Electric Motors & Generators: Converting Between Electrical and Mechanical Energy

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  • 0:01 Motors & Generators
  • 1:26 Electromagnetism
  • 3:11 Alternating & Direct Current
  • 4:52 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Both generators and electric motors are common devices in everyday life, but how do they work and what is the difference between them? In this lesson, we'll explore both of them, which may produce different results but are actually more similar than they may seem.

Motors & Generators

We've all been there - one moment you're watching your favorite program on TV and the next you're in the dark because the power's gone out. At such a time, you may wish you had a candle or a flashlight so that you could see in your dark house. But what would be really useful is a generator because this converts mechanical energy into electrical energy. If the generator were powerful enough, you could use it to restore electricity to your house, at least for a little while.

Now that you know how handy a generator is, you may be tempted to rush to the store and buy one. So, you get in your car and you drive there, pick one up and bring it home. Once you get your power restored, you realize that in the darkness you were clumsy and knocked your dinner plate onto the floor. So, you get the vacuum out and start cleaning it up. At this point you have done an amazing thing - you have essentially reversed what the generator did! Inside the vacuum is an electrical motor, which converts electrical energy into mechanical energy.

While the generator and the vacuum (or any other device powered by a motor) may serve different functions, they are actually two sides of the same coin. In fact, they are the same device! In an electrical motor, the input is electricity and the output is mechanical power. Contrary to this, a generator takes mechanical power and outputs electricity. In both cases, electricity is flowing - just in a different direction!


Both motors and generators run because of something called electromagnetic induction. Discovered by Michael Faraday, this is when a voltage is induced by a changing magnetic field. With electromagnetic induction, an electric current can be produced in a coil of wire by moving a magnet in or out of that coil, or by moving the coil through the magnetic field. Either way, voltage is created through motion.

The amount of voltage induced depends on the number of loops in the coil of wire, as well as the speed at which the magnet is moved through the coil. A greater number of coils means a greater amount of voltage is induced. Similarly, the faster the magnet is moved through the coil, the more voltage you get.

What does this have to do with motors and generators? Well, a generator produces electricity by rotating a coil in a stationary magnetic field, and in a motor, a current is passed through a coil, which forces it to spin. In both cases, Faraday's law of electromagnetic induction is employed, allowing you to produce electricity in your house and then use it to vacuum your floor, wash your dishes in the dishwasher, keep food fresh in your refrigerator and so much more.

Remember before how we said that a motor and a generator are the same device, but producing opposite results? What we mean here is that the flow of electricity is reversed, not that the machine itself operates in reverse. So, you can't just take a generator and turn it into a motor by 'reversing' the components of the machine. Likewise, with an electric motor you can't just flip a switch that makes the components operate in reverse to produce electricity. Instead, what you have to change is the direction the electricity flows: inward for a motor and outward for a generator.

Alternating and Direct Current

Ever heard of AC/DC? We're not talking about the Australian rock band - this is a physics lesson after all! When we're referring to AC and DC for motors and generators we're talking about alternating current and direct current. Like the name implies, alternating current alternates direction as it flows through a circuit. In contrast, direct current does not change direction as it flows through a circuit.

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