AQA Topic 7 - Magnetism and Electromagnetism 9-1

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Name the two poles magnets have
North and South - also known as North Seeking and South Seeking
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What is a magnetic field?
A region where other magnets or magnetic materials i.e Iron experience a force - This is a NON CONTACT force
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Describe the magnetic field lines of a bar magnet
The line circulates from the North Pole to the South Pole on the magnet
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How would the magnetic field lines change if a) Two South Poles were placed near each other b) A North and South pole
a) They would diverge into a star pattern because of the repulsion b) The lines in direct sight of the South from the North will link straight across but the others would form a global shape as they go from one pole to another
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Complete the sentences: The field lines show ____ way a force would ____ on a ____ pole if it was put at that ___ in the field.
1. Which 2. Act 3. North 4. Point
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How can field lines indicate a strong magnetic field?
The closer together the lines the stronger the fields so the further away from a magnet you get the weaker the field is
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Why are the magnetic forces strongest at the poles?
Because the magnetic field is strongest at the poles
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True or False: The force between a magnet and a magnetic material is always attractive, no matter the pole
True
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What happens when two poles are placed near each other?
They will exert a force on one another
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What do compasses show? Why is this?
The directions of magnetic fields, this is because there is a tiny bar magnet inside of a compass and the north pole of said magnet is attracted to the south pole of any nearby magnet. Therefore the compass points in the direction of the magnetic F
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Describe how you can use compasses to see what a magnet's field lines look like
Move a compass around a magnet and trace its position (the way its pointing) on some paper to build up a picture of what the magnetic field looks like
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Why do compasses always point North?
Because the earth generates its own magnetic field which also shows the core of the earth must be magnetic
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Name the two types of magnets
Permanent and Induced
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Which magnet produces its own magnetic field?
Permanent
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What are induced magnets?
Magnetic materials that turn into a magnet when they're placed in a magnetic field
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What happens when you take away the permanent magnet from the induced one?
It loses its magnetism (most of it) and stops producing a magnetic field
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What happens when a current flows through a wire?
A magnetic field is created made of concentric circles perpendicular to the wire w/ the wire in the centre
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Describe the Right Hand Thumb Rule
Using your right hand point your thumb in the direction of the current and curl your fingers. Your fingers point in the direction of the field but your thumb points in the direction of the current
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True or False: Compasses will not show the direction of a magnetic field surrounding a current-carrying wire
False, you can repeat the exact same experiment that you do with a normal bar magnet
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What increases the strength of the wire's magnetic field and why?
The strength produced changes with the current and distance from the wire and so can become stronger by a larger current flowing through the wire or being closer to the wire
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What is a solenoid?
A coil of wire
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Describe how to increase the strength of the magnetic field that a wire is producing (1)
Wrap the wire into a coil called a solenoid
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Why does (1) work?
The field lines around each loop of wire line up w/ each other which results in lots of field lines pointing in the same direction that are very close to each other, closer field lines = stronger field
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Complete the sentences: The magnetic field ____ a solenoid is strong and ____
1. Inside 2. Uniform
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Define uniform in a magnetic context
The same strength and direction at every point in that region
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What is the magnetic field like outside a solenoid?
Like a bar magnet
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How does putting an iron core into a solenoid increase the field strength?
The 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 because its magnetic field can be turned on and off with an electric current
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Name and describe two uses of Electromagnets
1. Used in scrap yards because they can attract magnetic materials and the magnet can be switched on and off 2. In circuits to act as switches i.e in motors as an electric starter
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Why does a current in a magnetic field experience a force?
When a current-carrying wire is placed between magnetic poles, the magnetic field around the wire interacts with the magnetic filed is has been placed in. This causes the magnet and the conductor to exert a force on each other
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Define the Motor Effect
The motor effect is the term used when a current-carrying wire in the presence of a magnetic field experiences a force and can cause the wire to move
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What degree must the wire be at to the magnetic field and why?
90 degrees because otherwise it won't feel the full force
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Complete the sentences: The force always acts at ____ angles to the magnetic field of the ____ and the ____ of the current in the ____
1. Right 2. Magnets 3. Direction 4. Wire
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How can you show the direction of the force in an electromagnet?
Apply a current to a set of rails inside a horseshoe magnet, a bar is placed on the rails which completes the circuit which generates a force that role the bar along the circuit
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Complete the sentences: The ____ of the force ____ with the strength of the magnetic field
1. Magnitude 2. Increases
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True or False: The force also increases with the amount of current passing through the conductor
True
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What is magnetic flux density?
How many field (flux) lines there are in a region, this also shows the strength of the magnetic field
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When can the equation F = BIL be used?
When the current is at 90 degrees to the magnetic field it is in
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Describe Fleming's Left Hand Rule
First finger = Field, seCond finger = Current, thuMb = Motion. Using your left hand you point your First finger in the direction of the Field, seCond finger in the direction of the Current and thuMb in the direction of the Motion (force)
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What does 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, useful for motors
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What happens to a current-carrying coil of wire in a magnetic field?
It rotates
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Name the components to a basic DC motor
Magnets, a spindle, a split-ring commutator, coil of wire and electrical components which touch the split ring
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What happens in a DC motor?
Forces act on the two side arms of a coil of wire thats carrying a current
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Why does the coil of wire rotate?
Because the coil is on a spindle and the forces act one up and one down so it rotates
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What does the split-ring commutator do?
It swaps 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 can be reversed?
Swapping the polarity of the dc supply (reversing the current) or swapping the magnetic poles over (reversing the field)
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How do loudspeakers work? (1)
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...
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How do loudspeakers work? (2)
. When the current reverses the force acts in the opp 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
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How do loudspeakers work? (3)
pressure that causes a sound wave. The freq of the sound wave is the same as the frequency of the AC so by controlling the freq of the AC you can alter the sound wave produced
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What happens when you cut field lines?
It induces a potential difference
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Define 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 do you induce the Generator Effect? (1)
By moving a magnet in a solenoid or moving a conductor (wire) in a magnetic field (cutting the field lines). Shift the magnet from side to side creates a 'blip' of current if the conductor is part of a complete circuit. If you move the magnet/con in
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How do you induce the Generator Effect? (2)
the opposite direction, then the potential difference/current will be reversed. Same if the polarity of the magnet is reversed to
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What happens if you keep moving the magnet or coil backwards and forwards?
You produce a potential difference that keep swapping direction - an AC
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Complete the sentence: Induced current ___ the change that made it
Opposes
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Why does the magnetic field produced by an induced current act against the change?
Because it is trying to return things to their original place, it can does this by reversing the movement of a wire or a change in the field that it's in
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How do you change the size of induced potential difference?
1. Increasing speed of movement i.e cutting more magnetic field lines in a given time 2. Increasing the strength of the magnetic field so there are MORE field lines that can be cut
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Why can you change the size of induced potential difference?
Because you can change the rate that the magnetic field is changing
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Complete the sentence: Alternators generate ____ current
Alternating
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Name the other term for an Alternator
AC Generator
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Describe the process of an AC Generator (1)
The generator rotates a coil in a magnetic field, as the coil/magnet spins a current is induced in the coil. This current changes direction every half turn. Instead of a split-ring commutator they have slip rings and brushes so the contacts DON'T
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Describe the process of an AC Generator (2)
swap every half turn. This means they produce an alternating potential difference
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True or False: Dynamos produce alternating current
False, they produce direct current
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Name the other term for a Dynamo
DC Generator
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Describe the process of a DC generator
Dynamos work in the same way as alternators but they have a split ring commutator not slip rings. This swaps the connection every half turn to keep the current flowing in the same direction
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How can you use an oscilloscope to see the generate P.D?
Oscilloscopes show how the p.d generated in the coil changes over time. For AC this is a line going up and down, crossing the X axis but for DC the line stays above the axis (p.d is always positive)
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Complete the sentence: The ____ of a line on a oscilloscope at a given point is the _____ potential difference at that ____
1. Height 2. Generated 3. Time
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What does increasing the frequency of revolutions do?
It increases the overall p.d but also creates more peaks
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How do microphones work? (1)
Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet. The wire then moves in the magnetic field which generates a current, the movement of the generated current depends on the properties of the sound wave
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How do microphones work? (2)
i.e louder sounds make the diaphragm move further. Therefore microphones can convert the pressure variations of a sound wave into variations in current in an electric circuit
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What current do transformers work with?
Alternating
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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 is a transformer made up of?
Two coils of wire, primary and secondary, joined with an iron core
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What happens when an alternating p.d is applied to the primary coil?
When an alternating p.d applied across the primary coil the iron core magnetises and demagnetises quickly. This changing magnetic field induces an alternating p.d in the secondary coil
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What happens if the secondary coil is part of a complete circuit?
This causes a current to be induced
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What is the ratio between primary and secondary p.d's the same as?
The ratio between the number of turns on the primary and secondary coils
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Why is iron used?
It is easily magnetised
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Describe what Step-Up Transformers do
They increase the potential difference and have more turns on the SECONDARY coil than the primary
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Describe what Step-Down Transformers do
They decrease the potential difference and have more turns on the PRIMARY coil than the secondary
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State a useful thing about the transformers equation
It can be used either way up, so Input P.D = No. of turns on primary coil can be on the bottom and top (switch)
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If you assume Transformers are 100% efficient what is the equation you can use?
P.D across secondary coil x Current through secondary coil = P.D across primary coil x Current through secondary coil
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Other cards in this set

Card 2

Front

What is a magnetic field?

Back

A region where other magnets or magnetic materials i.e Iron experience a force - This is a NON CONTACT force

Card 3

Front

Describe the magnetic field lines of a bar magnet

Back

Preview of the front of card 3

Card 4

Front

How would the magnetic field lines change if a) Two South Poles were placed near each other b) A North and South pole

Back

Preview of the front of card 4

Card 5

Front

Complete the sentences: The field lines show ____ way a force would ____ on a ____ pole if it was put at that ___ in the field.

Back

Preview of the front of card 5
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