Energy Transferred by Working
Electricity and Magnetism

Devices in general

Physics Narrative for 14-16 Supporting Physics Teaching

Devices switch pathways – so focus on power-in and power-out

Controlling the rate at which energy is shifting (the power) has been, and remains, a central concern of technology. Directing this power is achieved by selecting well designed devices. Here we promote this idea of device to centre stage, to focus on this engineering aspect, because studies of power are usually concerned with finding out what we cannot do, and then with doing the best job possible within the calculated constraints.

So what does a device do?

A device switches pathway (remember that a pathway is the means of emptying or filling a store). As there are four pathways, different combinations of input and output suggest that there will be many different devices.

Here are the four pathways:

  • Electrical working.
  • Mechanical working.
  • Heating by radiation.
  • Heating by particles.

These were first introduced in the SPT: Energy topic.

Some devices are better than others

Just concentrating on electric circuits, so starting or ending with electrical working, there are many possibilities. Lamps, heaters and motors come in many shapes and with different rated powers to suit the tasks for which they have been designed. To make a (qualitative) start in judging how good a device is, consider just the pathways and not the power in each pathway. This greatly reduces the possibilities. A well designed light, for example, is a device that switches most of the electrical working to the heating by radiation pathway, and very little to the heating by particles pathway, and none to the mechanical working pathway. This was discussed rather extensively in the SPT: Energy topic (episode 03). A good electrical motor switches nearly all of the electrical working to mechanical working, with insignificant amounts to heating by particles or heating by radiation. A perfect dynamo switches all of the mechanical working to electrical working. These are just a few of the more common examples.

Perfect wires don't switch at all (i.e. they do not switch the electrical working to any other pathway). This is an idealisation, but one that results in models that mimic the laboratory bench behaviour rather closely. It really is very hard to measure a potential difference across a standard connecting wire. This is, of course, planned: the resistance is low because the wire is thick. However, you'd have to make the transmission wires of the National Grid very, very thick for this idealisation to be so good an approximation. This has significant consequences for designing a grid to shift energy from power stations to our homes.

Devices that switch the pathway to or from electrical working are also called transducers.

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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