# A experiment on magnetism

1. Feb 13, 2014

### Antigone

Two parallel wires carrying currents can repel each other, if the currents flow in the opposite direction. What would happen to a magnet or magnetic compass, if brought to close to the magnetic field of the first current, and then the second current? What direction would it point to/or which part of the magnet, north or south, would be attracted/repelled by the magnetic fields of the currents?

a) The magnetic needle point towards north when close to "wire A", and when close to "wire B" the magnetic needle points to south.
b) nothing happens.
c) The magnet gets deflected by the magnetic field, in both cases.

So, I couldnt do this experment at home. But do anybody know for sure what happens?

Thank you

2. Feb 13, 2014

### Staff: Mentor

So which choice do you think is right and why. Remember at PF we must show some work before we can help you solve the problem.

As a hint try to draw the magnetic lines of force around the wires using the right hand rule and remember for this case the currents are going in opposite directions.

Next a compass will align itself along a magnetic line of force so how would it lineup around each wire and then how would it line up between the two wires.

3. Feb 13, 2014

### Antigone

I Think A. I Think that the North-pole will Point at wire A, and the South-pole at wire B. And I cant "draw the magnetic lines". Also I dont want to know how a compass is lined up "between the wires". I would like to know how the mangetic needle reacts individually to each wire.

4. Feb 13, 2014

### Okefenokee

The magnetic field lines are going to be circles around the wire. Where do you think the compass should point?

I think you'll have to drive a pretty significant DC current through a single wire to get a magnetic field that is significantly stronger than Earth's magnetic field.

5. Feb 13, 2014

### Antigone

When I look at videos on youtube, the Compass is perpendicular to the current. So if the current moves downwards, the Compass Points to West. If it moves upwards, the Compass Points to East. Now, this Compass is lying upon the wire, but I havnt seen how it reacts when moved around the magnetic field. I wonder what it would Point at. Would it stay perpendicular Always and not change its pointing direction?

6. Feb 13, 2014

### Staff: Mentor

7. Feb 13, 2014

### Staff: Mentor

Correct. When we do this in an undergraduate lab experiment, with a wire running vertically through a hole in a table, we have to use a current of about 30 A in order in order for the the wire's B field to overcome the Earth's B field inside a radius of about 12 cm.

If you use small compasses and put them very close to the wire, you don't need as much current.