Electromagnetism

Magnets reminder

• Opposite poles ___________• Like poles _____________• The north pole of a magnet is attracted to

Earth’s _______ _________.• A magnetic material is attracted to

_______s.• Magnetic materials include: ____, ______,

and _______. Steel is also magnetic, because it contains _______.

Electromagnetism

Electromagnets reminder.

• Whenever a current flows through a wire there will be an _______ _______.

• We can create a strong magnetic field by looping the wire into ______s.

• This can be made even stronger by adding a ____ _____ core.

• Increasing the number of coils ________ the strength of the magnetic ______.

• Increasing the current also increases the ______ of the field.

Electromagnets reminder.

• Whenever a current flows through a wire there will be an electromagnetic field.

• We can create a strong magnetic field by looping the wire into coils.

• This can be made even stronger by adding a soft iron core.

• Increasing the number of coils increases the strength of the magnetic field.

• Increasing the current also increases the strength of the field.

Rules for good field diagrams.

• Include at least 3 lines above the magnet/solenoid, and 3 below.

• Show three lines leaving the North Pole, and three lines arriving at the South Pole.

• Mark direction arrows on to every line, showing the field going from North to South.

• Show lines closer together where the field is strongest, and further apart where the field is weakest.

Uses for Electromagnets

• An electromagnet does all the things that ordinary magnets can do, but you can switch them on and off.

• An electric bell – uses an electromagnet to rapidly pull the hammer over to the gong then release it.

• For sorting scrap – an electromagnet can be used to pick up and put down magnetic materials, sorting them from non-magnetic scrap.

• In speakers – an electromagnet is used to move a cone very rapidly, causing sound waves.

• In switches – a small current can be used to operate an electromagnet, which in turn can control another circuit in which a much larger current might be flowing. This isolates the large current from the person operating the switch, making it safer.

Uses for Electromagnets

• Diagram of an electric bell

Uses for Electromagnets

• An electromagnet being used to pick up scrap

Uses for Electromagnets

• Relays are used in circuit control.

The Motor Effect

• To increase this force:– Increase the current– Increase the number of coils – Increase the strength of the

magnet – Increase the length of

conductor in the field

• To reverse this force:– Reverse the direction of the

current – Reverse the direction of the

(permanent) magnetic field

“A conductor carrying an electric current may experience a force

when placed into a magnetic field.”

NOTE: There is NO FORCE if the conductor is parallel to the field.

Keep the field the same

Reverse the field

Field

Current

Motion

Motion reverses

Reverse the current

Motion reverses

Keep the current the same

escience P3 3.1 The motor effect animation could be shown here.

Electromagnetic Induction

• A potential difference is induced across the ends of a conductor when it cuts across magnetic field lines. This is called Electromagnetic Induction.

• The same effect occurs if the conductor is held still and the magnetic field changes.

• The faster the conductor cuts the field lines (or the faster the magnetic field changes) the bigger the p.d. induced.

A simple dynamo• If the conductor

forms part of a circuit, a current will flow.

• In a dynamo, a coil is rotated inside a magnetic field, causing an alternating current to flow.

A simple dynamo• You can use the

right hand rule to prove to yourself that a current will flow all the way around the coil of wire when the coil is rotated.

A simple dynamo• The slip rings and

bushes allow the coil to be turned whilst still keeping a constant connection to the voltmeter in this case, or into a circuit.

Transformers

• This sort… not that sort!

Transformers

• A coil of wire is wound on to one side of a soft iron core. This coil is called the primary coil.

• When an alternating current flows through this wire, an alternating electromagnetic field is set up in the core.

Transformers

• If a secondary coil is then wound on to the other side of the core, this changing magnetic field will induce an alternating p.d. across the ends of the secondary coil.

Transformers

• Transformers step voltage up or down. The size of the induced voltage is given by the ratio:

s

p

s

p

N

N

V

V

or

secondaryon turnsofnumber

primaryon turnsofnumber

secondary across p.d.

primary across p.d.

Transformers and Mains supply

• Electricity is generated at the power station at about 33,000V.

• A step-up transformer steps this up to about 400,000V for transmission in overhead cables.

• This is then stepped down for use in homes, to 230V (or for industrial uses, to 11,000V).

• WHY?

Transformers and Mains supply

• When the p.d. is stepped up, the current is stepped down.

• So there is a lower current flowing through the wires.

• This means that less energy is lost to heat (P=I2R).

• So more of the power supply’s energy gets to the appliance, rather than being lost in the wires.