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June 2010, physics 4 question 11

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1. The question is about F=BILsin(theta). There is a wire with a current of 4A, length 0.05m, going through a magnetic field. This is on top of a balance and gives a reading of 0.8 'gram'. It then asks us to calculate the average flux density along the length of the wire.

I went about this by saying that 0.8 grams is 0.8x10^-3 Kg. and that this represents a force of (0.8x10^-3)/9.8 N (using F=mg)

I then substitue everything into F=BIL (It's at right angles) and get '4.08 x10^-4'.
=>
(0.8x10^-3 / 9.8) / (0.05 * 4)

Looking at the markscheme, it just says that it is 0.04T. And i can't see why i am a factor of 100 out.

2. I worked it out in the end. Silly mistake on my part. I quoted F = mg and then in my calculations, and trust me i have no idea why, i did F = m/g. Durrr.

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