher the temperature of a substance, the more internal energy it possesses

As the temperature of a substance is increased, the total energy of the molecules (the internal energy) increases

• Since temperature is a measure of the average kinetic energy of the molecules, only an increase in the average kinetic energy of the molecules will result in an increase in temperature of the substance

  • Due to thermal expansion, when the temperature of a substance increases, the potential energy of the molecules also increases

• Temperature and internal energy are directly proportional to each other

  • A decrease in temperature will result in a proportional decrease in internal energy

• Where:

  • ΔU = change in internal energy (J)

  • ΔT = change in temperature (K)

Internal Energy and Phase Change

• A phase change is another way of saying a change of state

  • The states of matter are solid, liquid, and gas

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

• This breaks the bonds between the molecules, causing them to move further apart and

  • This leads to a change of phase (e.g. liquid to gas)

• When a substance is heated and changes phase (from solid to liquid, or liquid to gas):

  • electrostatic potential energy increases as bonds are broken, and the separation of the molecules increases

  • kinetic energy remains the same, hence the temperature remains the same, even though the substance is still being heated

• When a substance is cooled and changes phase (from gas to liquid, or liquid to solid):

  • electrostatic potential energy decreases as bonds are formed, and the separation of the molecules increases

  • kinetic energy remains the same, hence the temperature remains the same, even though the substance is still being cooled

An increase in internal energy from heating can cause a change of state