Internal energy

—> a system has many energies associated with it
—> values such as kinetic and gravitational potential energies are known as mechanical

—> on a molecular level, the molecules are vibrating or moving in random directions.
—> the molecules also have binding energies between them

—> the combinations of all these energies if called the internal energy
—> the sum of all kinetic and potential energies of the molecules in a system

Out of a solid, liquid and a gas which one has the greatest value of internal energy?

• Gas -> increases in temperature which means an increase in pressure
• a hotter material will have a higher internal temperature

Internal E = Sum of all kinetic
{symbol} Potential energy

—> gas with a thermal energy ‘Q’
—> piston had been pushed down a distance ‘x’ by some force

Name of the term used for ‘force x distance in the direction of the force’

- work done

And what type of quantity does this represent? 
- energy

Equation where the change in internal energy is given by this equation and is known as the first law of thermodynamics :

△U = Q - W

(△U = change in internal energy
Q = heat added to the system
W = work done by the system (force x work done) -> energy)

The relevance of having positive or negative values in the change in internal energy and the work done in the first law of thermodynamics :

• having a + value is when the internal energy increases
• having a - value is when the internal energy decreases

(b) (I) A student uses the foot pump shown in Figure 2 to inflate a bicycle

The temperature of the air in the barrel increases as the tyre is pumped up.
The friction in the pump is negligible.
Explain why the temperature of the air in the barrel increases.

• The air is compressed. When the user of the pump pushes down on the pump, the volume decreases. So the air pressure increases. (1 mark)
• Work is done in the gas. So the particles increase kinetic energy. (2 mark)