Discovery of Magnetic Induction: Faraday's Experiments - Prof. Renat R. Letfullin, Study notes of Advanced Physics

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Magnetism

Magnetism

Lecture 29: Electromagnetic Induction

x x x x x x

x x x x x x

x x x x x x

v

F

B

q

v

F

B

q

v

F = 0

B

q

F qE qv B

2

Review of the Test

The History of Induction

Faraday’s Discovery

Experimental Demonstrations

Motional EMF

Induced Current in a Circuit

Today

Today

Faraday’s Discovery

Faraday’s Discovery

Faraday had wound two coils around the same

iron ring. He was using a current flow in one coil

to produce a magnetic field in the ring, and he

hoped that this field would produce a current in

the other coil. Like all previous attempts to use a

static magnetic field to produce a current, his

attempt failed to generate a current.

However, Faraday noticed something strange.

In the instant when he closed the switch to start

the current flow in the left circuit, the current

meter in the right circuit jumped ever so slightly.

When he broke the circuit by opening the switch,

the meter also jumped, but in the opposite

direction. The effect occurred when the current

was stopping or starting, but not when the

current was steady.

Faraday Investigates

Induction

Faraday Investigates

Induction

Faraday placed one

coil above the other,

without the iron ring.

Again there was a

momentary current

when the switch

opened or closed.

Faraday replaced the

upper coil with a bar

magnet. He found that

there was a momentary

current when the bar

magnet was moved in or

out of the coil.

Was it

necessary to move

the magnet?

Faraday placed the

coil in the field of a

permanent

magnet. He found

that there was a

momentary current

when the coil was

moved.

Conclusion: There is a current in the coil if and

only if the magnetic field passing through the coil is

changing.

The current in a circuit due to a changing magnetic

field is called induced current.

Separating Charge

Separating Charge

top bottom

0 0

l l

y

 V  V  V  E dy   vB dy  vlB

E  vlB

Clicker Question 1

Clicker Question 1

The square conductor moves upward through a

uniform magnetic field that is directed out of the

diagram.

Which of the figures shows the correct distribution of

charges on the conductor?

Example: Potential

Difference along a Rotating

Bar

Example: Potential

Difference along a Rotating

Bar

A metal bar of length l rotates

with angular velocity  about a

pivot at one end. A uniform

magnetic field  is perpendicular to

the plane of rotation.

What is the potential difference

between the ends of the bar?

v   r E  Bv  B  r

tip pivot

0

1 2

2

0 0

( )

l

r

l l

V V V E dr

B  r dr B  rdr B  l

   

   



 

Induced Current in a

Circuit

Induced Current in a

Circuit

The figure shows a conducting wire

sliding with speed v along a U-shaped

conducting rail. The induced emf E will

create a current I around the loop.

E  vlB

vlB

I

R R

E

Activity: Lighting A Bulb

Activity: Lighting A Bulb

The figure shows a circuit

including a 3 V / 1.5 W light bulb

connected by ideal wires with no

resistance. The right wire is pulled

with constant speed v through a

perpendicular 0.10 T magnetic field.

(a) What speed must the wire

have to light the bulb to full

brightness?

(b) What force is needed to keep

the wire moving? (3.0 V)

300 m/s

(0.10 m)(0.10 T)

v

lB

E

(1.5 W)

0.50 A

(3.0 V)

P

I

V

  



(3.0 V)

(0.50 A)

V

R

I



   

2 2 2 2

pull

3

(300 m/s)(0.10 m) (0.10 T)

5.0 10 N

vl B

F

R



Eddy Currents

Eddy Currents

Suppose that a rigid square copper loop is between the poles of a

magnet. If the loop moves, as long as no conductors are in the field

of the magnet there will be no current and no forces. But when one

side of the loop enters the magnetic field, a current flow will be

induced and a force will be produced. Therefore, a force will be

required to pull the loop out of the magnetic field, even though

copper is not a magnetic material.

However, if we cut the loop, there will be no force.

Eddy Currents (3)

Eddy Currents (3)

Now consider a sheet of

conductor pulled through a magnetic

field. There will be induced current,

just as with the wire, but there are

now no well-defined current paths.

As a consequence, two

“whirlpools” of current will circulate

in the conductor. These are called

eddy currents.

A magnetic braking system.