Lesson 31: Motors and Generators

Download here: Ontario Curriculum Expectations

Science investigates religion interprets. Science gives man knowledge which is power. Religion gives man wisdom which is control.

Martin Luther King Jr.

AC and DC Current:

The electricity that comes from batteries is DC, or direct current. This means that the electrical energy is carried around the circuit by the electrons that flow directly from the negative terminal of the battery around to the positive terminal of the battery. When the electrons pass through a load, they give up that electrical energy.

Electricity that is generated at a power plant and is transmitted to your home on high tension wires is AC, or alternating current. This means that electrons don’t actually travel to your home from the power station. Instead, they just vibrate back and forth passing the electrical energy from one electron to the next as they do so.

Review electricity by watching the music video below:

Electric Motors:

Electric motors work on the following premise: When electricity travels through a wire, it creates a magnetic field around that wire. If you pass the wire, with its surrounding magnetic field, through the magnetic field of a magnet, the two magnetic fields interfere with each other and the result is the generation of a force that causes motion.

To learn more about how motors work, complete the multimedia element electric motors. When you have completed it, be sure to return to this page.

Generators:

Generators are used to generate electricity; they work on the following premise: If you move a magnet near a coil of wire, the magnet will cut through the magnetic field lines and induce a current in the wire.

Watch the video below to learn about the first electric generator:

Factors that Affect the Induced Current:

Use the student exploration guide to answer the questions below on the factors that affect the induced current in electromagnetic induction.

1. The Number of Loops in a Helix:

 What effect does the length of the coil have on the magnitude of the induced current in the wire?

You should have observed that the helix with more loops generated more electricity. When the magnet passes near the wire, it induces an electrical current in the wire. If there are more wires present, more electrical current is induced into those wires.

2. The Speed at which the Field Lines are Cut:

What effect does the speed of the magnet have on the magnitude of the induced current in the wire?

You should have discovered that moving the magnet quickly through the helix results in a larger induced current. In fact, the same result can be achieved if the magnet were kept stationary and the helix was moved back and forth around the magnet.

3. The Polarity of the Magnet:

 What difference, if any, do you think this will have on the current induced in the wire?

You should have noticed that when the magnet is inserted into the helix north pole first, the arrow on the meter moved to the left but when the magnet was removed from the helix, the arrow moved to the right. Conversely, when the magnet was inserted into the helix south pole first, the opposite occurred. The arrow on the meter moved to the right as the magnet was pushed into the helix and to the left as the magnet was pulled out of the helix. We can actually predict the direction of the induced current using Lenz’s Law.

Lenz’s Law:

Lenz’s Law enables us to predict the direction of the induced current.

Lenz’s Law

An induced current is always in such a direction that its

magnetic field opposes the motion or change causing it.

In the diagram, the magnet is being pushed into the core of the helix north pole first. We know that a current will be induced in the wire. According to Lenz’s Law, the magnetic field of this current must oppose the motion of the magnet. That means that the left side of the helix must be the north pole. Using the Right Hand Rule for electromagnetic force, you need to grab the helix with your right hand in such a way that your right thumb points north – to the left. When you do this, your fingers curl over the helix in the direction of the current. In this case, the current will flow B to A.

As the magnet is pulled out of the helix, the magnetic field must again oppose the motion. This means that the left side of the helix must now be a south pole. Consequently, the current in the wire must flow A to B.

The AC Generator:

In the generator below, the magnetic field is stationary and is pointing towards the left. In order for a generator to work, something needs to turn the orange wire loop. Generally this is done using high pressure steam, or falling water. The green arrows illustrate the direction in which the orange loop is turning. By looking at the left hand side of the orange loop and using the Right Hand Rule, you can determine the direction of the induced current:

If you use the Right Hand Rule again, but instead look at the right hand side of the loop, you will find that the current, pointing in the direction of your thumb, is moving out of the screen towards you. This means that the current is travelling in a clockwise direction around the orange loop.

The black slip rings will turn with the orange loop. You can see that the longer end of the loop is connected to one of the rings and the shorter end to the other. Carbon brushes at the ends of the main circuit, the black lines, connect to the slip rings and allow the current to flow around the circuit. Here the current is flowing in a counter-clockwise direction.

Now the orange loop has turned 180?. You can tell this by looking and the long and short ends of the orange loop. If you repeat the steps for determining the direction of the induced current, you will find that it is still moving in a clockwise direction around the orange loop. Actually, the current in the loop has been reversed, but then so had the loop. Consequently, the direction in which the electricity flows around the circuit has been reversed. The current is now flowing through the circuit in a clockwise direction.

This is a current vs. time graph for the generator. The alternating positive and negative values correspond to continuous reversal of the induced current as we say in the diagrams above. As you can see, the current produced by a generator is alternating (AC).

As with the motor, most generators will have many coils of wire instead of just the one shown in the diagrams above. This will allow a much greater current to be produced.