Evans SPH 3U1

Unit 5: Electricity & Magnetism

Note 12: Electromagnetic Induction

Reference: Chapter 18

Demo: Changing magnetic field near a conductor inducing a current. Show on oscilloscope.

A changing magnetic field near a conductor induces a current in the conductor.

Fig. 18.4

Lenz's Law (OH):

When an electric current is induced in a coil by a changing magnetic field, the current is in such a direction that its own magnetic field opposed the change that produced it.

................based on the Law of Conservation of Energy:

The induced current could not create a magnetic field which supports the change in the inducing field, because that would make the induced current larger. This would start a cycle of continuously increasing magnetic fields inducing one another! Clearly this is not possible.

In simple terms, the induced magnetic field opposed that motion of the inducing magnetic field.

Example 1: page 610

Page 611 #1 (have answers placed on board, discuss)

Electric Generators:

Demo

Generators provide electricity that runs many devices (computers, TVs).

They convert mechanical energy by rotational motion into electric current.

Its construction is just like a motor but it operates in the opposite manner.

Input                                                Output

Motors                Current flow                                Rotational mechanical energy

Generators        Rotational mechanical energy                Current flow

Transformers: Using Electricity to Generate Electricity

Closing the switch in the primary circuit changes the magnetic field in the ring and therefore induces current flow in the secondary circuit for an instant.

Problem: one would have to continuously open and close the switch to maintain an induced current flow.

Solution: use AC (alternating current) in the primary circuit. (therefore a magnetic field is reversed whenever the current is reversed).

AC is the only type of current that can be transformed from one electrical potential (voltage) to another. This can be done with transformers.

Draw transformers: Step up and Step down.

The power dissipated in the transformer's primary and secondary sides is the same..

where P is power, V is voltage, I is current, N is number of turns

Investigate and make notes on: 'Why it is advantageous to transmit at high voltages and low current rather than high current and low voltage?' see page 618

VIDEO: Alternating Current (Beyond the Mechanical Universe)

-describes the competition between Edison and Tesla to commercialize DC vs. AC electricity

-skip the math explanation in the middle of the video