# Motors, generators and transformers - Physics

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• Created by: becky.65
• Created on: 02-01-16 14:02
What is the space around a magnet called?
A magnetic field
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Where is the Earth's magnetic field most concentrated?
At the magnetic north pole and south pole
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What is a plotting compass?
A tiny magnet that lines up with the Earth's magnetic field and its tip points to the Earth's magnetic north pole
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What are the lines of force of a magnetic field?
The lines along which a plotting compass would point
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When two magnets are held near each other what do like and unlike poles do?
Like poles repel and unlike poles attract
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What happens when a current flows through a wire?
A magnetic field is produced around the wire
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What shape are the magnetic field lines that surround the wire?
Circles
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What are the circles in relation to the wire?
They are centred on the wire and are in a plane perpendicular to the wire
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What does increasing the current do?
Make the magnetic field stronger
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How is an electromagnet made?
By wrapping insulated wire around a piece of iron called the core
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How do electromagnets work?
When a current flows through the wire the iron becomes strongly magnetised and when the current is switched off the iron loses its magnetism. The temporary magnetism makes electromagnets very useful
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What are some devices electromagnets are used in?
Scrapyard cranes, circuit breakers, electric bell and relays
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What is the motor effect?
When we place a wire carrying an electric current in a magnetic field, it may experience a force and this is called the motor effect
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When is the force at its maximum?
If the wire is at an angle of 90 to the magnetic field
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When is the force at zero?
If the wire is parallel to the magnetic field
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What is Fleming's left-hand rule used to determine?
The direction of the force
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How can the size of the force be increased?
By increasing the strength of the magnetic field and/or increasing the size of the current
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How is the direction of the of the force on the wire reversed?
If either the direction of of the current or the direction of the magnetic field is reversed
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How is the speed of a motor increased?
By increasing the size of the current
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How can the direction of the motor be reversed?
By reversing the direction of the current
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When a current passes through the coil, why does the coil spin?
A force acts on each side of the coil due to the motor effect and the force on one side of the coil is in the opposite direction to the force on the other side
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What does the 'split-ring' commutator in a motor do?
Reverses the direction of the current around the coil every half turn
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Why is the coil always pushed in the same direction?
Because the sides swap over each half-turn
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What is the generator effect?
The effect of inducing a potential difference using a magnetic field
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What is the process of electromagnetic induction?
When an electrical conductor such as a wire or a coil 'cuts' through magnetic field lines, a potential difference is induced across the ends of the conductor
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When is the only time a potential difference is induced?
While there is movement of the conductor relative to the magnetic field and the conductor cuts across the lines of force of the magnetic field
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When does a current pass through a conductor?
If it is part of a complete circuit
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How is the size of the induced pd increased?
By increasing the speed of movement, the strength of the magnetic field and the number of turns on the coil
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What does a simple alternating current generator consist of?
A rectangular coil which is forced to spin in a magnetic field. The coil is connected continuously to a centre-reading meter via metal 'brushes' that press on two metal slip rings
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What happens when the coil turns steadily in one direction?
An alternating potential difference is induced in the coil
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When is the magnitude of the induced potential difference at its greatest?
When the plane of the coil is parallel to the direction of the magnetic field and the sides of the coil cut directly across the magnetic field lines
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When is the magnitude of the induced potential difference zero?
When the plane of the coil is perpendicular to the magnetic field lines and the sides of the coil do not cut the field lines
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What is the difference between an alternating current generator and a direct current generator?
A direct current generator has a split-ring commutator
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What happens as a result of the split-ring commutator?
It reconnects the coil the opposite way round in the circuit every half-turn and this happens each time the coil is perpendicular to the magnetic field lines. Therefore the induced pd does not reverse its direction and varies from zero to a maximum
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What happens in a cycle dynamo?
A magnet spins near the end of a fixed coil so an alternating pd is induced in the coil
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What does a transformer consist of?
Two coils of insulated wire, called the primary coil and the secondary coil. These coils are wound on to the same iron core.
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What happens when an alternating current passes through the primary coil?
It produces an alternating magnetic field in the core. This field continually expands and collapses
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Why are the coils of wire insulated?
So that the current does not short across either the iron core or adjacent turns of wire, but flows around the whole coil
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Why is the core made of iron?
So it is easily magnetised and demagnetised
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What does a step-up transformer do to the potential difference
It makes the pd across the secondary coil greater than the pd across the primary coil. Its secondary coil has more turns than its primary coil.
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What does a step-down transformer do to the potential difference?
It makes the pd across the secondary coil less than the pd across the primary coil. Its secondary coil has fewer turns than its primary coil
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What core does a switch mode transformer have?
A ferrite core
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Compared with traditional transformers, what do switch mode transformers do?
Operate at a much higher frequency, are lighter and smaller an use very little power when there is no device connect across its output terminals
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When are step-up transformers used in the National Grid?
To step-up the pd from power stations to grid cables
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Why does the pd at which electrical energy is transmitted across the Grid need to be higher?
So the smaller the current through the cables so the smaller the energy wasted
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When are step-down transformers used in the National Grid?
To step-down the pd from the grid cables so that it is safe to be used by consumers
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## Other cards in this set

### Card 2

#### Front

Where is the Earth's magnetic field most concentrated?

#### Back

At the magnetic north pole and south pole

### Card 3

#### Front

What is a plotting compass?

### Card 4

#### Front

What are the lines of force of a magnetic field?

### Card 5

#### Front

When two magnets are held near each other what do like and unlike poles do?