Topic 7 - Magnetism and Electromagnetism Flashcards
Magnets
> All magnets have 2 poles - north and south.
All magnets produce a magnetic field - a region where other magnets or magnetic materials (e.g. iron, steel, nickel and cobalt) experience a non-contact force.
The force between a magnet and magnetic material is always attractive, no matter the pole.
If the 2 poles of a magnet are put near each other, they will exert a force on each other.
This force can be repulsive or attraction.
Same - like poles - repel.
Unlike poles - attract.
Magnetic field lines
> You can show a magnetic field by drawing magnetic field lines.
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 the field.
The closer together the lines are, the stronger the magnetic field. The further away from a magnet you get, the weaker the field is.
The magnetic field is strongest at the poles of a magnet. This means that the magnetic forces are also strongest at the poles.
Compasses
> Compasses show the direction of magnetic fields.
Inside a compass is a tiny bar magnet (the needle).
The north pole of this magnet is attracted to the south pole of any other magnet it is near. So the compass needle points in the direction of the magnetic field it is in.
You can move a compass around a magnet and trace the needle’s position on some paper to build up a picture of what the magnetic field looks like.
When they’re not near a magnet, compass needles always point north. This is because the Earth generates its own magnetic field, which shows that the inside (core) of the Earth must be magnetic.
Types of magnets
> There are 2 types of magnet - permanent magnets and induced magnets.
The force between permanent and induced magnets is always attractive.
Induced magnets
> Induced magnets are magnetic materials that turn into a magnet when hey’re put into a magnetic field.
When you take away the magnetic field, induced magnets quickly lose most or all of their magnetism.
Permanent magnets
> Permanent magnets produce their own magnetic fields.
Magnetic field - charge
> When a current flows through a wire, a magnetic field is created around the wire.
The field is made up of concentric circles perpendicular to the wire in the centre.
You can see this by placing a compass near a wire that is carrying a current. As you move the compass, it will trace the direction of the magnetic field.
Changing the direction of the current changes the direction of the magnetic field - use the right-hand thumb rule to work out which way it goes.
The strength of the magnetic field produced changes with the current and the distance from the wire. The larger the current through the wire, or the closer to the wire you are, the stronger the field is.
The Right-Hand Thumb Rule
> Using your right hand, point your thumb in the direction of current and curl your fingers.
The direction your fingers is the direction of the field.
Solenoid
> You can increase the strength of the magnetic field that a wire produces by wrapping the wire into a coil called a solenoid.
This happens because the field lines around each loop of wire line up with each other. This results in lots of field lines pointing in the same direction that are very close to each other. The closer together field lines are, the stronger the field is.
The magnetic field inside a solenoid is strong and uniform (it has the same strength and direction at every point in that region).
Outside the coil, the magnetic field is just like the one around the bar magnet.
You can increase the field strength of the magnet even more by putting a block of iron in the centre of the coil. This iron core becomes an induced magnet whenever the current is flowing.
If you stop the current flowing, the magnetic field disappears.
Electromagnet - definitions
> A solenoid with an iron core is called an electromagnet.
>A magnet whose field can be turned on and off with an electric current.
Electromagnets - uses
> Magnets you can switch on and off are really useful.
They’re usually because they’re so quick to turn on and off or because they can create a varying force (like in loudspeakers).
Electromagnets are used in some cranes to attract and pick up things made from magnetic materials like iron and steel, e.g. in scrap yards. Using an electromagnet means the magnet can be switched on when you want to pick stuff up, then switched off when you want to drop it.
Electromagnets can also be used within other circuits to act as switches.
When the switch in circuit one is closed, it turns on the electromagnet which attracts the iron contact on the rocker. The rocker pivots and closes the contacts, completing circuit 2, and turning on the motor.
Motor effect - definition
> When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other.
This is called the motor effect and can cause the wire, to move.
Motor effect
> To experience the full force, the wire has to be at 90 degrees to the magnetic field. If the wire runs parallel to the magnetic field, it won’t experience any force at all. At angles in between, it’ll feel some force.
The force always acts at right angles to the magnetic field of the magnets and to the direction of the current in the wire.
Force on a conductor equation
> For a conductor at right angles to a magnetic field and carrying a current:
-force = magnetic flux density × current × length
F = B I L
force, F, in newtons, N
magnetic flux density, B, in tesla, T
current, I, in amperes, A (amp is acceptable for ampere)
length, L, in metres, m
the factors that affect the size of the
force on the conductor
> The force acting on a conductor in a magnetic field depends on 3 things:
1. The 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 that’s in the magnetic field.
When the current is at 90 degrees to the magnetic field it is in, the force acting on it can be found using the equation - F = BIL
Fleming’s left-hand rule
> You can find the direction of the force with Fleming’s left-hand rule:
1. Using your left hand, point your first finger in the direction of the field.
2. Point your second finger in the direction of the current.
3. Your thumb will then point in the direction of the force (motion).
The rule shows that if either the current of the magnetic field is reversed, then the direction of the force will also be reversed.
Electric Motors
> Electric motors use the motor effect to get them and keep them moving. IMPORTANT:
The diagram in the book shows a basic dc motor.
Forces act on the two sides arms of a coil of wire that’s carrying a current.
These forces are just the usual forces which act on any current in a magnetic field.
Because the coil is on an axle and the forces act one up and one down, it rotates.
The slit-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction.
The direction of the motor can be reversed either by swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field).
Use left-hand rule to work out which way coil will turn.
Generator Effect - background
> The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.
The generator effect induces a potential difference in a conductor (and a current if the conductor is part of a complete circuit).
Generator Effect - definition
> The induction of a potential difference (and a current if there’s a complete circuit) in a wire which is experiencing a change in magnetic field.
Generator effect - how to
> You can do this by moving a magnet in a coil of wire or moving a conductor (wire) in a magnetic field (cutting magnetic field lines).
Shifting the magnet from side to side creates a little ‘blip’ of current in the conductor if it’s part of a complete circuit (the current can be shown on an ammeter in the circuit).
If you move the magnet (or conductor) in the opposite direction, them the pd/current will be reversed. Likewise if the polarity of the magnet is reversed, then the pd/current will be reversed too.
If you keep the magnet (or coil) moving backwards and forwards, you produce a pd that keeps swapping direction, which producing an alternating current.
Generator effect - rotation
> Rotation can also cause the generator effect.
You can create the same effect by turning a magnet end to end in a coil, or turning a coil inside a magnetic field. This is how generators work to produce AC or DC.
1. As you turn the magnet, the magnetic field through the coil changes. This change in the magnetic field induces a PD, which can make a current flow in the wire.
2. Every time the magnet moves through half a turn, the direction of the magnetic field through the coil reverses. When this happens, the potential difference reverses, so the current flows in the opposite direction around the coil of wire.
3. If you keep turning the magnet in the same direction - always clockwise, say - then the potential difference will keep on reversing every half turn and you’ll get an alternating current.
Induced current
> A change in magnetic field can induce a current in a wire. But, when a current flows through a wire, a magnetic field is created around the wire (2nd magnetic field).
The magnetic field created by an induced current always acts against the change that made it (whether that’s the movement of the wire or a change in the field it’s in.) Basically, it’s trying to return things to the way they were.
This means that the induced current always opposes the change that made it.
Changing the size of the induced potential difference.
> If you want to change the size of the induced pd, you have to change the rate that the magnetic field is changing.
Induced potential difference (and so induced current) can be increased by either:
1. Increasing the speed of movement - cutting more magnetic field lines in a given time.
2. Increasing the strength of the magnetic field (so there’s more field lines that can be cut).
Alternators
> Alternators generate alternating current.
Generators rotate a coil in a magnetic field or a magnet in a coil.
Their construction is pretty much like a motor.
As the coil or magnet sins, a current is induced in the coil. This current changes direction every half turn.
Instead of a split-ring commutator, alternators have slip rings and brushes so the contacts don’t swap every half turn.
This means they produce an alternating potential difference.
Dynamos
> Dynamos generate direct current.
Dynamos work in the same way as alternators, apart from one important difference.
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.
What can you use to see generated pd?
An oscilloscope
Oscilloscope
> Oscilloscopes show how the pd generated in the coil changes over time.
For ac this is a line that goes up and down crossing the horizontal axis.
For dc the line isn’t straight like you might expect, but it stays above the axis (the pd is always positive) so it’s still direct current.
The height of the line at a given point is the generated pd at that time.
Increasing the frequency of revolutions increases the overall pd, but it also creates more peaks too.
See book.
Loudspeakers - how they work
> Loudspeakers and headphones work because of the motor effect and both use electromagnets:
- An ac 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 current causes a force on the coil (which causes the cone to move).
- When the current reverses, the force acts in the opposite 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 the variations in pressure that cause a sound wave.
- The frequency of the sound wave is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced.
Microphones - how they work
- Microphones are basically loudspeakers in reverse.
- Sound waves hit a flexible diaphragm that is attached to a coil of wire , wrapped around a magnet.
- The movement of the coil (and so the generated current) depends on the properties of the sound wave (louder sounds make the diaphragm move further).
- This is how microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit.
Transformers
> Transformers change the size of the potential difference of an alternating current.
They all have two coils of wire, the primary and the secondary, joined with an iron core.
When an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating pd in the secondary coil.
If the second coil is part of a complete circuit, this causes a current to be induced.
The ratio between the primary and secondary potential differences is the same as the ratio between the number of turns on the primary and secondary coils.
Iron is used because it is easily magnetised.
Step-up transformers
> Increase the potential difference.
>More turns on secondary coil than primary coil.
Step-down transformers
> Decrease the potential difference.
>More turns on the primary coil than the secondary coil.
Transformer equation
> As long as you know the input pd and the number of turns on each coil, you can calculate the output pd from a transformer using the transformer equation:
input pd on primary divided by output pd on secondary = turns on primary coil divided by turns on secondary coil.
Equation can be used either way up.
Transformers are almost 100% efficient so you can assume the input and output power are equal:
pd across secondary x current through secondary = pd across primary x current through primary.
Why are transformers used?
> Low current means less energy is wasted heating the wires and the surroundings, making the national grid an efficient way of transmitting power.
