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Electromagnetic Induction

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Electromagnetic Induction

Practical and predict direction of the Iinduced

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• Flemings right hand generator rule:

• First finger (Index) point to the direction of magnetic Field

• Thumb is the direction that the conductor is moving

• SeCond (Middle) finger=direction of the induction Current flow

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The Fleming’s Right Hand Rule

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• Are there any readings on Galvanometer if we move the metal bar in the direction which is parallel to the B lines?

No!

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Bav

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B

a

b

v

I

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E = Blv sinθ

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An airplane with a wing span of 30.0 m flies parallel to the Earth’s surface at a location where the downward component of the Earth’s magnetic field is 0.60 ×10-4 T. Find the difference in potential between the wing tips is the speed of the plane is 250m/s。

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Any other methods can create Iinducted?

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Switch on/off

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Electromagnetic induction

When a magnet is pushed into a coil of wire an electric current is produced in the wire. We say that a current was induced in the wire.

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EM Induction explained•The coils of wire are full of ‘free’ electrons randomly moving from atom to atom.

•When the magnetic field of the magnet moves past the coils of wire the magnetic field exerts a force on the ‘free’ electrons.

•We call this force an EMF ‘Electro Motive Force’.

•The force on the electrons causes them to move through the wire, this is an electric current.

•The changing magnetic field induces an electric voltage across the circuit.

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Moving up and down

When we change the direction of the magnet we are changing the direction of the force on the ‘free’ electrons in the wire.

The electric current is induced in the opposite direction.

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Electromagnetic induction

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Stationary magnet = no current

•When the magnet is stationary there is no electric current.

•This is common sense we cannot generate electrical energy without some energy going into the system.

•Without the kinetic energy of the moving magnet the electrical energy of the current will not be created.

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Standard equipmentA suitable meter, either an a.c. meter or a galvanometer,

•A coil,

•Two connecting wires,

•A magnet.

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Factor’s affecting induction

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Increasing the speed

•Moving a magnet in and out at a faster speed will increase the voltage.

•The overall electrical energy from each movement will be the same but there will be more movements per second and hence more power. We will need to put in more kinetic energy every second.

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Increasing the magnetism

•If we increase the strength of the magnet passing into the coil there will be a more voltage.

•However, if this makes the magnet heavier we will need to provide more kinetic energy.

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Increasing the coils

•If we increase the number or area of the coils there will be more voltage.

•With more coils of wire we will have more ‘free’ electrons, there will be a greater current; but a longer wire also means more resistance – and something called Lenz’s law (not in examination).

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Increasing the voltage

•There are three mains ways to increase the size of the induced electric voltage.

• Use a stronger magnet,

• Use more coils of wire (sometimes called turns),

• Move the magnet quicker.

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What’s the conditions for creating induced current /e.m.f. ?

Change of magnetic field in the loop

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Lenz’s law

• The direction of the induced e.m.f. is such as to cause effects to oppose the change producing it.

• The INDUCED current creates an INDUCED magnetic field of its own inside the conductor that opposes change of the original magnetic flux density.

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1/Direction of the original magnetic flux density

1/φo point to downwards

2/Change of the original magnetic flux density

2/φo increased , △φo>0

3/Direction of the INDUCED magnetic field

3/BInduced point to upwards

4/By using right hand grip rule, find out Iinduced.

4/IInduced Anti-clockwiseIInduced

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IInducedIInducedIInduced

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A B

CD

BAny magnetic force on the coil?

Iinduced

F

A B

CD

BIinduced

F

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• the current is always induced in such a direction that the magnetic force it produces opposes the relative motion between the conductor and the magnetic field.

Magnitude of E?

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b

R

× × × ×

× × × ×

× × × ×

× × × ×

× × × ×

B

a mL

Metal bar ab keep in touching with the P and Q, Mass of ab is m ， length of it is L ， there is a resistor R in the circuit ， the resistance in the wire is negligible ,the length of the P and Q is infinite , ab fall down in a uniform magnetic field, find out its maximum speed.

P Q

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Electromagnetic Induction

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I1I2 I1

I2

•An e.m.f. and a current induced in a circuit by changing magnetic flux

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Induction cooker

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Applications• Moving coil microphone

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A microphone works when sound waves enter the filter of a microphone. Inside the filter, a diaphragm is vibrated by the sound waves which in turn moves a coil of wire wrapped around a magnet. The movement of the wire in the magnetic field induces a current in the wire. Thus sound waves can be turned into electronic signals and then amplified through a speaker.

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Applications• Electric Guitar Pickups

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AC and DC

A quick review

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A.C. and D.C.

•Direct current is present in one direction. The charge flows in one direction only, e.g. a battery.

•Alternating current is present in both directions. Charge flows first in one direction and then in the opposite direction. It has a typical frequency of 50 Hz (UK) or 60 Hz (Bermuda and the USA).

•There are advantages and disadvantages of both types of electrical power.

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A.C. and D.C.

•Batteries always supply direct current.

•Mains electricity is always supplied as alternating current.

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Alternating Current

•An electric current is called an alternating current if the charge flows in one direction then changes to flow in the opposite direction.

•When looking at a diagram it is important to see if the line goes both above and below the zero line.

Forwards

Zero line

Backwards

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0 Volts

+

-

a.c. sine wave

When electricity is generated in power stations it takes the form of a sine wave. Many electrical signals look like a sine wave.House current is 120 volts, 60 Hz in the US and Bermuda.House current is 240 volts, 50 Hz in the UK.

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0 Volts

+

-

Is this a.c. or d.c.

Although this electrical signal has a sine wave shape it is still a d.c. signal.

The wave increases and decreases but never reverses direction. Think about a car moving along speeding up and then slowing down.

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Generators

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The dynamo

A dynamo has a rotating magnet inside a coil.

Simple examples can be found in old bicycle lamps.

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Generator

•Rather than move a magnet inside of a coil we can turn the situation around.

•This time we will move a coil inside of a magnetic field.

•We call this arrangement a generator.

•Most generators produce a.c. electricity consequently they are sometimes called alternators.

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a.c. generator

Moving a coil in a circle between two magnets produces an induced electric current in the coil.

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Overall picture

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One circle = one wave

When the coil is turned through one complete circle one complete sine wave of alternating current is produced.

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Generator animation

coil

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Generator

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Generators

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Generator theory

•As the coil turns it cuts across the magnetic field,

•An EMF (Electro-Motive Force) pushes free electrons along the wire.

•This is an induced electric current.

•An a.c. is transmitted through the brushes to the rest of the circuit.

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Generation by hand

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Generators

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Power Production

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Transformers

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Applications

• My never dying flashlight

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What are the four ways in which the (induced) current from an AC generator can be increased?

1. _________________________

2. _________________________

3. _________________________

4. _________________________

faster movement

stronger magnetic field

more coils

larger area of coils

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Summary - Induction

•When a conductor moves through a permanent magnetic field a voltage is induced in the conductor.

•When a magnet spins inside a coil of wire the magnetic field at any place in the coil is changing. The changing magnetic field induces a current in the wire. coil.

•Induced voltage can be increased by using a stronger magnetic field, using more coils of wire or by moving the generator faster.