1Lectures for Conceptual Physics, 8th Ed.

2

Electromagnetic Induction p4361820 Hans Oersted showed that current affected a magnet.1831 Michael Faraday and Joseph Henry made electricity from magnets. Made it possible to light up cities at night and ruined the sleep habits of the new era.It was simple…just rotate (move) a loop of wire in a magnetic field and electricity was produced.

OR

AND

3

Here is a critical concept!

Magnet being moved.

Coil of insulated wire

Resistor

The resistor is converting electric power into heat (work).

The source of that power is the person. She has to forcibly push the magnet into the coil.

The power she puts into making current comes out as heat (work).

The electricity just connects her work (input) to the work done (output).

The magnet resists being moved!

4

Faraday’s Law p438

The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops.

= - N d

dtB

Lenz’s Law: Heinrich Lenz 19th cent.

The induced voltage and the magnetic flux have opposite sign.

i.e. the magnetic field created by the induced voltage opposes the magnetic flux in the loop…you have to push.

5

More on Lenz’s Law:

Moving the magnet to the right increases the magnetic field lines in the loop.

The induced current creates a magnetic field which opposes this increase.

That’s it! Tricky but true.

The repelling magnetic fields explain why you have to push.

Picture from Serway, vol 2 p990

6

Generators and Alternating Current p439

By the way, when you push current goes one way. Then, when you pull, current goes the other way.

Starting and stopping is hard. It’s easier to make something rotate…the loop. (fig 24-5)

Rotating the loop induces current in the loop. This is the input.

The heat and light from the bulb is the output.

7

Motor effect: moving charges create a force.

fig 24-6

Generator effect: pushing the wire down creates a current.

8

It is rate of change of the magnetic field, the magnetic flux, that matters.

The rate of change, flux, is greatest at c.

So, the strongest current is produced at this part of the cycle.

AND, the next time the loop assumes the “c” position, the top will be on the bottom and current will be going the other way.

top

Fig 24-7

9

0

+

-

Maximum current in one direction.

Maximum current in the other direction.

Nikola Tesla, the scientist, along with George Westinghouse, the astute business person, provided the world with abundant electrical power.

10

Turbogenerator Power p441

This is the “turbo” part.

This is the generator part.

Power in. Power out.

Fig 24-9

11

MHD Power p441 MagnetoHydroDynamic generator

Loud but very efficient.

No moving parts. But, only recently have we had materials that can stand the high temp.s.

Heat from exhaust is used to boil water for a conventional generator.

N

S

12

Transformers p442

Transform voltage from low to high or high to low.

A galvanometer (voltmeter)

Putting an iron core through the coils focuses the magnetic field lines.

13

More on Transformers

On the left side (the primary side) a 1 VAC input is operating.

It is critical that the input voltage is alternating, changing directions.

The magnetic field produced by the primary coil causes an opposing magnetic field in the secondary coil. (Lenz’s Law)

The fluctuating secondary magnetic field creates an alternating current in the secondary coil.

Primary voltage

Number of primary turns=

Secondary voltage

Number of secondary turns

14

The answers are on p 445.

Study the answers…hint, hint!

15

Self-Induction p445

Imagine a large coil with a current running through it. Things are normal. The current is steady and creating a large magnetic field.

Did I mention that this is called an electromagnet?

Then you suddenly open the switch. (Cut off the current.)

The magnetic field with all its energy collapses quickly producing a giant current in the other direction which zaps across the switch.

This phenomenon is also called “back emf”, or “back electromotive force”.

16

Power Transmission p446

Large currents in wire create heat and loss of power.

So “electricity” is sent long distances at high voltage and small currents.

I V = I V

Transformers “step-up” the voltage at the production plant.

And, transformers “step-down” the voltage at the customer end.

17

Field Induction p447

A more fundamental way of looking at all the preceding is to focus on fields not currents and voltages. Fields are not encumbered by wires.

The whole electricity/magnetism thing was officially unified by James Clark Maxwell in the 1860’s:

“ A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.”

Note the comment about Richard P. Feynman at the bottom of pag 447…”among the most brilliant physicists of his time.”

And, finally, read the “In Perspective” section. The end