AC Electric Circuits

AC Generator Theory

13 questions By Tony R. Kuphaldt

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Question 1 of 13

If an electric current is passed through this wire, which direction will the wire be pushed (by the interaction of the magnetic fields)?

Is this an example of an electric motor or an electric generator?

Reveal answer

The wire will be pushed up in this motor example.

Notes:
A visual aid to understanding the interaction of the two magnetic fields is a diagram showing the lines of flux emanating from the permanent magnets, against the circular lines of flux around the wire. Ask those students who came across similar illustrations in their research to draw a picture of this on the board in front of the class, for those who have not seen it.
Question 2 of 13

If this wire (between the magnet poles) is moved in an upward direction, what polarity of voltage will the meter indicate?

Describe the factors influencing the magnitude of the voltage induced by motion, and determine whether this is an example of an electric motor or an electric generator.

Reveal answer

The voltmeter will indicate a negative voltage in this generator example.

Notes:
Ask your students to explain their answers regarding factors that influence voltage magnitude. Where did they obtain their information? Are there any mathematical formulae relating these factors to induced voltage?
Question 3 of 13

If this wire (between the magnet poles) is moved in an upward direction, and the wire ends are connected to a resistive load, which way will current go through the wire?

We know that current moving through a wire will create a magnetic field, and that this magnetic field will produce a reaction force against the static magnetic fields coming from the two permanent magnets. Which direction will this reaction force push the current-carrying wire? How does the direction of this force relate to the direction of the wire’s motion? Does this phenomenon relate to any principle of electromagnetism you’ve learned so far?

Reveal answer

The reaction force will be directly opposed to the direction of motion, as described by Lenz’s Law.

Follow-up question: What does this phenomenon indicate to us about the ease of moving a generator mechanism under load, versus unloaded? What effect does placing an electrical load on the output terminals of a generator have on the mechanical effort needed to turn the generator?

Notes:
If you happen to have a large, permanent magnet DC motor available in your classroom, you may easily demonstrate this principle for your students. Just have them spin the shaft of the motor (generator) with their hands, with the power terminals open versus shorted together. Your students will notice a huge difference in the ease of turning between these two states.

After your students have had the opportunity to discuss this phenomenon and/or experience it themselves, ask them why electromechanical meter movement manufacturers usually ship meters with a shorting wire connecting the two meter terminals together. In what way does a PMMC meter movement resemble an electric generator? How does shorting the terminals together help to protect against damage from physical vibration during shipping?

Ask your students to describe what factors influence the magnitude of this reaction force.