# Homework Help: Current carrying wire and Earth’s magnetic field directions

1. Oct 11, 2011

### Qaztgbe

1. The problem statement, all variables and given/known data
A telephone cable at a place has 4 long straight horizontal wires carrying a current of 1 ampere in the same direction east to west. The earth's magnetic field at the place is .39G, the angle of dip is 35°, and the magnetic declination is nearly zero. What are the resultant magnetic fields (magnitude and direction) at points 4cm below the cable?

2. Relevant equations
μI/2∏r

3. The attempt at a solution
Magnitude of the field due to the current carrying wires is 2x10-5T
Components of the earth’s magnetic fields –
Along the horizontal is BHcos35° = 3.19x10-5T
Along the vertical is BVsin35° = 2.24x10-5T

According to the right hand rule the magnetic field above the wires should be going into the plane and below should be coming out. I can’t ‘visualise’ the directions of the Earth's magnetic field so I can’t go any further.

I have numerous questions like this one but I can never see the direction of the earth’s field when it has different angles of dip.

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Last edited: Oct 11, 2011
2. Oct 11, 2011

### grzz

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3. Oct 12, 2011

### Qaztgbe

Definitely helped. If only I could 'see' it for myself everytime I get one of these questions. Thanks.