Measuring Earth's magnetic field

[hasan_researc]

Am reading the attached labscript. Finding it difficult to understand at places. Any good sources of further explanation on the Net or textbooks?

 

[cragar]

could you tell us what your stuck on .

 

[hasan_researc]

I have to measure the deflection of the centre of the wire. I did so using a microscope but the results are all spread about zero.

Secondly, I have to use a microscope graticule. I don't know why I have to and how to use it.

 

[Bob S]

Your measurement of the Earth's magnetic field uses the same technique as the floating wire method used to map large experimental physics magnets. Look under floating wire magnet in Google.
This a difficult measurement, even in large magnetic fields. Yo need to use a non-magnetic wire with low mass per unit length, low resistance per unit length, and high melting temperature. Copper might be a better choice than tungsten*. Also, a multi-filament wire is better than a solid wire, because it is more flexible. Finally, minimizing pulley bearing friction is very important. Pulley should be electrically insulating if possible. I vaguely recall using a 40Ga. multi-filament copper wire at currents up to ~ 3 amps (it gets very hot). (Maybe tungsten wire is better because you can run it red hot, even though the resistance goes up??).

Finally, measure wire deflection with both polarities of wire current.

* The room-temperature resistivity of tungsten is about 3 x the resistivity of copper.

Bob S

 

[marcusl]

A graticule is a ruled scale on a transparent background that goes inside the microscope so you can measure absolute distances.

Talk to your TA or prof to see if you've set things up correctly. Try taking the mean of your measurements and see if they really are close to zero.

 

[hasan_researc]

Thanks, everyone.

Now, I'd like to turn to another statement made in section 2 of the labscript:

"In the experiment described here a current carrying wire is held under tension by a small weight (≈ 20 g) and passes over fixed pulleys at either end. It is thus deflected in an arc with the maximum deflection at the centre of the wire. "

I don't understand why the wire is deflected in an arc, and assuming the wire is deflected, why does it have to be max at the centre?

Thanks for all your help!

 

[hasan_researc]

Secondly,

"You will need a power supply to control the current, and an ammeter and voltmeter to monitor the current through the wire and voltage across it. The power supply can stabilise either the voltage or the current. Wire your circuit and investigate the two modes of operation. Which one is more suitable to do the experiment? "

I am wondering which one would be more suitable?

 

[marcusl]

Secondly,

"You will need a power supply to control the current, and an ammeter and voltmeter to monitor the current through the wire and voltage across it. The power supply can stabilise either the voltage or the current. Wire your circuit and investigate the two modes of operation. Which one is more suitable to do the experiment? "

I am wondering which one would be more suitable?

What causes the deflection?

 

[Bob S]

The force on the wire is due to the Lorentz force law; F = I x B, (vector cross product, so force is perpendicular to both current and field). Because the force is perpendicular to current, the wire path is a circular arc with maximum displacement at center. The displacement at the center is proportional to current, proportional to arc length squared*, and inversely proportional to wire tension.

What kind of power supply do you need if the wire resistance varies with wire temperature?

You need a pulley at only one end.

*Using your knowledge of circles, arcs, and chords you learned in plane geometry, calculate the distance between arc and chord at the center. How does separation depend on the length of chord?

Bob S