P15 Electromagnetism Flashcards
which poles attract and which poles repel?
like poles repel
unlike poles attract
what is a magnetic field?
the region arround a magnet where the force of magnetism acts
what can be used to show a magnetic field?
iron filings
plotting compasses
what is a magnetic field line?
line in a magnetic field along which a magnetic compass points
what are the magnetic field lines of a bar magnet like?
they curve around from the north to the south pole
what is induced magnetism?
magnetism created in an unmagnetised magnetic material when the material is placed in a magnetic field
why is steel used t make permanent magnets?
steel does not lose its magnetism easily
what are the magnetic field lines around a wire like?
circles centred on the wire in a plane perpendicular to the wire
what is a solenoid?
a long coil of insulated wire
what are the magnetic field lines like in a solenoid?
parallel to its axis and are in the same direction
what are the magnetic field lines like on the outside of a solenoid?
bend around from the north to the south pole
what is an electromagnet?
a solenoid that has an iron core
how do you know the direction of the magnetic field of a coil of wire?
use the right hand grip rule
how do you know the north and south pole of a solenoid?
look at the direction the current flows in around the coil
think of an S and an N and put arrows on the ends of the letters
if the current is flowing in the same direction as either of the letters, the letter tells you which pole it is
what can electromagnets be used for?
scrapyard cranes
circuit breakers
electric bells
relays
how do electromagnets work in relays, bells or a circuit breaker?
attracting an iron armature
what is the motor effect?
when a current is passed along a wire in a magnetic field, and the wire is not parallel to the line of the magnetic field, a force is exerted by the magnetic field
how can the size of the force exerted by the motor effect be increased?
- increasing current
- using a stronger magnet
how can the direction of the force exerted by the motor effect be determined?
using fleming’s left hand rule
what is magnetic flux density?
a measure of the strength of a magnetic field
what is a split-ring commutator?
metal contacts on the coil of a direct current motor that connects the rotating coil continuously to its electrical power supply
it reverses the current in order to reverse the direction of force to keep the motor spining
what is the generator effect?
the effect of inducing a potential difference using a magnetic field
what happens when a conductor crosses the lines of a magnetic field?
a potential difference is induced across the ends of the conductor
what increases the size of the potential difference produced by a generator?
- the speed the conductor crosses through the lines of a magnetic field
- the strength of the magnet
what is a key fact about the direction of an induced current?
it always opposed the original change that caused it
what is an alternator?
an alternating current generator
what is a d.c generator?
dynamo
what is the difference between an alternator and a dynamo structurally?
a dynamo has a split ring commutator instead of two slip rings
what is a transformer?
electrical device used to change an alternating voltage
what is a step-up transformer?
a transformer used to increase the size of an a.c. p.d.
what is a step-down transformer?
an electrical device used to decrease the size of an a.c. p.d.
how does a transformer work?
- an alternating current passing through the primary coil generates an alternating magnetic field in the iron core
- the lines of the magnetic field pass through the secondary coil
- the magnetic field in the secondary coil induces an alternating p.d. between the terminals of the secondary coil
why do transformers not work with d.c?
there is no changing of magnetic field so the secondary p.d. is zero
what are the structures of a transformer?
primary coil
secondary coil
iron core
why is an iron core used in a transformer?
iron is easily magnetised and demagnetised
how do the number of turns of the primary coil compare to the secondary coil in a step down transformer?
less in the secondary than the primary
how do the number of turns of the primary coil compare to the secondary coil in a step down transformer?
less in the secondary than the primary
how do the number of turns of the primary coil compare to the secondary coil in a step up transformer?
more in the secondary than the primary
why is the p.d. stepped up?
it reduced the current needed so reduced power loss and makes it more efficient
