From fuels to energy stores

Associating energy stores to fuels

You can describe the ability of a fuel, such as petrol, to get a job done by saying that it has an energy store associated with it. As the fuel does its job (maybe propelling a car along the road), energy is shifted from the store. Given the source of the energy (the petrol), we say that this is a chemical store.

We could describe this process purely in terms of the familiar everyday things involved: I put petrol in the car, and the car goes along the motorway.

Alternatively, we can look at the process from a different perspective (in precisely the same way that we encouraged you to look at the world through forces spectacles). When looked at from the energy perspective, we describe the process in different terms: As the car travels along the motorway, energy is shifted from the chemical store associated with the petrol.

It's important to remember that fuels such as petrol or chocolate need to react with other chemicals to shift energy, often by burning in oxygen. This shifting of energy is what empties the store. To have a store, both the fuel and the oxygen are needed. That is why rockets designed to work in space must carry both – they cannot rely on the oxygen being around once they have left the atmosphere.

Consider the burning of methane(methane + oxygen  →  carbon dioxide + water) as shown by the equation:

CH₄ + 2O₂  →  CO₂ + 2H₂O As the fuel is burned, energy is shifted from the chemical store associated with the methane and oxygen. If there is no reaction, there is no change in the store of energy.

In general terms you can say:

Teacher: Energy is shifted from a store when a fuel is burned

(Remembering that both fuel and oxygen are needed.)