When a force causes a body to move, work is being done on the object by the force. Work is the measure of energy transfer when a force ‘F’ moves an object through a distance ‘d’. So when work is done, energy has been transferred from one energy store to another, and so:
energy transferred = work done
Energy transferred and work done are both measured in joules (J).
Calculating work done
The amount of work done when a force acts on a body depends on two things:
- the size of the force acting on the object
- the distance through which the force causes the body to move in the direction of the force
The equation used to calculate the work done is:
work done = force × distance
E=f×d
This is when:
- work done (E) is measured in joules (J)
- force (F) is measured in newtons (N)
- distance (d) is in the same direction as the force and is measured in metres (m)
Example
Calculating energy changes
Calculating kinetic energy
The amount of kinetic energy of a moving object can be calculated using the equation:
This is when:
- kinetic energy (KE) is measured in joules (J)
- mass (m) is measured in kilograms (kg)
- speed (v) is measured in metres per second (m/s)
Calculating gravitational potential energy
The amount of gravitational potential energy stored by an object at height can be calculated using the equation:
change in gravitational potential energy = mass × graviational field strength × change in vertical height
Example
Galileo takes a 5 kg cannonball to the top of the Tower of Pisa for one of his experiments. The tower is 56 m high. How much gravitational potential energy has the cannonball gained? (g = 10 N/kg)
Energy dissipation
No system is perfect. Whenever there is a change in a system, energy is transferred and some of that energy is dissipated.
Dissipation is a term that is often used to describe ways in which energy is wasted. Any energy that is not transferred to useful energy stores is said to be wasted because it is transferred to the surroundings.
Electrical cables warming up are a good example of this. It is not useful to have hot wires behind a television as energy is dissipated to the surrounding air.
In a mechanical system, energy is dissipated when two surfaces rub together. Work is done against friction which causes heating of the two surfaces – so the internal (thermal) energy store of the surfaces increases.
New types of electrical component can be more energy efficient such as LED light bulbs as opposed to filament lamps – using these cause less energy to be wasted.
Examples of dissipation
Energy is usually transferred to the internal energy store of the surroundings.
The ways in which energy is dissipated depends on the system:
- for a radio or set of speakers the electrical work is transferred into useful sound waves and thermal energy is dissipated causing a rise in the internal energy store (temperature) of the surroundings
- for a tumble dryer, the electrical work is transferred into useful internal (thermal) energy which helps to dry clothes and energy is dissipated wastefully by sound waves which cause a rise in the internal energy store (temperature) of the surroundings
