Energy Transferred by Working
Electricity and Magnetism

An electric car travelling at steady speed

Teaching Guidance for 14-16 Supporting Physics Teaching

What's happening in terms of energy?

Wrong Track: Chemical energy in the battery is transferred to kinetic energy of the car, and this goes to heat energy in the surroundings: chemical energy (battery)  →  kinetic energy (car)  →  heat energy (surroundings: air, road, car).

Right Lines: When the eco–friendly electric car is going at a steady speed along the road, energy is shifted from the chemical store of the battery to the thermal store of the surroundings: energy (chemical store: battery)  →  energy (thermal store: surroundings)

Matching the teaching to the challenge

Thinking about the teaching

In thinking about this question it's helpful to start by distinguishing between the physical (real-world) and energy descriptions of the electric car.

The physical picture to have in mind here is one of the car travelling along at a steady speed with the car battery gradually going flat.

In energy terms we'd say that the chemical store of the battery is gradually being depleted of energy. If energy is being taken from the chemical store, the question is, where is it going to balance the energy books?

It's very tempting to take the wrong track approach here and to argue that the chemical energy from the battery is being transferred continuously to the kinetic energy of the car. However, the energy in the kinetic store of the car is not changing – it's not building up or accumulating, because the car is moving at a steady speed.

In thinking carefully about the situation, it becomes clear that the energy being shifted from the chemical store of the car battery can only be shifted to the thermal store of the surroundings. As the car moves along the road at a steady speed, it simply warms up the surroundings.

You might be thinking that the energy description seems to miss out the key practical point – that the car is moving. In some ways we'd agree with you. Energy descriptions focus attention on the energy in the stores at the beginning and end of processes and in so doing draw attention to some features of each process and not to others. In this example, the car's speed is constant, there is no change in the energy in the kinetic store associated with the car and so it doesn't feature in the energy description.

In terms of pathways, the chemical store of the battery is depleted, or gradually empties, along an electrical working pathway, while the thermal store of the surroundings is filled through a heating by particles pathway.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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