Electromagnetic Induction

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# Electromagnetic Induction - PowerPoint PPT Presentation

Electromagnetic Induction. WS 19.1. What have we learnt?. A current carrying conductor in a magnetic field experiences a force. From EN 19.1: Motion of an electrical conductor in a magnetic field generates e.m.f in the conductor. For those who are interested….

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### Electromagnetic Induction

WS 19.1

What have we learnt?
• A current carrying conductor in a magnetic field experiences a force.
• From EN 19.1:Motion of an electrical conductor in a magnetic field generates e.m.fin the conductor
For those who are interested…

Why will a conductor moving in a magnetic field cause e.m.f to be generated in the conductor?

(go to s3phytanwl.pbworks.com to find additional notes. Discuss online!)

http://s3phytanwl.pbworks.com/w/page/55762133/Force%20to%20Induction

Faraday discovered that when a coil (or any electrical conductor)

experiences a change in magnetic field strength (either the magnet moving or the conductor moving)

e.m.f. is induced in the conductor.

For the induced e.m.f….

Magnitude

Direction

Magnitude of e.m.f. induced

The magnitude of e.m.f. induced depends on:

The rate of change of magnetic field

• How fast the magnet is moving (or how fast magnetic field is changing)
• How fast the conductor is moving
Magnitude of e.m.f. induced

The e.m.f. induced in a (closed loop) conductor is proportional to the rate of change of magnetic field experienced within the loop.

Consider the loop below: Is there e.m.f. induced?

Before

After

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Is there e.m.f. induced?

Before

After

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ● ● ●

● ● ● ● ●

● ● ● ● ●

● ● ● ● ●

● ● ● ● ●

● ● ● ● ●

● ● ● ● ●

Is there e.m.f. induced?

Pushing magnet near solenoid

S N

Is there e.m.f. induced?

A metal ring is moving to the right in a uniform magnetic field.

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Magnitude of e.m.f. induced

The e.m.f. induced in a (straight ) conductor is proportional to the rate at which the conductor cuts the magnetic field lines.

Is there e.m.f. induced?

A straight conductor is moving to the right.

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Fixed set of U-shaped wires.

(Not a closed loop)

Is there e.m.f. induced?

(Magnetic field lines pointing to the right.

A straight conductor is moving to the left.

Is there e.m.f. induced?

Wire moving upwards

Top view of a straight wire

N

S

Direction of e.m.f. induced

The conductor, when experiencing a changing magnetic field, will induce an e.m.f.

such that if current flows, the magnetic field thus produced willoppose the change

in magnetic field which was experienced.

Direction of e.m.f. induced

Lenz’s Law

The direction of the induced e.m.f., and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.

Direction of e.m.f. induced

A closed loop experiencing a change in magnetic field,

The e.m.f. will be induced in a direction which will compensate for the change in the magnetic field strength or to oppose the motion which causes it.

Closed loop conductor

Increasing north pole

Increasing south pole

S N

N S

Closed loop conductor

Decreasing north pole

Decreasing south pole

S N

N S

Direction of e.m.f. induced

A straight conductor moving in a uniform magnetic field,

The e.m.f. will be induced in a direction which will cause a force to be exerted on the conductor in the opposite direction to its motion

Straight conductor

A straight conductor is moving to the right.

How will direction of induced current flow?

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Fixed set of U-shaped wires.

(Not a closed loop)

Argument (1)
• In order to oppose the motion of the metal rod, the magnetic field will want to induce a force acting on the conductor towards the left.
• If the magnetic field want the force to be to the left, then the direction of the induced current will be upwards, using Fleming’s Left Hand Rule.
Argument (2)
• If you can live with this idea of opposing motion, then Fleming’s Left Hand Rule is good enough for you.
• If you find this confusing, I will introduce to you: Fleming’s Right Hand Rule
Fleming’s Right Hand Rule
• For induced current

Motion

B-field

Induced current

What is direction of induced current?

Wire moving upwards

Wire moving diagonally

Top view of a straight wire

Top view of a straight wire

N

S

S

N

What is direction of induced current?

Before

After

Top view of rectangular coil

Top view of rectangular coil

N

S

S

N

Comparison

Fleming’s Left Hand Rule

Fleming’s Right Hand Rule

EM Induction

No current supplied to the conductor. E.m.f. is induced (produced) in this conductor, then supplied to external circuit (if any)

Conductor is forced to move by another agent.

Motor Effect

• Current is supplied to the conductor through an external circuit.
• Force is produced, which causes motion.
EM Induction (RH) or Motor Effect (LH)?

A metal rod connected to a circuit starts accelerating upwards when placed in the magnetic field. What is the direction of the current flowing in the conductor?

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Connected to battery

EM Induction (RH) or Motor Effect (LH)?

A metal rod is being pushed to the right at a constant rate. What is the direction of current flowing in the metal rod?

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

× × × × × × ×

Fixed set of U-shaped wires.

(Not a closed loop)

EM Induction (RH) or Motor Effect (LH)?

A metal rod when rolling into the magnetic field is found to decelerate to a stop. Explain why.

× × × × × × × × ×

× × × × × × × × ×

× × × × × × × × ×

× × × × × × × × ×

× × × × × × × × ×

× × × × × × × × ×

Fixed set of U-shaped wires.

(Not a closed loop)

EM Induction (RH) or Motor Effect (LH)?

The current carrying coil experiences a net clockwise moment when placed in the magnetic field and rotates clockwise. Find the direction of current flowing in the coil.

Top view of rectangular coil

N

S

EM Induction (RH) or Motor Effect (LH)?

I turn a generator wheel to rotate the coil in a clockwise direction. What is the direction of the induced current?

Top view of rectangular coil

N

S

EM Induction (RH) or Motor Effect (LH)?

The wind turbines attached to the coil to rotates and generates current in the direction as shown. In what direction is the coil rotating?

Top view of rectangular coil

N

S

Summary
• Two versions of Faraday’s Law: (1) For looped conductor; (2) For straight conductor.
• When the conductor is connected to a closed circuit, the e.m.f. induced will cause current to flow.
• The direction of current flow can be determined by using both Fleming’s Left Hand Rule or Right Hand Grip rule in a different manner or by using Fleming’s Right Hand Rule.