Work & Energy Transfer

The physical processes of working and energy transfer are inseparable. Work requires an energy source, for example, a fuel, an electric cell or a wound-up spring. As the work is done, energy leaves the source and ends up elsewhere, this is what is meant by energy transfer.

Energy is the ability to do work.
The energy transfer from a source is equal to the amount of work done.
Like work, energy is measured in joules.

An object has energy if it can exert a force that causes movement of the point of application of the force.

Here are some examples of everyday energy transfers.

A bus accelerates away from a bus stop and then maintains a steady speed

Image

As the bus speeds up, some of the energy from the fuel and oxygen is transferred to the kinetic energy of the bus, i.e. the energy it has due to its movement. The rest of the energy is transferred to the surroundings. This takes place in a number of ways: the exhaust gases transfer energy as heat directly into the surrounding air and some energy is also transferred to heat wherever there are resistive forces.

Once the bus is maintaining a steady speed there is no increase in its kinetic energy, so all the energy transferred from the fuel and oxygen is going to the surroundings.

Image

The load gains gravitational potential energy as it is lifted, i.e. the energy it has due to its position above the Earth’s surface. All the movement is at a constant speed, so there is no change in the kinetic energy of the system. As with the bus travelling at a steady speed, work done against resistive forces causes heating, and this energy is transferred as heat to the surroundings.