# The magnitude and direction of the minimum magnetic field

#### themountain

Problem Statement
A .76 meter long wire runs horizontally and carries a current of 28 A from left to right. What would be the magnitude and direction of the minimum magnetic field to suspend the wire in mid air if the wire has a mass of 46.6 g/m
Relevant Equations
F=ILB
F=ILB
F=(48)(.76)B
F/36.48=B
I am stuck at how to find F, is there a different formula I am missing?

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#### kuruman

Science Advisor
Homework Helper
Gold Member
How about a different force? If the current were turned off, would the wire still be suspended in mid air?

• themountain

#### themountain

Ok, so F=ma so F=(46.6)(-9.8) so the force is 456.68. Using my past calculations, 456.68/36.48=B so B=12.52 T and the direction is perpendicular to the wire, so direction is 90 degrees. Is this correct? Thanks!

#### kuruman

Science Advisor
Homework Helper
Gold Member
Ok, so F=ma so F=(46.6)(-9.8) so the force is 456.68. Using my past calculations, 456.68/36.48=B so B=12.52 T and the direction is perpendicular to the wire, so direction is 90 degrees. Is this correct? Thanks!
Not correct.
1. The mass 46.6 is incorrect. It has no units and it's the wrong number.
2. The direction of the B field is ambiguous, 90 degrees relative to what? Use the directions of gravity and the current to specify the direction of the field or, even better, use unit vectors.

I would recommend getting an expression in terms of symbols and then put in the numbers at the very end.

• themountain

#### themountain

46.6 x .76 =35.416g
so F=(35.416)(-9.8) which means F=347.07
347.07/36.48= 9.514 T and it is towards bottom of the page

#### kuruman

Science Advisor
Homework Helper
Gold Member
46.6 x .76 =35.416g
so F=(35.416)(-9.8) which means F=347.07
347.07/36.48= 9.514 T and it is towards bottom of the page
Please put units next to numbers. Your F is still incorrect and so is what you divide it by.
Once more, I would recommend getting an expression in terms of symbols and then put in the numbers at the very end. It would be much easier to find your mistakes.

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