Download Electromagnetism leaflet

ELECTRIC GENERATORS
Electromagnetism
1.When a wire is moved (perpendicular) through a
magnetic field it produces voltage. If the wire is a
conductor a current will flow through it.
HISTORY
2.The direction of the current can be worked out using
Fleming’s right hand rule.
1.1819- Danish physicist Hans Christian Oersted
found that when electricity flows through a wire it
deflects a magnetic needle
ELECTRIC TRANSFORMERS
2.1831- English scientist Michael Faraday found that
placing a magnet near a wire induces a magnetic current
3.1832- Joseph Henry made the same discovery
4.1856- English physicist James Clerk Maxwell
predicted electromagnetic waves
The magnetic field around a solenoid
A wire which encounters a changing magnetic field gains
an induced current if part of a complete circuit. The wire
does not have to move. Instead the wire can be
stationary and the magnetic field varied in strength or
direction. The second magnetic field can be controlled
by changing the current.
A transformer has two coils of wire (primary and
secondary) wound around a core of magnetic material.
The two wires are separate and only linked by the
common core.
5.1925- Goudsmit and Uhlenbeck showed that
electrons have spin and behave like little bar magnets
Electromagnets
The current in the first coil differs from that in the
second depending on the n umber of coils
A solenoid (coil of wire) with an iron core inside
it.
SELF INDUCTION
The magnetic field around a wire
A solenoid
When a current in a wire varies it produces a
changing magnetic field. The changing magnetic
field induces an emf across the wire. The induced
emf opposes the change that causes it.
This happens because when electricity flows
through the solenoid it produces a magnetic effect,
which lines up the domains in the iron core. As the
current in the wire is increased the magnetic field
becomes stronger up to a saturation point (when all
domains have been lined up). When the current is
switched off the Iron retains a weak residual
magnetism
The domains in an electromagnet
Electromagnets can be made stronger
by:
1.
Making more coils in the wire
2.
Turing up the current flowing through the
wire
3.
Adding an Iron core (or any magnetic
metal)
Electric Bell
1. Pressing the switch completes the circuit and
the Iron striker is attracted to the
electromagnet and strikes the bell
2. As the striker moves towards the bell, the
contact is broken, breaking the circuit.
Electricity stops flowing through the coil which
loses its magnetism.
3. The spring returns the striker to its original
position, which makes a new contact and so
electricity flows again
RELAYS
Back to number 1 and the cycle repeats
itself.
The bell will continue to ring as long as the
switch is held closed.
Relays are used in car ignitions so that, for
safety reasons, the car can be started with a
small voltage, which controls a much larger
circuit
USES
1. Relays
2. Electric bells
3. Circuit- breakers
4. Electromagnetic clutches and brakes
5. Scrap- yards
6. Cyclotrons (particle accelerators)