# A Level Physics Capacitance question

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Thread starter 4 months ago
#1
The Earth's magnetic field at a certain position on the Earth's surface has a horizontal component of 18 micro T due north and a downwards vertical component of 55 micro T. Calculate:

a the magnitude of the Earth's magnetic field at this position,

b the magnitude and direction of the force on a vertical wire of length 0.80 m carrying a current of 4.5 A downwards.

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I'm stuck on part b currently. Any help would be greatly appreciated.

(For anybody who looks at this thread for part a, you can find the magnitude of the magnetic field strength by using Pythagoras's theorem on the components of the field, as they are perpendicular to each other, so sqrt[(18x10^-6)^2 + (55x10^-6)^2])
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4 months ago
#2
Do you know the equation for calculating the force on a current carrying conductor in a magnetic field? Because if you do, all that's left is to decide which numbers to put into it.....
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Thread starter 4 months ago
#3
(Original post by phys981)
Do you know the equation for calculating the force on a current carrying conductor in a magnetic field? Because if you do, all that's left is to decide which numbers to put into it.....
Yeah, I tried using F = BIL using the magnitude of the magnetic field calculated in part a) but that gives: F = 5.8x10^-5 * 4.5 * 0.80 = 2.1x10^-4 N. The answer in the back of the text book is 6.5x10^-3 N due East, so I'm either applying the equation wrong or missing the point of the question entirely.
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4 months ago
#4
Aha! But which bit of the magnetic fields should you be considering here? Or rather, which component? And why?
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Thread starter 4 months ago
#5
Ok, I've done it now. You just need to use the horizontal component of the force, 18x10^-6, and substitute that into F=BIL.
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4 months ago
#6
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