Energy Transferred by Working
Electricity and Magnetism

Things that warm

Physics Narrative for 14-16 Supporting Physics Teaching

Warming is a kind of accumulating

Good room heaters warm the particles in a room, so they switch pretty much all of the power from the electrical pathway to a heating by particles pathway. Electric grills, ovens, hobs and irons are also very efficient, so the switch from one pathway to another is nearly 100 % efficient.

It might not always be the thermal store that you want that gets filled by the heating by particles pathway, but it will be a thermal store. Maybe the air outside gets significantly warmed because you didn't insulate well enough, or block off enough of those draughts. So you need to allow for this misdirection in the final description to make sure that when you do the calculations, you consider all the things that may be warmed and not just the target output.

Even simpler than electrical bulbs

These devices are also electrically simple: you've met them in circuits as simple resistors. Choosing the resistance, and using mains voltage, so a potential difference of 230 volt, sets the current and therefore the power dissipated. Remind yourself how by looking again at episode 02 if you need to.

Heating by switching power from the electrical pathway is very convenient, but it has a cost. Much electrical power production is driven by the burning of fuels, so why not burn them directly where you want the warmth? This is more efficient, but perhaps less convenient (although cooking on gas is preferred by many, and gas patio heaters are more portable – no trailing wires for the guests to trip over – so the convenience arguments aren't all one-sided).

In generating electrical power, only 20–60 % of the energy extracted from the chemical store is available to the electrical pathway. These are very substantial inefficiencies (more exact information, and techniques for calculating efficiencies, are given in episode 04).

There is another cost. Transmission wires must be long enough that we don't all need localised power stations. Then the wires are no longer perfect: they function as devices that switch some of the power in the electrical pathway to a heating by particles pathway (typically 5–15 %). The more capital you spend on the wire (and therefore also its supports, if you string it up across the countryside), the lower the running costs will be. Thick wires, of highly-conducting, yet stiff and strong, materials, are best.

In practice, the long transmission wires now present a significant resistance, so not all of the potential difference appears across the heating element. In fact, this is a simple circuit with series connections and we have already learnt how to model these so as to find the power dissipated in each resistor (episode 02). This model doesn't mimic how electrical power gets delivered to the householder, but it's an important step on the way to understanding why the system needs to be more complicated. The result of the complications is that the electrical pathway for long-distance transmission is chosen to be of a particular character: large potential difference and small current, rather than the other way round. We come back to this later.

One final cost, often not included, is that the fuel used in the power stations may not be sourced from anywhere near that power station – it might be cheaper to pay miners in another country to dig up the coal and ship it to a power station in this country than to pay the miners closer to the power station. Working out exact efficiencies from chemical store to thermal store depends on accurate information about the larger picture.

Radiating is a kind of accumulating

There are times when remote and directed warming is wanted – a microwave or therapeutic infrared lamp – and here switching is from the electrical pathway to the heating by radiation pathway. These devices are not so simple, electrically, but the energy descriptions are still simple.

The energy descriptions provide both an overview of what the device does and a focus for the engineer trying to improve the design. She can see where the inefficiencies are by studying the Sankey diagram. It then requires considerable skill and cunning to improve the performance of the device. That's where an understanding of the detailed physics comes in useful. The energy description remains helpful, because it provides limits on what can happen, and it offers a mechanism-free overview of the whole process. Doing physics well is often a question of having the experience to select the appropriate point of view, and then developing that description in order to understand what is going on in the process.

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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