Electromagnetic Induction

Induced EMF and Induced Currents

When there is no relative motion between the coils of wire and the magnet there is no current produces

Current is created in the coil when the magnet is moved towards the coil

Current also exists when you pull it away from the coil, just in the opposite direction.

The current in the coil is called an induced current. The coil itself acts as a source of emf known as an induced emf.

Another way to look at it.

Changing the area of a coil, in effect, reduces/increases the B field that the coil is subject to. Changing the B field strength experienced by the coil. This will also create a current.

Motional EMF

The EMF Induced in a Moving Conductor

A rod is being pushed to the right with constant speed v. Suddenly the bulb lights. Why?

Where is the current coming from ?

Where is this opposing force coming from?

We have been using the term emf, ε, or electro motive force.

ε=BLv

Potential Difference

Magnetic Flux

Motional EMF and Magnetic Flux

cosBAA

B

By definition therefore BA

Of course the angle with the field is important

It is convenient express emf in terms of area when using induction in motors and generators.

0

0

0

0

0

0

0

0

tt

BABAB

tt

AA

tt

LxxLBL

tt

xx

BAttt

0

0since

Faradays Law actually reads

tN

Where N is the # of turns in the coil. But what is the negative all about?

Consider the field created by the counterclockjwise loop in our previous problem. What is the direction of its field?

Lenzs’ Law

The induced emf resulting from a changing magnetic field will produce a current in such a way that the induced magnetic field will oppose the original change in flux.

Like “magnetic inertia”

Transformers

Later, I will ask you why we need

ALTERNATING CURRENT

to make this work!

Basically, this is a transformer!

How many of those little Power Cube thingies do you have around your house? Here's how many I found around mine:

The answering machine The cordless phone The cell phone The electronic anti-flea ultrasonic noisemaker (we have 2 dogs) The digital camera The nicad battery recharger The video camera battery replacer/recharger The digital clock in the bedroom The electric toothbrush The electric razor The electric screwdriver The electric drill Both sides of the baby monitor The laptop computer The office phone The ink jet printer The speaker system on the computer The tape recorder that records notes in the office The calculator recharger The 9-volt battery replacer for the radio The radio shack univeral power cube

So let's take one of these apart and see what's inside. Here's the transformer we will be exploring today:                                                                                                                                

Under the cover

• This transformer came with a rechargeable electric screwdriver. This particular transformer is rated at 3 volts and 240 milliamps. Once you convince the cover to come off here is what you find inside:

• What you can see here are two windings. The purpose of a transformer is to convert one AC voltage to another AC voltage. In this case the transformer converts the normal 120 volt AC current in your house down to three volts.

Primary Winding

• The 120 volts comes in on the primary winding on the left. Running down the middle of that winding (as well as around the outside) is an iron core. The AC current in the primary winding creates an alternating magnetic field in the iron just as it would in an electromagnet.

Iron Core

Secondary Winding• The other winding, known as the secondary winding

wraps around the same iron core. In the secondary winding the magnetic field in the core creates current. The voltage in the secondary is controlled by the ratio of the number of turns in the two windings. So ifthe primary and secondarywindings have the same number of turns, the primary and secondary voltage will be the same. If the secondary winding has half as many turns as the primary then the voltage in the secondarywill be half that of the voltage in the primary.

• You can see in the following figure that the primary in this particular transformer uses very fine wire while the secondary uses much thicker wire. To drop down to 3 volts, there needs to be 40 times more turns in the primary than in the secondary.

• On the other side of the transformer you find two diodes wrapped in rubber insulation. The diodes act as a rectifier, turning the AC current into DC current. Most transformer cubes that you find around the house produce a low-voltage DC current (3 to 12 volts, and less than an amp of current).

DC current is necessary because rechargeable batteries store DC current, because most electronics require low-voltage DC current and because small DC motors run directly from batteries and are the least expensive motors available.

Turning AC into DC

•On the other hand, the picture tube in your TV requires 15,000 V to accelerate the electron beam, and a transformer is used to obtain this from a 120 V wall outlet.

The current coming into the house is 30 amps, find a way to tell me the current in the High-voltage wire.