# Bar magnet determining north pole

• Flyx
In summary, you could determine the polarity of a magnet by passing current through a solenoid and feeling the direction of the resulting magnetic field.f

#### Flyx

I came across this problem:

Explain how you would determine which was the north pole of a bar magnet, without using any other magnet.

I was thinking of using a solenoid, and dropping the magnet in. Then you can determine whether it was a north or south pole by whether the current flows clockwise or anticlockwise. I was thinking that one of these would produce a negative current, but I am not sure which. Is there any way to do this?

ProfuselyQuarky
Wouldn't it just be a matter of hanging the magnet from a string?

Wouldn't it just be a matter of hanging the magnet from a string?

That won't work if the magnet isn't strong enough.

Well if there are no restrictions on the magnet strength then it is a solution!

Just float it in water...

https://www.kjmagnetics.com/images/blog/water-compass400.jpg

davenn, Nidum and Flyx
Does the Earth count as another magnet?

Just float it in water

What if it is a horse-shoe magnet?

Does the Earth count as another magnet?

No

What if it is a horse-shoe magnet?

No

In the end all you are trying to do is create a moment to rotate the magnet, so again, if you have both poles facing the ground there will still be a torque trying to rotate the magnet...from a string!

Flyx
What if it is a horse-shoe magnet?
You will still get a small torque that will align the magnet. There is an easy trick to make the torque for a horseshoe magnet much larger in the Earth's magnetic field. Can you think what that trick might be?

ProfuselyQuarky
You will still get a small torque that will align the magnet. There is an easy trick to make the torque for a horseshoe magnet much larger in the Earth's magnetic field. Can you think what that trick might be?

Vacuum?

What if you were on a planet with no magnetic field?

Vacuum?
Water is pretty frictionless for slow movements. No, I had a different improvement in mind...
What if you were on a planet with no magnetic field?
Then you would need to wave one pole of the magnet past a coil of wire and measure the polarity of the voltage induced in the coil. Have you read about magnetic induction yet in your studies?

cnh1995
I was thinking of using a solenoid, and dropping the magnet in.
The solenoid will have induced current and hence, it will momentarily act as another magnet (electromagnet). What if you passed current through the solenoid using a battery and brought the magnet near its axis?

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Suspend the magnet horizontally using a thread. Connect a metre or so of wire from the + to - of a 1.5 volt battery. Hold the magnet closely over the wire. It will point according to the direction of the current using the Cork Screw Rule.

Then you would need to wave one pole of the magnet past a coil of wire and measure the polarity of the voltage induced in the coil. Have you read about magnetic induction yet in your studies?

We're doing that. Would the north pole make a positive current or a negative current first?

The solenoid will have induced current and hence, it will momentarily act as another magnet (electromagnet). What if you passed current through the solenoid using a battery and brought the magnet near its axis?

I don't know

Suspend the magnet horizontally using a thread. Connect a metre or so of wire from the + to - of a 1.5 volt battery. Hold the magnet closely over the wire. It will point according to the direction of the current using the Cork Screw Rule.

thanks

I don't know
What if you passed current through the solenoid using a battery and brought the magnet near its axis?
The solenoid will become an electromagnet. Using the direction of current, you can determine the poles of the electromagnet. Then take the magnet near its axis. You will feel either attraction or repulsion, from which you could tell the polarity of the magnet.

Flyx
The solenoid will become an electromagnet. Using the direction of current, you can determine the poles of the electromagnet. Then take the magnet near its axis. You will feel either attraction or repulsion, from which you could tell the polarity of the magnet.

That's what I was thinking, but I was wondering which direction the current would flow.

That's what I was thinking, but I was wondering which direction the current would flow.
Current direction will be simply from +ve terminal to the -ve terminal of the battery(conventional current,of course).

Current direction will be simply from +ve terminal to the -ve terminal of the battery(conventional current,of course).

Thanks