# Magnetic field strength quick question

1. Apr 11, 2007

### debwaldy

1. The problem statement, all variables and given/known data
Two very long parallel wires which are 20cm apart ,carry currents of 1.5A and 2.0A respectively, flowing in opposite directions. Calculate the magnetic field strength at following points in plane of two wires
(i) midway between wires
(ii)at a point 10cm from the wire carrying the larger current and 30 cm from the other wire

2. Relevant equations
I have been using the formula H = I/2*pi*R

3. The attempt at a solution
I think i could do part (i) as I knew that the magnetic field strengths add in the middle so i just plugged into the formula using R=0.1 and got an answer of
5.6 A m^-1

However in part (ii) I am slightly confused.
I have said the following:
H = 1.5/2*pi*.3 + 2.0/2*pi*0.1
Is this equation correct or should i be subtracting one magnetic field strength
from the other.andif so which one?i dont really understand
any help or tips would be v much appreciated thanks

2. Apr 11, 2007

### daniel_i_l

The fields aren't going in the same direction. Do you know how to find the direction of the field generated by a wire (the "thumb rule") ? You put your thumb of your right hand in the direction of the wire and the direction that your fingers are curling is the direction of the field. In the middle of the wire the fields go in the same direction even though the currents are in opposite directions because the middle of the wire is to the right of one and to the left of the other one. But in (ii) the point is on the same side as both of the wires so since the fingers curl in opposite directions, the fields will have opposite directions.

3. Apr 11, 2007

### debwaldy

oh i think i understand now thanks a million,looked in loads of books but none explained it in the simple straightforward manner that you did
cheers debs:tongue: