TRANSFORMERS and elctromagnets

two coils of INSULATED wire - primary coil, secondary coil

coils both wound round same iron core

when an alternating current pases through the primary coil, it produces an alternating magnetic field in the CORE.  this field continually expands and collapses. 

the alternating field lines pass through the secondary coil and induce an alternating potential differenceacross its ends.  if the secondary coil is part of a complete circuit, an alternating current is produced.

the coils of wire are insulated so: current does not short across either the iron core or adjacent turns of wire, but flows around the whole core.  core is iron because it is easily magnitised.

transformers are used in the national grid

STEP-UP: makes the pd across secondary coil greater than primary's.  more turns of wire on 2 than 1. step-up the pd from power stations, because higher pd transmitting electrical energy across the grid means less energy is wasted in the cables.

STEP-DOWN: makes the pd across secondary coil LESS than primary's.  fewer turns of wire on 1 than 2.  reduce the pd so that it is safe to use in homes/ by consumers. 

SWITCH MODE TRANSFORMER: has a ferrite core. 

• operates at much higher frequency
• lighter and smaller
• uses very little power when there is no device connected across its output terminals

Vp/ Vs = Np/ Ns

V = pd across (p or s) coil

N = number of turns on (p or s) coil

100% efficient:

Vp * Ip = Vs * Is

ends of a magnet = magnetic poles north and south

the region around a magnet in which a peice of iron or steel will be attracted to it (mag materials will experience a force on them) = magnetic field

lines of force = magnetic field lines

if two magnets are brought close to each other....

• opposite poles ATTRACT
• like poles REPEL

when a current flows through a wire, a magnetic field is produced around the wire. 

an elctromagnet is made by: wrapping insulated wire around a piece of iron (core). 

when a current flows through the wire the iron becomes strongly magnetised - when the current is switched off, the iron loses its magnetism.  TEMPORARY MAGNETISM MAKES ELECTROMAGS V USEFUL. 

EGS:

• scrapeyard cranes
• circuit breakers
• electric bells
• relays

when we place a wire carrying an electric current in a magnetic field, it may experience a force - MOTOR EFFECT.

the force is MAX if the wire is at a 90 angle to the mag field, and is ZERO if the wire is parallel to the magnetic field.

flemming left hand rule:

(all held at right angles to each other)

• First finger = mag Field (N to S)
• seCond finger = Current (+ to -)
• thuMb = Motion (direction of force)

the size of the force can be increased by:

• increasing stength of mag field
• increasing size of current

speed of motor increased by increasing size of current

direction of motor reversed by reversing direction of current

when a current passes through the coil, the coil spins because:

• the force acts on each side of the coil due to motor effect
• the force on each side of the coil is in the opposite direction to the force on the other side

split-ring commutator reverses the direction of the current around the coil every half-turn.  because the sides swap over each half-turn, the coil is always pushed in the same direction. 

electromagnetic induction: is the process of creating a pd using a magnetic field

if an electrical conductor 'cuts' through magnetic field lines a PD IS INDUCED ACROSS THE ENDS OF THE CONDUCTOR. 

if a magnet is moved into a coil of wire a pd is induced across the ends of the coil (conductor).  if the wire or coil is part of a complete circuit, a current passes through it. 

direction of the induced pd is reversed:

• the direction of movement of conductor is reversed (coil or wire)
• polarity of the magnet is reversed

a pd is only induced while there is MOVEMENT

size of the pd is increased by increasing:

• the speed of the movement
• the strength of the magnetic field
• the number of turns on the coil