Energy stores and transfers

A system is an object or group of objects. There are changes in the way energy is stored when a system changes.

That sentence sets the language for the whole of this topic. Energy is not created or used up — it moves between stores, and a question asks you to say which store filled and which emptied.

You should be able to describe all the changes involved in the way energy is stored when a system changes, for common situations.

Throughout this section on energy you should be able to calculate the changes in energy involved when a system is changed by:

• heating • work done by forces • work done when a current flows.

Those three are the only ways energy gets into or out of a system at GCSE, and each has its own equation later in the course.

You should also be able to use calculations to show, on a common scale, how the overall energy in a system is redistributed when the system is changed. "On a common scale" means the numbers must add up: the energy leaving one store equals the energy arriving in the others.

You should be able to calculate the amount of energy associated with a moving object, a stretched spring, and an object raised above ground level — three equations, of which two must be recalled and one is given on the equation sheet.

The quantities and their units:

• kinetic energy, Ek, in joules, J; mass, m, in kilograms, kg; speed, v, in metres per second, m/s • elastic potential energy, Ee, in joules, J; spring constant, k, in newtons per metre, N/m; extension, e, in metres, m — assuming the limit of proportionality has not been exceeded • gravitational potential energy, Ep, in joules, J; mass, m, in kilograms, kg; gravitational field strength, g, in newtons per kilogram, N/kg; height, h, in metres, m.

In any calculation the value of the gravitational field strength g will be given, so it is not something to memorise.

The amount of energy stored in or released from a system as its temperature changes can be calculated using the equation:

change in thermal energy = mass × specific heat capacity × temperature change ∆E = m c ∆θ

change in thermal energy, ∆E, in joules, J; mass, m, in kilograms, kg; specific heat capacity, c, in joules per kilogram per degree Celsius, J/kg °C; temperature change, ∆θ, in degrees Celsius, °C.

This equation is given on the Physics equation sheet, so it does not have to be recalled.

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius. That definition is worth learning word for word — it is asked for directly, and it explains the units.

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.

That single sentence is the principle of conservation of energy, and it is the reason every energy question must balance.

You should be able to describe, with examples, where there are energy transfers in a closed system, that there is no net change to the total energy. In a closed system nothing enters or leaves, so however the energy is rearranged between stores, the total stays exactly the same.

You should be able to describe, with examples, how in all system changes energy is dissipated, so that it is stored in less useful ways. This energy is often described as being 'wasted'.

Note the word 'all'. Dissipation is not a fault in badly made machines — every system change dissipates some energy. A motor warms up, a ball bouncing warms the floor and the air, a phone warms in your hand. The energy is still there, which is why it is called dissipated rather than lost; it has simply spread into the thermal store of the surroundings, where it is too spread out to be useful.