PHYS 1402 – General Physics II

Introduction to Electromagnetism

 

Materials

 2 x Genecon                                                    2 x Alligator patch cords (light weight)

Small coil with iron rod                                     Wood block with two screw eyes

Compass                                                          Stiff wire bent in shape of U

Masking tape                                                    Horseshoe Magnet

Ring stand with clamp                                       Tangent Galvonometer

DC power supply (0-28 V)

 

Introduction

      In this activity we will explore some of the basic ideas of electromagnetism.   Electromagnetism refers to the magnetic field produced by an electric current.

 

Procedure

The Magnetic Field of a Solenoid

      Begin by plugging the Genecon cable into the Genecon.  Connect the alligator clips on the Genecon cable to the connectors on the small solenoid.  Place the solenoid on the table with the bore oriented horizontally.  Place the small iron rod so that it is a little more than halfway into the bore of the solenoid. 

 

1)  Turn the crank of the Genecon one time rapidly while observing the iron rod.  Record what happens to the iron rod.  (If nothing happens notify your instructor to assist you.)

 

 

2)  Repeat the procedure only this time turn the crank of the Genecon in the opposite direction.  Record what happens to the iron rod.

 

 

3)  Keeping in mind that the iron rod is a temporary magnet, suggest an explanation for what is happening to the iron rod when you turn the crank.

 

 

4)  Explain how you might use an iron rod inside of a solenoid to make a switch.

 

 

      It appears that when we turn the crank we produce a magnetic field in the solenoid.  We would like to verify that observation and determine some properties of the resulting magnetic field.  Connect the alligator clips on the Genecon cord to an alligator clip patch cord.  Wrap the patch cord in a line around the center of the compass so that it makes three turns around the compass.  You may want to use some tape to secure the cord. Orient the compass so the compass needle is parallel to the turns of the wire and turn the Genecon handle at a constant rate.  Be careful not to turn the crank handle too fast as this can break the Genecon. 

 

5)  What happens to the compass needle when you turn the crank?

 

Wrap the entire patch cord around the compass and turn the Genecon handle at the same rate as in the previous question. 

 

6)  Is it easier to produce the same effect as you described in question 5)?

 

Turn the crank of the Genecon in the opposite direction. 

 

7)  Record what happens to the compass needle. 

 

 

8)  How does this observation differ from the previous?

 

 

Orient the compass so that the compass needle is perpendicular to the coil of wires. 

9)  Turn the crank of the Genecon and record what happens to the compass needle.

 

 

10)  Turn the crank in the opposite direction and record what happens to the needle in this case.

 

 

11)  Remembering that the compass needle is a magnetic dipole and will align with the direction of the magnetic field,  what do questions 7) - 10) tell you about the direction that the magnetic field of a solenoid points.  Specify the direction in terms of the orientation of the coils.

 

12)  Does the magnetic field depend on the direction that the current flows in the solenoid?  If so, how?

 

 

13)  Summarize your observations about the strength and direction of the magnetic field of a solenoid.

 

 

Electromagnets

      Wrap the patch cord tightly around the iron rod with as many turns as you can fit.  Have one person turn the crank while another holds the electromagnet. 

 

14)  Use the compass needle to demonstrate that the wrapped iron rod behaves like a permanent magnet when current flows through the wire.  Record your observations demonstrating this result.

 

 

 

Remove the iron rod from the coil but leave the coil the same.  Repeat your procedure with the compass to verify that the coil without the iron rod acts like a permanent magnet. 

15)  Record your observations demonstrating that the coil without the rod behaves like a permanent magnet.

 

 

16)  Does the magnet seem to be stronger with the iron rod or without?  Explain.

 

 

17)  Remembering that the iron rod is a temporary magnet, explain why the magnetic field of the electromagnet is stronger when the iron rod is present.

 

 

A Very Simple Motor

      Set the U-shaped magnet on the table with one pole on the table and one pole in the air.  Use the ring stand and the clamp to hold the wood base with two hooks.  Suspend the U-shaped wire from the hooks, adjusting the height so that bottom of the wire is halfway between the poles of the magnet. Make sure that the wire is free to swing on the hooks.

      Turn the crank of the Genecon back and forth about a quarter turn each way. 

 

18)  Record what happens to the wire.

 

 

19)  Explain this behavior in terms of the magnetic fields of the U-shaped magnet and of the field due to the current in the wire.  Draw a suitable picture.

 

 

 

Stop the wire and turn the Genecon continuously in one direction. 

20)   Record what happens in this case.

 

 

 

21)  Why is the result different when you turn the crank continuously in one direction than when you turn the crank back and forth?

 

 

The Equivalence of a Generator and a Motor

      Connect the alligator clips of two Genecons together.  Place one on the table and turn the crank of the other. 

 

22)  Describe what happens?

 

Turn the crank in the opposite direction.

23)  Describe what happens.  How is it different than previously?

 

Note that when you turn the crank of the Genecon, it acts like an electric generator.  The generator converts mechanical work into electrical energy.

 

24)  What process is happening in the second Genecon?

 

 

25)  How do a generator and a motor seem to be related?

 

 

We will explore this connection more in ch. 22.

 

The Magnetic Field of Earth

      In this brief activity we will determine the magnetic field of Earth.  We will use an apparatus called a tangent galvanometer.  The tangent galvanometer is essentially a coil with a compass at its middle.

 

Procedure and Questions

1.  Set-up

Make sure that the DC power supply is turned off, the knob on the right is set to the 0 – 28 V setting and the knob on the left is fully counter clockwise.  Connect the coil to the DC output of the power supply between the middle and an outer terminal.  This means that the power supply is connected across 5 turns in the coil.  Rotate the apparatus so that compass needle is oriented in the plane of the coil.  Identify which side of the compass needle points north, and note it for later reference.  Rotate the compass case so that the needle aligns with N on the case.

 

2.  Turn on the power supply and very slowly turn up the current.   Be careful to monitor the current meter on the power supply and not the voltage.  Do not exceed 5 A, and probably you will need considerably less.

 

26)  Do you observe any effect?  Describe it.

 

 

3.  Turn up the current a little more. 

 

27)  What change is there?

 

 

4.  Turn down the current and reverse the leads to the coil.  Slowly turn up the current again. 

 

28)  Does the effect change in way?

 

 

5.  Turn down the current and connect the power supply across the outer terminals on the tangent galvanometer.  The power supply is now connected across 10 turns in the coil..  Very slowly turn up the current.

 

29)  Was there any change in the effect?

 

 

30)  When current flows in the coil, what is produced?

 

 

31)  In the center of the coil, what direction does the magnetic field produced by the coil point compared to the coil?

 

 

32)   How does the strength of the produced magnetic field depend on the current in the coil?

 

 

33)  How does the strength of the magnetic field produced by the coil depend on the number of turns with the coil?

 

 

34)  Draw a vector diagram showing the sum of the magnetic field due to the coil and the magnetic field due to Earth.

 

 

 

35)  Use your diagram to find an expression for the angle, q,  through which the needle  deflects in terms of Earth magnetic field and the magnetic field of the coil.

 

 

      An expression for the magnitude of the magnetic field, in units of teslas, at the center of the coil is given by , where B is the magnetic field strength, N is the number of turns in the coil, r is the radius of the coil, I is the current flowing in the coil, and m0 = 4p x 10-7 Tm/A is a fundamental constant called the permeability of free space. 

 

36)  Describe a procedure to measure Earth's magnetic field using this apparatus.

 

 

 

37)  Carry out your procedure and record your results and calculations below.