p7 magnetism and electromagnetism Flashcards
what are the poles of a magnet?
where the magnetic forces are the strongest
what happens when two magnets are bought together?
they exert a force on eachother.
two like poles repel, whereas two opposite poles attract
what is a permanent magnet?
a magnet that produces its own magnetic field
what is an induced magnet?
a material that becomes a magnet when placed in a magnetic field
what happens to an induced magnet when it is taken out of a magnet field?
as an induced magnet always causes a force of attraction, when a magnet is taken out of a magnetic field, it loses its magnetism very quickly
what are examples of magnetic materials?
nickle, iron and colbalt
what do magnets produce?
- description
magnetic fields.
- the lines always go from north to south and they show which way a force would act on a north pole if it was put at that point in a field
what do close lines mean in a magnetic field?
- where is a magnetic field strongest at?
that the magnetic field is stronger.
- at the poles
what do compasses do?
they show the direction of a magnetic field
how does a compass work?
1) inside a compass is a tiny magnet needle.
2) the north of this magnet will be attracted to the south pole of any other magnet near it.
3) so the compass needle will point in the direction of the magnetic field.
how can you plot a magnetic field pattern of a magnet using a compass?
by moving a compass around a magnet and trace the needles position on some paper to build up a picture of what the magnetic field looks like
how is the behaviour of a compass related to the fact that the earth’s core may be magnetic?
because when they are not near a magnet, compasses always face north.
this is because the earth generates its own magnet field, which shows that the core of the earth must be magnetic
what creates a magnetic field?
a moving charge (current)
explain how current creates a magnetic field?
1) when a current flows through a wire, a magnetic field is created.
2) the field is made of concentric circles perpendicular to the wire.
3) you can see this by placing a compass near a wire that is carrying current. as you move the compass, it will trace the direction of the magnetic field
this can be described using the right hand rule
what is the right hand rule?
using your thumb, point in the direction of the current and curl your fingers.
- the direction of your fingers is the direction of the field
how can you change the direction of a magnetic field?
by changing the direction of the current
what does the strength of the magnetic field depend on?
changes with the current and the distance from the wire.
- the larger the current through the wire, or the closer you are, the stronger the field is
what is a solenoid?
an electromagnet IF it has an iron core. coil of wire
what does a solenoid do?
it increases the strength of a magnetic field of a wire
how does a solenoid increase the strength of a magnetic field of a wire?
1) the field lines around each loop of wire line up with one another.
2) this results in lots of magnetic field lines pointing in the same direction that are very close.
3) the magnetic field inside a solenoid is strong and uniformed.
4) outside the coil, the magnetic field is just like a bar magnet
how can you increase the magnetic field strength of a solenoid further?
1) putting an iron core in the centre of the coil.
- it becomes an induced magnet whenever the current is flowing.
2) increasing coils/ length of wire
3) increasing current
what is an electromagnet?
a magnet whose magnetic field can be turned on and off with an electric current
what are some uses of an electromagnet?
1) they can create a varying force, eg loudspeakers.
2) used in cranes to attract and pick up magnetic things.
3) can be used within other circuits to act as switches
how can an electromagnet be used in a circuit as a switch?
(in the electric starters of motors)
1) when the switch is closed in circuit 1, it turns on the electromagnet.
2) the electromagnet attracts the iron contact on the rocker.
3) the rocker pivots and closes the contacts, completing circuit 2, and turning on the motor
what happens when a current- carrying wire or any other conductor is placed between magnetic poles?
the magnetic field around the wire interacts with the magnetic field it has been placed in.
- this causes the magnet and conductor to exert a force on each other.
- this is called the MOTOR effect and can cause the wire to move
what is the condition for the wire’s placement to experience the full force from the other magnetic field? (motor effect)
it has to be at a 90 degree angle to the magnetic field.
- if the wire runs parallel to the magnetic field, it wont experience any force at all.
- however in angles in between, it feels some force
what is the rule of force and magnet fields in the motor effect?
the force will ALWAYS act at right angles to the magnetic field of the magnets and to the direction of the current in the wire
what is a good method of displaying the direction of force in the motor effect?
1) apply a set of rails inside a horseshoe magnet.
2) a bar is placed on the rails, completing the circuit .
3) this generates a force that rolls the bar along the rails.
- the magnitude of the force increases with the strength of the magnetic field.
- the force also increases with the amount of current passing through the conductor
what 3 things is the force acting on a conductor in a magnetic field dependent on?
1) magnetic flux density- how many field lines there are in a region.
-this shows the strength of the magnetic field.
2) the size of the current through the conductor.
3) the length of the conductor thats in the magnetic field
how can you find the direction of force in the motor effect?
flemmings left hand rule
- first finger points in the direction of the field.
- second finger in the direction of the current.
- the thumb will then point in the direction of the force (motion)
what does flemmings left hand rule show and why is it important?
shows that if either the current or magnetic field is reversed, so will the direction of the force.
- this can be used to find the direction of the force in all sorts of things, like motors
on a dc motor, where do forces act?
what forces are they?
on the two arms of a coil of wire thats carrying a current.
- they’re just usual forces which act on any current in a magnetic field
how does a dc motor work?
1) because the coil is on an axle and the forces act one up and one down, it rotates.
2) the split ring commutator changed the direction of the current every half-turn to keep the motor rotating in the same direction
how can the direction of the motor be reversed?
1) reversing the current
2) swapping magnetic poles (reversing the field)
what is the generator effect?
the induction of potential difference (and current if theres a complete circuit) in a wire which is experiencing a change in magnetic field
describe the generator effect and how you can create it?
1) the generator effect induces a pd in a conductor and a current in a complete circuit.
2) you can do this by moving a magnet in a coil of wire.
3) you can also create this by moving a conductor (wire) in a magnetic field
brief summary of how loudspeakers work?
they use the motor effect to convert vibrations in current in electrical circuits to pressure vibrations in sound waves
how do coil-moving loudspeaker and headphones work?
1) an alternating current is sent through a coil of wire attached to the base of a paper cone.
2) the coil surrounds one pole of a permanent magnet, and is surrounded by the other pole.
3) so a current causes a force on the coil, so the cone vibrates.
4) when the current reverses, the force acts in the opposite direction, which causes the cone to move in the other direction too.
5) 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
what is the frequency of a soundwave in a loud speaker the same as?
the frequency of the alternating current. so by controlling the frequency of the ac, you can alter the sound wave produced
what happens when there is a change in magnetic field around a conductor OR if a conductor moves relative to a magnetic field?
a potential difference is induced along the ends of the conductor.
- if the conductor is part of a complete circuit, then a current is also produced in the conductor
how can you change a magnetic field in the generator effect?
1) moving a magnet into a coil of wire.
2) moving a conductor in a magnetic field
how is a current in the generator effect induced?
by moving a conductor through a magnetic field. when a conductor stops moving, so does the current
when is a current or PD not induced?
when the conductor is moving along the magnetic field instead of THROUGH it
how can you increase induced PD and current in the generator effect?
1) using a stronger magnetic field
2) move the conductor (wire) more rapidly
3) if we shape the wire into a coil, (as the greater number of coils in a wire, the greater the induced PD and current is)
what happens to the induced current and PD when moving a magnet in and out of a coil?
it reverses each time
when a current is induced, what does it create?
- what is the effect of this?
its own magnetic field which opposes the movement of the magnet.
- so when the north pole of a magnet is inserted into a coil, that end of the coil also becomes a north pole.
- this repels the magnet, making it harder to push in
- however when we pull the magnet back out, the coil turns into a south pole, attracting the coil back in. and harder to pull out.
this means we are DOING WORK
how is energy transferred throughout the generator effect?
energy is transferred from the movement of the magnet to the movement of the current
what do generators do?
they make use of the generator effect to induce a current.
- whether this current is alternating or direct depends on how the coil is connected to the circuit
what type of current alternators generate?
alternating
how do alternators work?
1) generators rotate a coil in a magnetic field (or a magnet in a coil)
2) as the coil (or magnet) spins, a current is induced in the coil, and every half-turn, the current changes direction.
3) alternators have slip rings and brushes so the contacts dont swap every half turn.
4) this means that they produce an alternating current and PD
what type of current do dynamos generate?
1) they work in the same way as alternators, except they have one key difference.
2) they have a split-ring commutator instead of slip rings.
3) this swaps the connection every half turn to keep the current flowing in the same direction
what do oscilloscopes do?
they can be used to see generated PD in a coil over time
what does alternating current look like in an oscilloscope?
a line that does up and down, crossing the horizontal axis
what does direct current look like in an oscilloscope?
it isnt a straight line, however it stays above the horizontal axis. this is because the PD is always positive, so its still direct current
what is the height of a line at any given point in an oscilloscope representative of?
the potential difference generated at that time
how can you increase PD?
increasing the frequency of revolutions. however it does create more peaks too
how do microphones work?
1) sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
2) this causes the coil of wire to move in the magnetic field, which generates a current.
3) the movement of the coil and generated current depend on the properties of the sound waves (eg louder sounds make the diaphragm move further).
4) this is how microphones can convert the pressure variations of a sound wave into variations of current in an electrical circuit
what do transformers do?
they change the size of PD, but only for alternating current
what is the structure of a transformer?
two coils of wire, the primary and secondary. these are joined with an iron core
how does a transformer work?
1) the primary coil is connected to an alternating current.
2) this creates a changing magnetic field and magnetises the iron core.
3) this magnetic field is transmitted along the metal core to the secondary coil.
4) when it passes through the secondary coil, it induces a PD
why is iron used in a transformer?
because it is easily magnetised
why is an alternating current needed in a transformer?
because a changing magnetic field is needed to induce a potential difference.
- eg direct current wouldnt work because it produces a constant magnetic field
what does the iron core do to the magnetic field?
it increases the strength of it
what is the ratio between the primary and second coil the same as?
the ratio between the number of coil turns on the primary and secondary coils
what does a step up transformer do?
steps the PD up. they have more turns on the secondary coil than primary coil
what does a step down transformer do?
step the PD down. they have more turns on the primary coil than the secondary coil
what is the transformer equation?
V(p) N(p)
—– = —–
V(s) N(s)
1) what is V(p) in the transformer equation?
2) what is V(s) in the transformer equation?
3) what is N(p) in the transformer equation?
4) what is N(s) in the transformer equation?
1) input PD
2) output PD
3) the number of turns on primary coil
4) the number of turns on secondary coil
what would happen if transformers were 100% effective?
the power input and power output would be equal
