Energy Transferred by Working
Electricity and Magnetism

Making a difference

Physics Narrative for 14-16 Supporting Physics Teaching

Change the world – one light bulb at a time

We have all seen this slogan, but does it make a difference? And in terms of the science, surely it's those using filament light bulbs who are keen on changing the world? After all, it's the rate of dissipation of stores that is thought to be a significant trigger for world change, or at least climate change.

The best option of all is, of course, to turn the light off: a circuit containing a break costs nothing to run. Not buying newly made things (manufacturing has significant energy costs) and not depleting energy resources are the most certain ways to avoid depriving future generations.

But there is no doubt that, if we need lighting, filament lamps do the worst job of switching power in an electrical pathway to power in a heating by radiation pathway (this is a bit of a mouthful, so for shorthand we might say switching electrical power to luminous power). This is not the whole story – a more sophisticated audit would include the environmental costs of construction, and of disposal. Both fluorescent and LED lights are much more demanding in these respects. Just looking at the construction of the compact fluorescent and listing the chemicals involved gives an indication of the challenges. However, these differences are more controversial, further from the simple physics story, and harder to quantify rigorously. So here we stick to actually running the lights.

What action could individuals take, and would it make a difference? Would the money they had saved be spent on something else that dissipated even more energy? The answers will not be simple, but physics can provide some insights.

Actually making a difference

A simple set of sums can provide some numbers to help us decide what to do. Changing from an incandescent to a compact fluorescent reduces the switching power to 40 % of the original input power, for the same luminous power. Calculate or estimate the total wattage installed in a typical house. 60 % of that could be saved. Then multiply up by the number of houses. For every 1000 MW, one less major power station.

You can work through the numbers for a city the size of Worcester:

lightbulbs/household, say 30;

average wattage/bulb, say 75 W;

installed wattage is therefore 30  ×  75 W;

power saved/household is therefore 30  ×  75 W  ×  60100;

households, say 40 000;

power saved is therefore 40 000  ×  30  ×  75 W  ×  60100;

Therefore 0.05 power stations saved. So persuade 100 conurbations of this size to change, and we would lose the need for five new power stations.

There are all kinds of simplifications in this kind of calculation. For example, you might want to add a line to estimate the load factor per bulb: it is unlikely that all 30 are on all of the time. Similarly, you might think that persuading the local council to turn the street lights off will make more of a difference. The point is that some calculation is necessary to the argument, and the argument is either made with real numbers or else is of little use.

Across the EU, the average number of bulbs per household is 24, with about 33 % of these being incandescent.

Global estimates put lighting at about 19 % of the total energy switched from the electrical pathway. Some 10 % of the total could be saved by changing to much more efficient lamps.

The financial picture is equally rosy: CFL and LED lamps save the user significant amounts over their lifetimes, due to the savings in electricity bills. But what will they do with the savings? Real-life decisions can be constrained by insights from physics, but in many cases they aren't determined by these insights.

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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