Magnetism and Electromagnetism

Warning!  The permanent magnets that are being used in this exercise are extremely strong and brittle. 

  1. Be careful as you separate and join the magnets.  Their strength can cause painful pinches and even blood blisters. 
  2. They are brittle and can shatter upon impact.  Unless you are careful, eye damage may occur. Do not let them move freely.  Pay attention to where you place them. 
  3. The magnetic fields of these magnets can damage floppy disks, hard drives, credit cards, ID cards and other magnetic materials.  Be careful where you place them.

Introduction

Look at the magnetic field demonstration model for the permanent magnet-compass demo in Dana 126. Notice that the magnetic field is 3-dimensional. The tiny compasses orient themselves along the magnetic field lines. The direction of the magnetic field lines external to a permanent magnetic originate at the north pole and terminate at the south pole.

Part 1    Field lines of an electromagnet

Equipment needed: spool of wire, battery pack, alligator clips and compass

  Using the compass and your knowledge of direction, determine the N pole of the compass.  (It may be reversed from what is written on the compass. The "north" geographic pole of the earth is near the south magnetic pole.)  

Connect the battery pack to the spool of wire and use the compass to determine the direction of the magnetic field lines. Use a pencil (pens may become magnetized) to draw an outline of the spool on the paper and note the position of the N and S poles.  Place the compass near, not necessarily at, the N pole of the magnet.  Make two dots in line with the compass arrow as close as possible to its head and tail.  Move the compass in the direction that the N pole is pointing until the S pole is above the dot made previously at the N pole.  At the new position of the N pole of the compass, make a new dot.  Continue in this manner until the S pole or the edge of the paper is reached.  Draw a smooth line through these dots and indicate by arrows the direction of the magnetic field.  Pick other places near the N pole and follow the same procedure until the field on both sides of the magnet is represented. Sketch these lines on a piece of paper indicating the direction of the field.  The axis of the spool is analogous to the axis of the magnet.  It's a little more difficult in this case because the spool is bigger.  Ideally you would want to put the axis of the spool in the plane of the paper.

Reverse the direction of the current and note the changes in the magnetic field directions. 

Part 2    Magnetic forces 

Equipment needed: same as Part 1, add 1 permanent magnet

Place the permanent magnet flat on a table. Identify the north and south poles of this magnet using the compass.  With the current running through the coil, move the coil above magnet.  When looking from above, the spool should look like a circle. Which pole of the coil will pick up the magnet?   You may have to flip the magnet over in order for the coil to pick up the magnet.  Disconnect the battery.  What happens? Do like poles repel or attract?  How can you tell with this setup?

 Part 3    Electromagnetic induction

 Equipment needed: 4 permanent magnets and steel rod, spool, galvanometer, alligator clips

Connect the spool to the - and G0 connections on the galvanometer.  Place the spool upright on the table.   Assemble the magnets so that the two pairs of magnets are separated by the steel rod. 

1. Move the magnet combination toward the center of the spool. What direction does the galvanometer deflect?  Move the magnet away from the spool and note the direction the galvanometer deflects. 

2.  Vary the speed of approach and departure of the magnet.  How does speed affect the deflection in each case?

3.  Reverse the direction of the poles of the magnet and repeat the first two steps in this Part.  How do your answers compare to your previous ones?

4. Return the magnet orientation as you had it in first step.  Bring the magnet to the opposite side of the spool.  How do your answers compare to your one for the first step?

5.  What do you expect to happen to the galvanometer as you pass the magnet in one side of the coil and out the other?  Try it.  Is it what you expected?

6.  Put one end of the magnet within the coil.  Can you hold the magnet still enough so that there is no deflection?

7.  From your results of #1-6 of this Part, what conclusions can you draw about the relative motion of the spool and the permanent magnet, and the induction of a current and voltage in the spool?

8.  Write a couple of summary sentences about electromagnetic induction.