Electromagnetism

Electromagnet

Wrapping an iron core with wire and sending a current through the wire creates a temporary magnet called an electromagnet.

When the electricity stops, so does the magnetism.

If a current generates a magnetic field, can a magnetic field generate a current?

Yes.

This is called electromagnetic induction (EMI).

EMI - Inducing an electric current using magnets.

An electric current induces a magnetic field which in turn

can induce an electric current.A magnet must be moved through a wire coil or the coil must be moved within the magnetic field.

Do the EMI lab prepared by your teacher.

Click here for internet demonstration

Faraday's Magnetic Field Induction Experiment

When Michael Faraday made his discovery of electromagnetic induction in 1831, he hypothesized that a changing magnetic field is necessary to induce a current in a nearby circuit.

To test his hypothesis he made a coil by wrapping a paper cylinder with wire. He connected the coil to a galvanometer, and then moved a magnet back and forth inside the cylinder just like you did with the bar magnet and the pipe coil. You repeated this experiment with the computer simulation you just did.

Continued next slide

Faraday continued

• When the magnet was moved back and forth, you noticed that the galvanometer needle moves indicating that a current is induced in the coil. Notice also that the needle immediately returns to zero when the magnet is not moving. Faraday confirmed that a moving magnetic field is necessary in order for electromagnetic induction to occur.

• The term electromagnetic induction means inducing an electric current using magnets. Of course we also know that a magnetic field is induced with electric current. Both of these processes are considered electromagnetic induction.

A motor is really just a generator operating backwards.

Generators make electricity using EMI. They turn mechanical energy into electrical energy.

Motors use electricity. They turn electrical energy into mechanical energy.

Motors & Generators

Generators

Instead of sending electricity through a coil to make it spin, spinning the coil by some other means will instead create an electric current. The magnetic field surrounding the coil pushes electrons through the wire creating a current. This is another example of electromagnetic induction.

DC Generators push the electrons in one direction only. It’s not a steady stream. The electrons are actually pushed in series of rapid pulses as shown by the graph.

DC Generation

http://www.phys.unsw.edu.au/~jw/HSCmotors.html#mandg

DC Generation

Click here for an DC Motor Demonstration from Molecular Expressions

AC Generation

Click here for an AC Generator from Molecular Expressions

An alternating current generator, or AC generator, produces an alternating current, which means the voltage produced alternately reverses from positive to negative polarity, producing a corresponding change in the direction of current flow.

As the electrons change direction the magnetic field around the coil alternates. The coil is alternately repelled and attracted by the permanent magnets that surround it and this keeps it turning.

Notice that an AC generator (alternator), does not have a commutator. Instead it has two slip rings. These rings allow electrons to be chased back and forth in the loop. Every time the loop turns 180o, the electrons reverse direction.

AC Generation

http://www.phys.unsw.edu.au/~jw/HSCmotors.html#mandg

An“Awesome”: AC and DC Generator

Website

Switch between

•with commutator (DC) and

•without commutator (AC)

Another Awesome AC and DC Motor and Generator

Website

http://www.phys.unsw.edu.au/~jw/HSCmotors.html#mandg

Watch how the polarity in the surrounding magnets keeps changing. Be able to explain how this keeps the armature spinning.

N

S

On

N

S

On

Loop rotates due to repulsion of like poles.

Direction of current

N

S

Off

The enamel shuts off the current so that unlike poles won’t attract anymore.

N

S

Off

Inertia keeps it rotating again.

The cycle continues.

South pole

North pole

Current in loop

so that repulsion can push the loop again.

Otherwise the motor would freeze up. Inertia keeps the loop spinning…..

On / Off Motor

If there were no switch, the north pole in the loop would rotate into position next to the south pole of the magnet. They two would attract each other and the loop would not continue to rotate. It would freeze in this position. When the current shuts off, the loop is able to rotate through until it can repel again and continue spinning.

Simple DC Motor

Polarity in the armature changes due to the split commutator. If polarity did not change the armature would stop spinning.

Permanentmagnet B

S

N

S

N

N

S

The attracting of unlike poles and repelling of like poles keeps it turning.

A

SN S

N

N

S

electromagneticarmature

The south pole of the bar magnet needs to repel the south pole of the coil and attract the north pole in order to make it start spinning.

Once it gets here the polarity of the coil must change in order for the south pole of the bar magnet to repel the south pole of the coil (previously the north pole) and attract the north pole (previously the south pole). If polarity did not change the unlike poles would

stay attracted and the motor would freeze.

C

S

N

N

S

N S N

S

D

S

N

N

S

Opposite poles once again attract and like poles repel keeping it spinning.

DC

Motor

Awesome DC MotorInternet Demonstration

Polarity in the armature changes due to the alternating current. This keeps the polarity of the ends of the armature and the poles of the permanent magnets always the same. The armature keeps getting pushed around. If polarity did not change the armature would stop spinning.

electromagneticarmaturePermanent

magnet

S N

A

N SN S

S N

B

N SN S

AC Motors

http://www.phys.unsw.edu.au/~jw/HSCmotors.html#mandg

A

C

B

D

Notice how the polarity of the surrounding magnets keep changing to keep the armature spinning.

AC Motor

Pictures

Motors Summary

Whether dealing with DC motors or AC motors, the basic idea is that if electricity is sent through a coil of wire that sits in a magnetic field, an opposing magnetic force is created around the coil. This force interacts with the surrounding magnetic field and this makes the motor spin.

Magnets are made from solid materials that have polar molecules that are all lined up in the same direction.

You can make a magnet by placing magnetic material in a strong magnetic field or by pounding a magnetic bar while it is lined up in the earth’s magnetic field.

A compass is a magnet that is balanced at its center and allowed to swing free. It aligns itself with the earth’s magnetic field.

Electromagnetism Summary

A magnetic object such as a nail can be made into a temporary magnet by holding it against a permanent magnet. Once the permanent magnet is taken away the nail loses its magnetism.

A current through a wire creates a magnetic field around the wire. The direction of the current determines the direction or polarity of the magnetic field.

Moving a magnet over a wire creates an electric current. Moving a wire through a magnetic field also creates an electric current.

A moving magnetic field induces an electric current. (Faraday’s law)

Electromagnetic induction (EMI) - generating an electric current using magnets.

Sending current through a wire that goes through a magnetic field causes the two magnetic fields to repel or attract. The wire will be pushed up or down. If the current is reversed, the direction the wire is pushed is also reversed. This is the basis for how motors work.

Pushing a magnet through a wire coil creates an electric current in the coil while the magnet is moving because the magnet chases electrons through the wire. Pulling the magnet out of the coil creates a current in the other direction.

The larger the coil, the larger the electric current because there are more electrons to move around. More electrons moving means a larger current is created.

Pushing the magnet in and out of the coil creates alternating current since electrons are chased back and forth.

Spinning a loop of wire in a magnetic field also creates an electric current. The magnetic field will push electrons through the wire. This is essentially a generator.

On the other hand, if electricity is sent through the loop of wire that is in a magnetic field, the loop will spin. This is essentially a motor.

The End