PHYSICS 106/1116

Electromagnetic Induction.

Text Reference:  Giancoli 21.1; Haliday, Resnick, Walker 30.2

Introduction:

If a charge is moving relative to a magnetic field, that charge feels a magnetic force perpendicular to its motion. One way we have seen this is by shooting a charged particle into a magnetic field. We extended that idea to show that there must be a force on a current carrying wire. The second part of this lab (building an electric motor) takes advantage of this latter point.

There are other less obvious ways to move charges relative to an external magnetic field which give rise to the same kinds of effects. Lets say that instead of moving the charge, we move the magnetic field (by moving the magnet). A stationary charge in the vicinity of a moving magnet is still moving relative to the magnetic field, and so it feels a force, and will begin to move! This is very exciting because all of a sudden we have a very easy way to create electric currents. If we move a magnet relative to a wire (initially carrying no current), the resting charges already in the wire will feel a magnetic force, and thus they will begin to move (if the directions work out right). Currents created in this way are called induced currents. The process is called magnetic induction. This is how the electric company creates the electric current that is sent to your house. In short, wires moving relative to magnetic fields have currents induced in them. Note a few things here: it does not just have to be wires, it can be any material with free charges (metal); your magnetic field can be created by a permanent magnet or by other current carrying wires.

The first part of this lab allows you to observe the induction of electric currents by moving magnetic fields near wires. You will observe this effect using a magnetic field created by a permanent magnet and also by other current carrying wires.

Experiment:

If a solenoid is connected in series with a galvanometer (a sensitive ammeter) so as to form a closed circuit and if a permanent magnet is inserted into or withdrawn from the solenoid, a current is induced in the solenoid and is maintained as long as the motion of the magnet continues. The direction of the current in the solenoid (clockwise or counterclockwise when viewed from one end of the coil) is determined by the direction of the motion of the magnet and the position of its poles relative to the end of the coil.

The rule is as follows: If the solenoid is viewed on end, as shown in the Figure, and if the north pole of the magnet is moved toward the observer so that it approaches the far side of the solenoid, then a positive clockwise current is induced in the coil. A single reversal of any single one of the italicized words reverses the direction of the current. For example, if the north pole of the magnet is moved away from the observer on the far side of the solenoid, the current induced is counterclockwise. Or if the north pole is moved toward the observer on the near side, the current is counterclockwise.

The magnet need not be permanent. Identical results are produced by the magnetic fields associated with currents. In particular, you can replace the bar magnet with a second solenoid carrying current which acts like a bar magnet in that one end of the coil is a magnetic north and the other a magnetic south. The rule is that if the observer views the solenoid on end, when the positive current is clockwise, the far end of the solenoid is the magnetic north end.

A magnetic field surrounding a current carrying solenoid may be changed in a given region of space by moving the solenoid or by altering the current in its coils. For example, a decrease in the current in the solenoid weakens the magnetic field and is equivalent to withdrawing the solenoid from the region.

Part I: Induction

Using the theory of electromagnetic induction, describe and explain the behavior of the galvanometer in each of the following exercises. Use diagrams freely. Note that the galvanometer needle will deflect in opposite directions for currents moving in opposite directions.

1. Connect the larger coil in series with the galvanometer and hold the coil still against the magnet. Does electricity flow in the wires? Explain.
2. With the magnet fixed, move the coil slowly so that the magnet passes through the coil. Withdraw the coil slowly. Repeat with a more rapid motion. Is there a current in the wires? Explain. Repeat this exercise with the poles of the magnet reversed.
3. With the coil fixed, move the magnet in and out of the coil. Repeat this exercise with the poles of the magnet reversed.
4. Connect the larger coil, henceforth called the primary, to the DC power supply. Connect the other coil, henceforth called the secondary, to the galvanometer.  Place the coils next to one another along a common axis.
   1. Start the current in the primary.
   2. Stop the current in the primary.
   3. Start the current in the opposite direction in the primary.
   4. Stop the current in the primary.

Observe and discuss the results.

Part II: Magnetic Force

In this part we will build an electric motor using the concept of the magnetic force on a current carrying wire. We might do 3 or 4, on the left menu, time permitting.

Write-Up

1. If this is a formal lab (as indicated on the lab syllabus), you have been instructed as to whether this is an individual or group write-up, or an oral presentation. If it is a write-up, each person must submit the lab electronically as a word-processed document in Moodle before the next lab meeting. If it is a group write-up, you should all be submitting the same document. For written formal labs, remember to check the "write-up hints" page to be sure everything is included and check your write-up against the grading rubric.
2. If this is an informal lab, work on the results together in your groups, and be sure to have your complete informal lab in your lab notebook and checked by the instructor before you leave.
3. Remember to read the next lab and do the pre-lab before you come to lab next week! You may work on the pre-lab with others, but each person must submit her or his own work.

Department of Physics