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The Big Ideas Behind Electromagnetic Induction

We have seen in the last two weeks that moving electric charges create magnetic fields. This is Biot-Savart law or Ampere's law.

Since electric charges always produce electric fields, one can think of Ampere's law as saying that a changing electric field creates a magnetic field . If the charges don't move, the electric field pattern in space is static, but if the charges move the electric field changes and that change is what causes a magnetic field.

It turns out the converse is true, a changing magnetic field B creates an electric field E.

Again let us repeat the main message that you should remember.

A changing electric field E creates a magnetic field B

and

A changing magnetic field B creates an electric field E

The first is how a current of electrons can create an electromagnet. The latter is the key to understand the electric generator and the transformer as we will see later.

Download the following simulation directly from the Phet website . This is a Java application and you will need to download java (on a Mac, you may need to right-click the file to open and this is not a recognized app by the system).

1. First, look at the bar magnet and move it around. You can see how the magnetic field (represented by small compasses at each point in space) changes. At each point in space the magnetic field has some value and some direction.
2. Second, look at the pickup coil tab, try to move the bar magnet. As you move the bar magnet around, the magnetic field in the wire of the coil changes. As the magnetic field changes, it creates an electric field that then imparts a force on the electron and makes it move. We get a current, the light bulb lights up!

Now look at the electromagnet tab. There you will find a battery that generates a current through many coils of wire. This electromagnet behaves in exactly the same way as the bar magnet in the first tab! If you change the polarity of the current by changing the sign of the voltage on the battery you can reverse the north and south pole of the magnet (look at the red and white parts of the compass). If you stop the current by reducing the battery voltage to 0 V, the magnetic field will disappear everywhere.

Generator