Optional Unit VII: Electromagnetism

C. The Motor Principle

Key Concepts

When a current-carrying conductor is located in an external magnetic field perpendicular to the conductor, the conductor experiences a force perpendicular to itself and to the external magnetic field. (The Motor Principle)

The right-hand rule for force on a conductor can be used to determine the direction of the force experienced on the conductor. If the right thumb points in the direction of the current in the conductor and the fingers of the right hand point in the direction of the external magnetic field, then the force on the conductor is directed outward from the palm of the right hand.

The motor principle is used to form a precise definition of the ampere. 1 ampere is the amount of current flowing through two straight parallel conductors 1 metre apart in a vacuum which produces a force of 2 X 10^-7 newtons per metre of conductor.

Analog electric meters (i.e., galvanometer, ammeter, voltmeter) operate on the motor principle.

Electric motors are an important application of the motor principle. An electric motor consists of a permanent external field magnet (stator) and a coiled conducting ammeter (rotor) which is free to rotate within the field magnet. Brushes and a commutator (designed differently if A.C. or D.C. current is supplied to the armature) connect the armature to an external voltage source.

The speed of rotation of a motor depends on the amount of current flowing through it, the number of coils on the armature, the strength of the field magnet, the permeability of the armature, and the mechanical load connected to the shaft.

Learning Outcomes

Students will increase their abilities to:

1. State the motor principle.

2. State the right-hand rule for force on a conductor.

3. Apply the right-hand rule for force on a conductor to determine the direction of force experienced on the conductor, the direction of the external magnetic field, or the direction of current flow through the conductor.

4. Apply the motor principle to explain important applications such as electric meters or electric motors.

5. Identify the main components of an electric motor.

6. Explain how an electric motor works.

7. Recognize important differences in the design of A.C. and D.C. motors.

8. State several factors which affect the speed of rotation of an electric motor.

Teaching Suggestions, Activities and Demonstrations

1. Build small electric motors using inexpensive materials. Try to make modifications in the design to produce the greatest amount of spin on the rotor, or the greatest torque on the shaft. The diagram below illustrates a simple set-up that does not require any elaborate equipment.

   Equipment used: paper clips, 22 to 30 gauge insulated wire, 1.5 V dry cell batteries or power supplies, connecting wires with alligator clips, bar magnets.

   
   Coil the wire around a 15 mL test tube or a marker pen, to produce a neat, circular coil. Once the insulated wire has been coiled, lay it on a flat surface and scrape the insulated wire on one side of each end. Make sure the wire has been scraped on the same side on each end. This is really important. The unexposed insulated wire acts as a commutator. It must deactivate the current in the wire as it spins.

   Have students experiment with a few variables which might affect the operation of the motor, such as the number of turns of wire on the rotor, the position and polarity of the bar magnets, the strength of the bar magnets (if many are

   available), the current and the voltage of the power supply, (or, if batteries are used, does connecting them in series or parallel make any difference?), the gauge and length of insulated wire used , and so on.
   

2. Explain how the motor principle is used to form a precise definition of the ampere.

3. (If the term electromotive force is used, clarify that it is not a force, but energy per unit charge.) Better still, avoid the use of the term entirely, because it confuses students.