# What is the magnitude of the magnetic field of the wire

## Homework Statement

a long straight 6mm diameter conducting wire is carrying a current of 4A.

1)What is the magnitude of the magnetic field at a point 1mm inside the surface of the wire.

2)what is the magnitude of the magnetic field at a point 2cm from the centre of the wire

## Homework Equations

using amperes law:
for part 1 i got 44*10^-9

and for part 2: 50*10^-9

I wnt to make sure i have done it right though, becuase it is a past exam question that i dont have solutions for. So if anyone is good at these questions, it would be helpful to check my answers in case i have stuffed up.

## The Attempt at a Solution

kuruman
Homework Helper
Gold Member
The answers to both parts are incorrect.

how do you calculate current inside the wire? And do you notice units of all parameters when you calculate?

what i tried was:

diameter =6mm. At a point 1mm inside the conducting wire, diameter is 4mm, therefore r=2mm

using amperes law:

∫B⋅ds=μ₀I{Through}

ds=2πr
I{Through}=I∗((πr²)/(πR²)) where R=6mm and r =2mm
B*2πr=I∗((πr²)/(πR²))

B=μ₀Ir/2πR²
B=μ₀(4)(0.002)/2π(0.006)²

You wrote diameter = 6mm but then when you put it into eq, you wrote R=6mm.

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