PHYS 2212 Module 9.6

Electric Generators and Back Emf

Learning Objectives

By the end of this section, you will be able to:

• Explain how an electric generator works
• Determine the induced emf in a loop at any time interval, rotating at a constant rate in a magnetic field
• Show that rotating coils have an induced emf; in motors this is called back emf because it opposes the emf input to the motor

Electric Generators

The electricity you are using right now is being generated at a power plant somewhere many miles away. What we are going to discuss in this module is how that electricity is made and transported to your home. And we’re going to start with Faraday’s law.

What I’ve just described is an electric generator and it is the basis for how all of our electricity is produced. The basic idea is to rotate something – a conducting loop in a B-field or rotate a B-field around a conducting loop. This rotation allows for a periodic change in flux which induces a periodic emf. This periodic emf will drive a current that will also look like a sine function. This type of current is called Alternating Current (AC) and is what you use every time you plug something into a household electrical outlet.

To generate electricity this way, we need to put mechanical energy (kinetic energy of rotation) into the system to convert it to electrical energy. And this is what happens at power plants. They all produce mechanical energy (usually by making something rotate, called a turbine) in some way: 

• heat water to produce steam, which uses pressure to rotate a turbine (the heating of water can be done with burning coal/oil, or through nuclear reactions)
• use pressure from a dam or a waterfall to rotate a turbine
• use wind to turn a windmill, which rotates a turbine 

In the U.S., our systems rotate with a frequency of 60 Hz, or 60 rotations per second.