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# Need help with this question! on Magnetism Watch

1. A wire, whose linear mass density is 150g/m, carries a current of 40A. The wire is suspended in the air by a flexible connection and lies above and parallel to another horizontal wire on a table 4.0cm below. A current is being passed through the bottom wire, thus creating a magnetic field and force on the top wire.
Determine the upward magnetic force on 1 metre length of top wire required to repel and support the top wire above the bottom wire?
2. Correct me if I am wrong but would not the force required to support top wire be its weight i.e length X linear mass density X g?
I didn't quite get the statement : 'The wire is suspended in the air by a flexible connection.'
3. (Original post by tangotangopapa2)
Correct me if I am wrong but would not the force required to support top wire be its weight i.e length X linear mass density X g?
I didn't quite get the statement : 'The wire is suspended in the air by a flexible connection.'
yeah I believe you are right, I think the formula is F=BIL, where F =mg, so mg = BIL. I think I'm right that m=0.15kg g=9.81. B is what i am after. As I=40A and L=1 metre. correct me if i'm wrong. yeah i don't quite understand that statement either as its an exam question.
4. (Original post by Lawalk97)
yeah I believe you are right, I think the formula is F=BIL, where F =mg, so mg = BIL. I think I'm right that m=0.15kg g=9.81. B is what i am after. As I=40A and L=1 metre. correct me if i'm wrong. yeah i don't quite understand that statement either as its an exam question.
Are you sure, that's the exact question? The question just asks for upward force required to keep the wire self suspended, which can be obtained by solving simple free body diagram. Had it been the current in the bottom wire which is asked for then you would have to solve for B and then solve for current in the bottom wire using Biot Savart law.
5. (Original post by tangotangopapa2)
Are you sure, that's the exact question? The question just asks for upward force required to keep the wire self suspended, which can be obtained by solving simple free body diagram. Had it been the current in the bottom wire which is asked for then you would have to solve for B and then solve for current in the bottom wire using Biot Savart law.
Well I'm not too sure, hence why I am asking on here. I find the that the question is not very well written. In the examination feedback sheet, it states that 'the electromagnetic force needed for the required balance must be equal to the force of gravity on the wire which can be calculated from the given info(knowing that g is a constant).' Also there is no answers provided for it either. I don't think that the question is that complicated as its 5 marks. So they way I said is the only way that I think it is solved, unless you have a better solution can you please explain. This is only 1st year, on a unit called engineering science
6. (Original post by Lawalk97)
Well I'm not too sure, hence why I am asking on here. I find the that the question is not very well written. In the examination feedback sheet, it states that 'the electromagnetic force needed for the required balance must be equal to the force of gravity on the wire which can be calculated from the given info(knowing that g is a constant).' Also there is no answers provided for it either. I don't think that the question is that complicated as its 5 marks. So they way I said is the only way that I think it is solved, unless you have a better solution can you please explain. This is only 1st year, on a unit called engineering science
Yes, you are right.
7. (Original post by tangotangopapa2)
Yes, you are right.
Okay thanks. So I am right about the method I stated earlier?

0.15kg x 9.81 = 1.4715

1.4715 = B*40*1
so 1.4715/40 = B
B= 0.0367875 T

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