# Labs from Chicago, Winter 1994 : Electricity and Magnetism.

Dr. Rich Kron, Dr. Heidi Newberg, and Luisa Rebull
Labs written for the CARA Space Explorers, Winter 1994

This is meant to be handed out to the students.

## I. Introduction

Most people think of permanent magnets when they think of magnets. For example, the magnets that stick to your refrigerator are permanent magnets. Permanent magnets are made out of materials (like iron) called ferromagnetic materials. You can think of ferromagnetic materials as being made up of lots of tiny magents (really groups of the atoms), each of which has a north and a south pole. If all the little magnets (also called domains) were oriented randomly, the piece of iron would not act like a magnet. (see figure below) Refrigerator magnets have had their domains aligned before you buy them at the store; the north pole of the magnet is where all of the north poles of the domains point, and the south pole is where all of the south poles of the domains point. (see figure below.)

Ferromagnet with domains aligned randomly.

Ferromagnet with domains aligned (magnetized).

Compasses are another common use for permanent magnets. Compasses are thin magnets that are mounted on a pivot. The north pole of the magnet is usually painted red and the south pole is usually painted white. Comapsses are often used for finding your direction because the earth has a central magnet. No one knows exactly why the earth is magnetic, but it has a south magnetic pole near the north geographic pole, and a north magnetic pole near the south geographic pole. The north pole of the compass is attracted to the south magnetic pole of the earth, so the north pole of the compass points north.

In this lab, we will see how electricity can be used to make a magnet. The electromagnet is physically completely different from a permanent magnet, but it makes the same magnetic field. If you coil a wire like the diagram, and run current through it, it will also act like a magnet. To figure out which end is the north pole, you use the right hand rule. Curl the fingers of your right hand so that your fingers point the direction the current goes around the coil. Then stick your thumb out, and it will point in the direction of the north pole. (see figure.)

To make a stronger magnet, you can put a piece of ferromagnetic material in the center of the coil. The magnetic field from the electromagnet aligns some of the domains in the ferromagnet, so you have both types of magnets working together. The electromagnet's strength is proportional to the number of loops of wire and to the current that flows through each loop.

## II. The Experiment

A. Making an electromagnet

Wrap about 3 yards of 22 gauge wire around a large nail. This is called a solenoid with ferromagnetic core.

Then, hook up the following circuit.

When you close the switch, the electromagnet will turn on. (And, of course, when you open the switch, the electromagnet turns off) Use the electromagnet to pick up and drop thumbtacks. Much larger electromagnets like these are used to pick up and drop scrap metal like old cars in a junk yard.

Bring your electromagnet near the compass.

Which end of the compass needle does the point of your electromagnet attract? _____________________

Which end of your electromagnet is the north pole? _____________________

Does your electromagnet obey the right hand rule? _____________________

Now, turn your battery around so that the current goes the other direction through your solenoid.

Which end of the compass needle does the point of your electromagnet attract? ______________________

Which end of your electromagnet is the north pole? _______________________

Does your electromagnet obey the right hand rule? ___________________

How do you think you can make a stronger electromagnet?

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B. Light Meter

Unwind the wire from your solenoid, and wrap it instead around the center of your compass, leaving about 6 inches of wire on each end. It helps to twist the two ends of the wire together, so it doesn't unravel.

Then, hook up the following circuit:

The resistor with arrows is a photoresistor. It is a variable resistor whose resistance varies with the amount of light that hits it. This circuit can be used as a light meter. Somewhere in the lab we have set up several different light bulbs for you to measure with your meter. First, leave the switch open. Align the dial of the compass so that the compass needle is parallel to the wires that are wrapped around it. This aligns the compass with the magnetic field of the earth. (Commercial gauges have a permanent magnet inside them, so that they do not need to be oriented with the earth.) To turn on your light meter, close the switch. The compass needle should deflect when the photoresistor is exposed to light - the more light, the more the deflection.

How does this gauge work?

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