Ampere's Law to find the magnitude of the magnetic field

[toochieboy]

Homework Statement

The wire in Figure 21-40 carries a current of 12 A. Suppose that a second long, straight wire is placed right next to this wire. The current in the second wire is 28 A. Use Ampère’s law to find the magnitude of the magnetic field at a distance of r = 0.72 m from the wires when the currents are (a) in the same direction and (b) in opposite directions.

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I1=12A
I2=28A
r= .72m
Mo= 4pi x 10^-7

Homework Equations

B = Mo/2(I1/pi(r)-I2/r)

The Attempt at a Solution

a.)

B= (4pi x 10 ^-7)/2 * (12/pi(0.72m)-28/.72m)
B=-2.11 x 10^-5 T

b.) not done yet

 

[cupid.callin]

in (a) current is in same direction ... so add them

 

[toochieboy]

in (a) current is in same direction ... so add them

So just change the sign - to sign + in [i2/pi(0.72m) - 28A/.72m)]

how about in letter B?