Electromagnetic Induction

Faraday Lenz

Why does Electromagnetic Induction Occur?

Horizontal Magnetic Field

Move wire down

--

-

I

-

+

I

Faraday’s Law

Magnetic Flux Density, B

Area, A

Flux, = B x A

Unit of Flux is the Weber (Wb)Flux is a vector term

Flux linkage of a coil = N x = N x B x A

Coil with N turns

Faraday’s Law• If you make a wire cut through magnetic flux then

an emf will be induced.• Faraday’s Law:

– The magnitude of the induced emf is equal to the rate of change of flux linkage

• This effect can be made by moving the magnetic field or moving the conductor

tN

tN

(s) Taken, Time

(weber) Linkage,Flux in Change(volts) emf, Induced

= gradient of the graph of Flux Linkage against Time

Whiteboard Challenge• On your whiteboard,

draw the axes as shown• Now sketch the shape of

the top graph as the car goes through the loop

• Now sketch the shape of the bottom graph, remembering that this graph is showing the gradient of the first graph.

N

t

t

tN

tN

(s) Taken, Time

(weber) Linkage,Flux in Change(volts) emf, Induced

-0.15

-0.1

-0.05

0

0.05

0.1

0.15

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Time/s

emf/V

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8

Time/s

F

lux

Link

age

/

Flux linkage

Rate of change of Flux linkage

This is the gradient of this

N

S

Now for you to get on with some work!

Questions:

• Page 241, Q4-8 • Extension: Question 10

Lenz’s Law• Do we need to apply Lenz’s law?

– Does moving wire, a moving magnet or a changing strength magnet mean the flux lines are being cut?

• Yes: Then an emf will be induced in the wire.

– Is the wire part of a complete circuit?• Yes: Then a current will flow through the wire

– Lenz’s law states that the direction of the induced current will produce a magnetic field to oppose the motion.

Lenz’s Law• Magnet moves towards the coil• Flux lines from the magnet are being cut by the coil• Faraday’s Law states that: An emf will be induced in the coil• The coil makes a circuit so there will be a current in the coil• Lenz’s Law states that: The Left hand end of the coil will

have an induced N pole to oppose the motion• Right-hand grip rule indicates that the current must flow down

at the back of the coil and up at the front.

S N N S

Now some tough questions…

• Page 278, questions 20 & 21• … and if you want a real challenge (yes I

know that you do!) have a go at 22 as well

Note: You can also use the Right Hand Grip rule to show the direction of flux inside a solenoid (from South to North), if your fingers indicate the direction of current

Q22