Electromagnetic Induction

AP Physics

Chapter 21

Electromagnetic Induction

21.1 Induced EMF

Induced EMF

Michael Faraday – a changing magnetic field can produce an electric current

This is an induced current

21.1

Induced EMF

A magnet moved quickly into a coil of wire induces a current

When it is quickly moved the other way, a current is induced in the opposite direction

21.1

Electromagnetic Induction

21.2 Faraday’s Law of Induction; Lenz’s Law

Faraday’s Law of Induction; Lenz’s Law

From experiments the induced emf depends on

1. Rate of change of the magnetic field

2. Loop’s area

3. Angle

Magnetic flux is the

total number of

magnetic field line

passing through

the are enclosed by the loop 21.2

Faraday’s Law of Induction; Lenz’s Law

Magnetic Flux

When is 90, no field lines pass through the loop

The unit for magnetic flux

is a weber

21.2

cosBAB

Faraday’s Law of Induction; Lenz’s Law

Since the induced EMF is the rate of change of flux

This is Faraday’s Law of Induction

For multiple loops in a magnetic field

Where N is the number of loops in the field 21.2

tB

tN B

Faraday’s Law of Induction; Lenz’s Law

Lenz’s Law – a current produced by an

induced emf moves in a direction so that its

magnetic field

(produced by

the current)

opposes the

original change

in flux

21.2

Faraday’s Law of Induction; Lenz’s Law

Steps in problem solving – Lenz’s Law

1. Determine if the magnetic flux inside the loop is decreasing, increasing, or unchanged

2. The magnetic field due to the induced current

a. Points in the same direction as the external field if the flux is decreasing

b. Points in the opposite direction if the flux is increasing

c. Is zero if the flux is not changing 21.2

Faraday’s Law of Induction; Lenz’s Law

Steps in problem solving – Lenz’s Law

3. Use the right hand rule-1 to find the direction of the induced current

4. Always keep in mind that there are two magnetic fields

a. An external field whose flux must be changed if it is to induce an electric current

b. A magnetic field produced by the induced current

21.2

Electromagnetic Induction

21.3 EMF Induced in a Moving Conductor

Faraday’s Law of Induction; Lenz’s Law

If the rod above is made to move at a velocity v

The area of the rod increases

So the change in area is

The induce EMF is21.3

tvx tlvxlA

tB

t

AB

t

tBlv

Blv

Faraday’s Law of Induction; Lenz’s Law

Sometimes called motional emf

True as long as components are mutually perpendicular

21.3

Electromagnetic Induction

21.4 Changing Magnetic Flux Produces an Electric Field

Changing Magnetic Flux Produces Electric Field

Since there is an induced current when there is an induced EMF, there must be an electric field

A changing magnetic field induces an electric field

So since

And

The electric field must be21.4

qvBF

q

FE

q

qvBE vBE

Electromagnetic Induction

21.5 Electric Generators

Electric Generators

Generator – converts mechanical energy (spinning) into AC current.

If we look at just one perpendicular arm of the loop

Since we have two arms in each loop

Now if we add loops

21.5

Generator Simulation

sinBlv

sin2Blv

sin2NBlv