2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet

1820 - Oersted - Electric current produces a magnetic field

1821 - Faraday - current carrying conductor in a magnetic field experiences a force

1831 - Faraday - electric current or emf can be generated through the use of a magnetic field

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet

1831 - Faraday - electric current or emf can be generated through the use of a magnetic field

Faraday wound coils of wire around wood and attached a battery (PRIMARY COIL)

Then he wound another circuit of coils which was attached to a galvanometer (SECONDARY COIL)

G

Faraday wanted to see if the magnetic field from one coil would set up a current in another.

He noticed that a temporary current registered when the battery was switched on or off but no current registered while the battery was left on

When he placed a needle in a secondary coil, he found that the needle became magnetised

His experiment with coils on opposite sides of an iron ring produce even more

noticeable currents when switching off and on

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WHEN THE MAGNETIC FIELD OF THE PRIMARY

COIL IS CHANGING, A CURRENT IS INDUCED IN

THE SECONDARY

M.Edwards 6/5/02

M.Edwards 6/5/02

Perform an investigation to model the generation of an electric

current by moving a magnet in a coil or a coil near a magnet

Outline Michael Faraday‘s discovery of the generation of an electric current by a moving magnet

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Jacaranda Experiments 7.1 & 7.2

Define magnetic field strength B as magnetic flux density

AB

BA

Magnetic flux, is the amount of magnetic field that

passes through an area

M.Edwards 23/5/02

Magnetic field strength, B, is the amount of flux per unit area, or ‘magnetic flux density’

M.Edwards 23/5/02

So if the same number of field lines pass through a smaller area, the magnetic field is stronger; like this:

Magnetic flux is measured in Webers (Wb)

‘Magnetic field strength’ or ‘magnetic flux density’ is measured in Tesla (T)

Describe the concept of magnetic flux in terms of magnetic flux density and

surface area

Describe generated potential difference as the rate of change of magnetic flux through a circuit

Review Text P.137-138

Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify the effect on a generated electric current when:- the distance between the coil and magnet is varied- the strength of the magnet is varied- the relative motion between the coil and the magnet is varied

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A student wanted to determine what factors would influence the current generated when a permanent magnet was moved inside a coil of wire as shown in the diagram below.

(a) Outline an experimental procedure that the student could use to determine what factors influence the induced current.

(b) Design a table that clearly shows and includes a quantitative sample of expected results for the experiment in part (a).

(a) 4 marks

Using a coil of 50 turns, arrange the magnet so that the north pole is in the coil and

the south pole is out of the coil. Then pull the magnet out quickly and record any

value of current. This is the control.

Repeat the experiment but pull the magnet out slowly.

Repeat the experiment but change the orientation of the magnet so that the south pole

is in the coil.

Repeat the experiment but use two magnets (with their north poles together) instead

of one.

Repeat the experiment but increase the number of turns in the coil to 100.

(b) 3 marks

speed of

extraction

pole

in coil

# magnets # turns Current

(A)

quick N 1 50 0.3

slow N 1 50 0.1

quick S 1 50 -0.3

quick N 2 50 0.5

quick N 1 100 0.5

Gather, analyse and present information to explain how induction is used in cooktops in electric ranges

Induction heating uses changing magnetic fields to induce eddy currents which cause an increase in temperature of the conductor (e.g. saucepan), due to collisions between moving electrons and atoms, and agitation of the atoms due to rapidly changing B field.

The eddy currents cause the saucepan to heat up directly (which then cooks the food), without the loss of energy to the air that occurs with gas heating. Induction cookers are 80% efficient, compared to 43% for gas.

The induction coils and the saucepan are separated by a ceramic top plate

The diagram below shows an induction cooktop.

(a) Explain how the coil induces a current in a saucepan placed on the cooktop.(b) Explain how this would reduce fire hazards in the kitchen.

2 (a) 2 marks

An alternating current through the coil of wire would generate a constantly changing

magnetic flux. When a large metal sheet such as a saucepan is placed on the cooktop,

eddy currents are produced which will cause the saucepan to heat up.

(b) 1 mark

Eddy currents will only be produced in conductors. Therefore organic material which

would burn on a conventional cooktop would not burn on an induction cooktop

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

This means that if a North pole of a magnet is brought near a solenoid, current will flowin the solenoid to produce a North pole that opposes the approaching North pole.

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I I I I

Or if a conducting ring is moving into or out of a field, a current will be set up in the ring that produces a magnetic field that opposes the change in flux.

e.g. this ring has a decreasing number of field lines into the page as it moves right. So the current will be clockwise to produce more field into the page.

current will stop when the ring has completely left the field

Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf in motors

Lenz’s Law must be true in terms of conservation of energy, otherwise if a change in flux produced a flux in the SAME direction as the original change, this would produce an even greater change in that direction, producing an even GREATER change etc. etc.... The current would keep increasing as this happened, thereby CREATING ENERGY WITHOUT DOING ANY WORK......

This clearly contradicts CONSERVATION OF ENERGY:Energy cannot be created nor destroyed, but it can be transformed from one form to another

To create electrical energy, work must be done - e.g. moving the magnet or the ring from the examples on the previous page

A rotating coil in a motor also sets up a current that opposes the change in flux. This will be in the opposite direction to the supply emf, so is called back emf. The back emf increases as the motor armature speed increases until the back emf equals the supply emf. Then the armature rotates at a constant rate.

Explain that, in electric motors, back emf opposes the supply emf

Explain the production of eddy currents in terms of Lenz’s Law

The magnetic fields set up by eddy currents oppose the changein flux that is occurring in that region of the conductor.

For example if an aluminium disc is moving to the left out of a B fieldwhich is directed into the page.....

What change is happening to the flux through the disc?

A. The amount of magnetic flux into the page (through the disc) is DECREASING

So how will the disc OPPOSE this change?

A. A current will be induced that produces amagnetic field into the page!

Which way will it go? Which way will it go?USE YOUR RIGHT HAND GRIP RULE!!! POINT THE FIELD INTO THE PAGE!!

Which way will it go? Which way will it go?

CLOCKWISE!

Gather secondary information to identify how eddy currents have been utilised in switching devices and electromagnetic braking

Review Text P.141

“GATHER”

The following diagram represents a disk spinning in a magnetic field.

Explain what will happen to the disk and how this may be overcome.

3 marks

Because the disk is spinning electrons in the metal are flowing. These are moving

charged particles in a magnetic field so they will experience a force. Therefore they will

move and other electrons will take their spot resulting in a current cycle. These cycles are

called eddy currents and multiple eddy currents will be set up throughout the disk. Now

because there is a current flowing in the disk this will induce a force on the disk slowing

it down. The eddy currents may be overcome by cutting slits in the disk so that the

electrons have nowhere to flow.