Physics - Topic 7 - Magnetism and Electromagnetism

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  • Created by: hannah.o
  • Created on: 12-02-18 21:30
What is a magnetic field?
A region where other magnets or magnetic materials experience a force.
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What does the magnetic field of a bar magnet look like?
.
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The closer together the lines are?
The stronger the magnetic field.
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The further away from a magnet you get?
The weaker the field is.
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What does an attraction magnetic field look like?
.
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What does a repulsion magnetic field look like?
.
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What do compasses show?
They show the directions of magnetic fields.
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What are the two types of magnet?
Permanent magnets and induced magnets.
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What are permanent magnets?
They produce their own magnetic fields.
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What are induced magnets?
They are magnetic materials that turn into a magnet when they're put into a magnetic field. When you take away the magnetic field, induced magnets quickly lose their magnetism and stop producing a magnetic field.
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What happens 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, with the wire in the centre.
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What does the strength of the magnetic field produced change with?
The current and the distance from the wire.
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What is a solenoid?
It is a coil of wire.
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How can you increase the strength of the magnetic field that a wire produces?
By wrapping the wire into a coil (solenoid).
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Why does this happen?
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.
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What is the magnetic field inside a solenoid like?
It is strong and uniform (it has the same strength and direction at every point in that region).
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What is the magnetic field outside the coil like?
Just like a bar magnet.
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How can you increase the field strength of the solenoid even more?
Put a block of iron in the centre of the coil. This iron core becomes an induced magnet whenever current is flowing.
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What is a solenoid with an iron core called?
An electromagnet.
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Where are electromagnets used?
They are used in some cranes to attract and pick up things made from magnetic materials like iron and steel. 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.
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How are electromagnets used within other circuits?
They act as switched, for example in the electric starters of motors, like this:
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What happens in the circuits?
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.
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What is the motor effect and when does it happen?
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. This causes the magnet and the conductor to exert a force on each other, called the motor effect.
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What does this look like from an aerial view?
.
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What musy be done for the wire 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. At angles in between, it'll feel some force.
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What increases with the strength of the magnetic field?
The magnitude (strength) of the force.
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How does the force also increase?
It increases with the amount of current passing through the conductor.
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What does the force acting on a conductor in a magnetic field depend on?
1) The magnetic flux density-how many field(flux) 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.
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What is the equation for force when the current is at 90' to the magnetic field it is in?
Force (N) = Magnetic flux density (T, tesla) x Current (A) x Length (m)
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How do you 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).
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What does a basic dc motor look like?
.
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What does this diagram show?
Forces act on the 2 side 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 a spindle and the forces act one up and one down, it rotates.
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What does the split-ring commutator do?
It is a way of swapping the contacts every half turn to keep the motor rotating in the same direction.
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How can the direction of the motor be reversed?
Either by swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field).
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What use electromagnets?
Loudspeakers and headphones.
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How do they use electromagnets? (2)
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, so the current causes a force on the coil (which causes the
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cone to move). 3) When the current reverses, the force acts in the opposite direction, which causes the cone to move in the opposite direction too. 4) 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. 5) The frequency of the sound wave is the same as the frequency of the ac you can alter the sound wave produced.
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What is the generator effect?
The induction of a potential difference (and current if there's a complete circuit) in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field.
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How can you produce a generator effect?
Moving a magnet in a coil of wire or moving a conductor (wire) in a magnetic field.
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How do generators work to produce ac or dc? (2)
1) As you turn the magnet, the magnetic field through the coil changes. This change in the magnetic field induces a potential difference, which can make a current flow in the wire. 2) When you've turned the magnet through half a turn, the direction
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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 then the pd will keep on reversing every half turn and you'll get an alternating current.
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How does the induced current always oppose the change that made it?
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. The magnetic field created by an induced current always acts against the change that made it.
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How do you change the size of the induced potential difference?
You have to change rate that the magnetic field is changing. Induced potential difference can be increased by either:1) Increasing speed of movement-cutting more magnetic field lines in a given time. 2)Increasing strength of magnetic field.
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How do alternators generate alternating current?
Generators rotate a coil in a mgnetic field. Their construction is pretty much like a motor. 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
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slip rings and brushes so the contacts don't swap every half turn. This means they produce an alternating potential difference.
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How do dynamos generate direct current?
Dynamos work in a similar way as alternators. 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.
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What do oscilloscopes shows?
How the potential different generated in the coil changes over time.
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What does an ac trace look like on an oscilloscope?
.
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What does a dc trace look like on an oscilloscope?
.
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What does increasing revolution look like on an oscilloscope?
.
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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.
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What does the movement of the coil depend on?
The properties of the sound wave (louder sounds make the diaphragm move further).
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What does this let microphones do?
Convert the pressure variations of a sound wave into variations in current in an electric current.
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What do transformers do?
They change the size of the potential difference of an alternating current.
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What do all transformers have?
2 coils of wire, the primary and the secondary, joined with an iron core.
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What happens 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.
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What does a step-up transformer do?
It steps the potential difference up. They have more turns on the secondary coil than the primary coil.
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What does a step-down transformer do?
Step the potential difference down. They have more turns on the primary coil than the secondary coil.
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What is the equation for the output pd from a transformer?
Input pd / Output pd = Number of turns on primary coil / Number of turns on secondary coil
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If you assume transformers are 100% efficient, what is the input power equal to?
The input power is equal to the output power.
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What is the equation for this?
Pd across secondary coil (V) x Current through secondary coil (A) = Pd across primary coil (V) x Current through primary coil (A)
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What does the magnetic field of a bar magnet look like?

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Card 3

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The closer together the lines are?

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Card 4

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The further away from a magnet you get?

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Card 5

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What does an attraction magnetic field look like?

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