Tuesday, 14 September 2010

Are high resistance bulbs brighter?

Energy is dissipated quicker in high resistance bulbs because the current has to try harder to get through - so theyre brighter, right?

Wrong - though it can be a seductive argument. One of the problems is the deeply ingrained idea that current flow is sequential. The incorrect story is that the electrons move happily through the low resistance wires but when they come to the bulb filament they find it difficult to squeeze through and all the jostling transfers energy to the filament, which we experience as heat and light. The higher the resistance, the more jostling and the brighter the bulb.

This is also an example of what you might call local reasoning. With circuits you have to think about what happens to every element in the circuit all at the same time: it normally causes problems if you focus in on one part and then another.

In fact what happens is that a higher resistance bulb decreases the current everywhere in the circuit. The slower moving charges transfer energy to the bulb at a lower rate and so the bulb is dimmer.

One other subtlety is that in a circuit energy is transferred quickest in the places where the resistance is RELATIVELY high. But the higher the actual (rather than the relative) resistance, the slower the overall energy transfer and the dimmer the bulb.

You can check out an animation explaining what happens when a very low resistance bulb is connected first to 12V made from joining small cells in series and then connected to 12V from a car battery.