# P3.3

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What are the ends of a magnet called?
Magnetic poles: north + south. The region around the magnet (in which a piece of iron/steel will be attracted to it) =its magnetic field.
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How can lines of force/magnetic field lines be indicated?
Iron filings placed near a magnet will form a pattern of lines that loop from one pole to another. A plotting compass placed in the magnetic field will always point along a field line.
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By wrapping insulated wire around a piece of iron (core). When a current flows through the wire, the iron becomes strongly magnetised because whenever a current flows through a wire a magnetic field is produced around the wire.
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How are electromagnets useful?
Their temporary magnetism makes them useful (as when the current is switched off it loses its magnetism). Electromagnets are used in scrapyard cranes, circuit breakers, electrical bells + 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. The force is a maximum if the wire is at a right angle (90) to the magnetic field + zero if the wire is parallel to the magnetic field.
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What does the first finger, second finger and thumb represent in the Fleming’s left hand rule?
First Finger represents magnetic field (north to south). Second finger represents the current (positive to negative). Thumb represents the direction of the force.
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How can the size of the force acting on a wire in the motor effect be increased and how can the direction of the force be reversed?
By increasing the strength of the magnetic field or increasing the size of the current. Direction of the force on the wire is reversed if either direction of current is reversed or direction of magnetic field is reversed.
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How can a speed of the motor be increased and the direction of a motor be reversed?
Speed of motor increased by increasing current. Direction of motor reversed by reversing direction of current.
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How is the direction of induced pd reversed (electromagnetic induction)?
If the direction of movement of the wire/coil is reversed or the polarity if the magnet is reversed. A is only induced while there is movement.
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Why does a coil spin when a current passes through it – motor?
As a force acts on either side of the coil due to the motor effect + as the force on 1 side of the coil is in the opposite direction to the force on the other side.
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What is the purpose of the ‘split-ring” commutator in a motor?
It reverses the direction of the current around the coil every half-turn. As the sides swap over each half-turn, the coil is always pushed in the same direction.
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What is electromagnetic induction?
Process of electromagnetic induction: If a magnet is moved into a coil of wire, potential difference is induced across the ends of the ends of coil. If the wire or coil is part of a circuit, a current is passed through it.
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How can potential difference be induced?
If an electrical conductor “cuts” through magnetic field lines, a pd is induced across the ends of the conductor. If a magnet is moved into a coil of wire = pd across ends of coil. A pd is only induced when there is movement.
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How is the size of induced pd increased (electromagnetic induction)?
By increasing the: speed of movement, strength of the magnetic field or number of turns on the coil.
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What does a transformer consist of?
2 coils of insulated wire, called the primary and the secondary coil are wound on to the same iron core. When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core-continually expands+ collapses
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How is potential difference induced in a transformer?
The alternating magnetic field lines pass through the secondary coil + induce an alternating potential difference across its ends. If the secondary coil is part of a complete circuit an alternating current is produced.
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Why are the coils of wire insulated in a transformer?
The coils of wire are insulated so that the current does not short across either the iron core or adjacent turns of wire, but flows around the whole coil. The core is made of iron so its easily magnetised.
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How are step-up transformers used in the National Grid?
Makes the pd across the secondary coil greater than the pd across the primary. Its secondary coil has more turns than its primary coil.
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How are step-down transformers used in the National Grid?
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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How is a switch mode transformer different to a traditional transformer?
Switch mode: has a ferrite core, operates at a much frequency, is lighter and smaller + uses very little power when there is no device connected across its output terminals.
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Why does the national Grid use step-up transformers?
As the higher the pd at which electrical energy is transmitted across the Grid, the smaller the energy wasted in the cables. Step-down transformers are used to reduce the pd so that it is safe to be used by consumers.
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What equation links the pd, the number of turns on the primary + secondary coils?
Vp (voltage across primary coil) / Vs = N (number of turns
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## Other cards in this set

### Card 2

#### Front

How can lines of force/magnetic field lines be indicated?

#### Back

Iron filings placed near a magnet will form a pattern of lines that loop from one pole to another. A plotting compass placed in the magnetic field will always point along a field line.

### Card 4

#### Front

How are electromagnets useful?

### Card 5

#### Front

What is the motor effect?