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# Potential Difference and Current question? watch

1. Hi everyone - I have a quick question to ask about the relationship between potential difference and current in a filament bulb.

As potential difference increases, current also increases in an ohmic conductor in a proportional manner. However, in a filament this doesn't happen due to increasing temperature leading to increasing resistance.

I know the second half of this answer is correct, but I'm not sure about the first half, particularly the bits highlighted in red. Can anyone clarify?

As pd increases, the energy supplied by the source to the electrons increases. This means that as the electrons move through the wire, they have more kinetic energy. This means that as pd increases, collisions between electrons and the atoms of the material are more likely. Kinetic energy is thus transferred to the atoms and raises the internal energy of the material, causing the atoms to oscillate more violently raising the temperature of the material. The effect of this is that it raises the probability of even more collisions – as the internal energy continues to increase, there is increased resistance against the flow of electrons. This means the rate of increase of drift velocity isn’t as high, thus the rate of increase of current isn't as high as it would be if the material was ohmic.

Thanks
2. That's fine. The electrons' average drift speed through the wire increases as you increase the pd.
3. Which bit are you struggling with exactly? The energy electrons carry is kinetic, the energy supplied to them is also kinetic. If you're confused about how a potential diference can give energy, the easiest thing to think about is something on a shelf falling off it. As it loses potential (and moves from a point of higher potential to one with lower) it gains kinetic energy. The only difference here is the electrons are being accelerated by an electric potential difference, instead of gravitational.

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Updated: February 15, 2010
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