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Physics - Topic 7 - Magnetism and Electromagnetism (Electromagnetism…
Physics - Topic 7 - Magnetism and Electromagnetism
Permanent and Induced Magnets
Magnets produce magnetic fields..
The closer together the lines are, the stronger the magnetic field. The further away from a magnet you get, the weaker the field is.
You can show a magnetic field by drawing magnetic field lines.
The magnetic field is strongest at the poles of a magnet. This means that the magnetic forces are strongest at the poles.
All magnets produce a magnetic field - a region where other magnets or magnetic materials experience a force.
The force between a magnet and a magnetic material is always attractive, no matter the pole.
All magnets have two poles - north and south.
If two poles of a magnet are put near each other, they will each exert a force on each other. This force can be attractive or repulsive.
Two poles that are the same - like poles - will repel eachother. Two unlike poles will attract each other.
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.
Compasses Show the Directions of Magnetic Fields.
You can move a compass around a magnet and trace its position on some paper to build up a picture of what the magnetic field looks like.
When they're not near a magnet, compasses always point north. This is because the Earth generates its own magnetic field, which show the core of the Earth must be magnetic.
Inside a compass is a tiny bar magnet. The north pole of this magnet is attracted to the south pole of any other magnet it is near. So the compass points in the direction of the magnetic field it is in.
Magnets can be Permanent or Induced
There are two types of magnet - permanent magnets and induced magnets.
Permanent magnets produce their own magnetic field.
Induced magnets are magnetic materials that turn into a magnet when they're put into a magnetic field.
The force between permanent and induced magnets is always attractive.
When you take away the magnetic field, induced magnets quickly lose their magnetism (or most of it) and stop producing a magnetic field
Electric Motors and Loudspeakers
Loudspeakers work because of the motor effect
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 current causes a force on the coil.
Loudspeakers and headphones both use electromagnets
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 waves is the same as the frequency of the ac, so by controlling the frequency of the ac you can alter the sound wave produced.
A current-carrying coil of wire rotates in a magnetic
Because the coil is on a spindle and the forces act one up and one down it rotates.
The split-ring commutator is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction.
These forces are just the usual forces which act on any current in a magnetic field.
The direction of the motor can be reversed either by swapping the polarity of the dc supply or swapping the magnetic poles over.
The diagram on the left shows a basic dc motor. Forces act on the two side arms of a coil of wire that's carrying a current.
You can use Fleming's left hand rule to work out which way the coil will turn.
Electromagnetism
A Moving Charge Creates a Magnetic Field
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 field is made up of concentric circles perpendicular to the wire, with the wire in the centre.
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.
When a current flows through a wire, a magnetic field is created around the wire.
The Right-Hand Thumb Rule
Using your right hand, point your thumb in the direction of current and curl your fingers. The direction of your fingers is the direction of the field.
Solenoids
This results in lots of field lines pointing in the same direction that are very close to eachother - 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.
This happens because the field lines around each loop of wire line up with eachother.
Outside the coil, the magnetic field is just like the one around a bar magnet.
You can increase the strength of the magnetic field that a wire produces by wrapping the wire into a coil called a Solenoid
You can increase the field strength of the Solenoid even more by putting a block of iron in the centre of the coil. This iron core becomes an induced magnet whenever current is flowing.
If you stop the current, the magnetic field disappears. A Solenoid with an iron core - a magnet whose magnetic field can be turned on and off with an electric current is called an Electromagnet.
Uses of Electromagnets
Magnets that can be switched on and off are very useful. They are used because they are so quick to turn off and on or because they can create varying forces - like in loudspeakers.
Electromagnets can also be used within other circuits to act as switches - e.g. in the starters of motors
The rocker pivots and closes the contacts, completing circuit two, and turning on the motor.
When the switch in circuit one is closed, it turns on the electromagnet, which attracts the iron contact on the rocker.
Electromagnets are used in some cranes to attract and pick up things made from magnetic materials like iron and steel.
Generators and Microphones
Oscilloscopes
For DC the line stays above the axis - the PD is always positive.
The height of the line at a given point is the generated potential difference at the time.
For AC this is shown as a line that goes up and down- crossing the horizontal axis.
Increasing the frequency of revolutions increases the overall PD - it also creates more peaks.
Oscilloscopes show how the voltage generated in the coil changes over time.
DC Trace
AC Trace
Alternators
As the coil - or magnet - spins, a current is induced in the coil. This current changes direction every half turn.
Instead of a split-ring communicator, AC generator have slip rings and brushes so the contacts don't swap every half turn.
Their construction is like that of a motor.
This means they produce an alternating potential difference.
Generators rotate a coil in a magnet - or a magnet in coil.
Microphones
This causes the coil of wire to move in the magnetic field, which generates a current.
The height of the line at any point shows the potential difference at the time.
Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet.
Increasing the frequency of revolutions increases the overall PD, this also creates more peaks.
Microphones are essentially loudspeakers in reverse.
Dynamos
A split-ring commutator swaps the connection every half turn to keep the current flowing in the same direction.
Dynamos work very similarly to alternators except one difference. They have a split-ring commutator instead of slip rings.
The Motor Effect
Finding the size of the force
The size of the current through the conductor.
The length of the conductor that is in the magnetic field.
The magnetic flux density - how many field (flux) lines there are in a region.
When the current is at 90° to the magnetic field it is in, the force acting on it can be found using the equation on the right.
The force acting on a conductor in a magnetic field depends on three things:
Force(N) = Magnetic Flux Density(T) + Current(A) + Length(m)
F=BI
l
Fleming's Left-Hand Rule
Point your se
C
ond finger in the direction of the
C
urrent.
Your thu
M
b will then point in the direction of the force (
M
otion).
Using your left hand, point your
F
irst finger in the direction of the
F
ield.
Fleming's left-hand rule show that if either the current or the magnetic field is reversed, then the direction of the force will also be reversed.
You can find the direction of the force with Fleming's left hand rule.
A Current in a Magnetic Field Experiences a Force
The force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire.
A good way of showing the direction of the force is to apply a current to a set of rails inside a horseshoe magnet.
A bar is placed on the rails, which completes the circuit. This generates a force that rolls the bar along the rails.
To experience the full force, the wire has to be at 90° to the magnetic field. If the wire runs parallel to the magnetic field, it won't experience any force at all.
The magnitude of the force increases with the strength of the magnetic field.
This causes the magnet and the conductor to exert a force on each other. This is called the motor effect and can cause the wire to move.
The force also increases with the amount of current passing through the conductor.
When a current - carrying wire is put between magnetic poles, the magnetic field around the wire interacts with the magnetic field it has been placed in.
The Generator Effect
Induced Current Opposes the Change that Made It.
The magnetic field created by an induced current always acts against the change that made it. It's trying to return things to the way they were.
This means that the induced current always opposes the change that made it.
A change in magnetic field can induce a current in a wire. When a current flows through a wire a magnetic field is created around the wire.
You can Change 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 can be increased by either:
Increasing the speed of the movement - cutting more magnetic field lines in a given time.
Increasing the strength of the magnetic field.
Cutting Field Lines Induces a Potential Difference
Shifting the magnet from side-to-side creates a little blip of current if the conductor is part of a complete circuit.
If you move the magnet in the opposite direction then the potential difference/current will be reversed. Likewise if the polarity of the magnet is reversed, then the potential difference/current will be reversed too.
You can do this by moving a magnet in a coil of wire or by moving a conductor in a magnetic field.
If you keep the magnet moving backwards and forwards, you produce a potential difference that keeps swapping direction - an alternating current.
The generator effect creates a potential difference in a conductor, and a current if the conductor is part of a complete circuit.
Transformers
Transformers only work in Alternating Current
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.
They all have two coils of wire, the primary and the secondary, joined with an iron core.
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.
Transformers change the size of the potential difference of an alternating current.
Step-Down Transformers - step the PD down. They have more turns on the primary coil than the secondary coil.
Step-Up Transformers - step the potential difference up. They have more turns on the secondary coil than the primary coil.
The Transformer Equation
You need to be able to relate both of these equations to power transmission in the national grid, to explain why and how the national grid transmits at very high voltage
A low current means that less energy is wasted heating the wires, making the national grid and efficient way of transmitting power.
Transformers are pretty much 100% efficient. It can be assumed that they are. The input power is equal to the output power.
The equation can be used to work out the number of turns needed to increase the voltage to the right levels.
The equation can be used either way up, there is less re-arranging if you put what you're finding on the top.
As long as 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.
