Topic 7 - Magnetism and Electromagnetism Flashcards
Define
magnetic field.
a region where other magnets or magnetic materials experience a force
this is a non-contact force
What is the
force between a magnet and a magnetic material?
always attractive
Where do
compasses point?
(when they’re not near a magnet) and why?
the point north because the Earth generates its own magnetic field
What are
permanent magnets?
magnets that produce their own magnetic field
What are
induced magnets?
magnetic materials that turn into a magnet when they’re put into a magnetic field
when you take away the magnetic field, the induced magnets quickly lose their magnetism and stop producing a magnetic field
What happens when
current flows through a wire?
a magnetic field is created around the wire
this field is made up of concentric circles perpendicular to the wire, with the wire in the centre
What is the
righ-hand rule?
a rule that is used to determine the direction of the field around a wire
your thumb points in the direction of the current and the direction in which you curl your fingers is the direction of the field
How can you
increase the strength of a magnetic field?
(of a wire) and how does this work?
by wrapping the wire into a coil (called a solenoid)
the field lines around each loop of wire line up with each other, resulting in lots of field lines pointing in the same direction that are very close to each other
How can you
increase the field strength of a solenoid?
and how does this work?
add a block of iron in the centre of the coil
this becomes an induced magnet whenever current is flowing
What is an
electromagnet?
a solenoid with an iron core
Why are
electromagnets very useful?
they’re very quick to turn on and off so can create a varying force
What are some
uses of electromagnets?
- cranes to attract and pick up things made from magnetic materials (and then drop it)
- within circuits as switches
What is the
motor effect?
and how does this work?
the force that is exerted on a current-carrying wire that is put between magnetic poles
the magnetic field around the wire interacts with the magnetic field it has been placed in
How can you
increase the amount of force produced by the motor effect?
(4)
- put the wire at 90° to the magnetic field
- increase the strength of the magnetic field
- increase the amount of current passing through the wire
- increase the length of the wire that’s in the field
What equation allows you to calculate the
force created by the motor effect?
force = magnetic flux density x current x length of wire in the field
F = BIl
F: newtons, N
B: tesla, T
I: amps, A
l: metres, m
What is
fleming’s left-hand rule?
a rule that is used to determine direction in which the force produced by the motor effect is going
your first finger points in the direction of the field
you second finger points in the direction of the current
your thumb will then point in the direction of the force
How do
loudspeakers work?
(4 steps)
- An alternating current is sent through a coil of wire attached to the base of a paper cone.
- The coil surrounds one pole of a permanent magnet and is surrounded by the other pole, so the cuttent causes a force on the coil (which causes the cone to move).
- When the current reverses, the force acts in the opposit direction, which causes the cone to move in the opposite direction too.
- So variations in the current make the cone vibrate, which makes the air around the cone vibrate and creates variations in pressure that cause a sound wave.
the frequency of the sound wave is the same as the frequency of the alternating current, so by controlling the frequency you can alter the sound wave produced
Define
generator effect.
the induction of a potential difference in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field
How can you
change the size of the induced potential difference?
- increase the speed of the movement - cutting more magnetic field lines in a given time
- increase the strength of the magnetic field (so there are more field lines that can be cut)
How do
alternators generate an alternating current?
- They rotate a coil in a magnetic field.
- As the coil spins, a current is induced in the coil. This current changes direction every half turn.
- Instead of a split-ring commutator, ac generators have slip rings and brushes so the contacts don’t swap every half turn.
How do
dynamos generate direct current?
- They work in the same way as alternators but they have a split-ring commutator instead of slip rings.
- This swaps the connection every half turn to keep the current flowing in the same direction.
How do
microphones generate current from sound waves?
- Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
- This causes the coil of wire to move in the magnetic field, which generates a current.
- The movement of the coil depends on the properties of the sound wave.
What do
transformers do?
they change the size of the potential difference of an alternating current
How do
transformers work?
- When alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly.
- This induces an alternating potential difference in the secondary coil.
- If the second col is part of a complete circuit, this causes a current to be induced.
The ratio between primary and secondary potential differences is the same as the ratio between teh number of turns on the primary and secondary coils.
Why does the
national grid transmit at very high potential differences?
a low current means that less energy is being wasted heating the wires and the surroundings, for a given power, a high pd is needed for a low current, this makes the national grid an efficient way of transmitting power
