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Physixs > P4 > Flashcards

P4 Flashcards

1
Q

What is a magnetic field?

A

A region where a magnetic material experiences a force.

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2
Q

What are magnetic materials?

A

Iron, nickle or cobalt

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3
Q

What are magnetic field lines?

A

They show the size and direction of magnetic fields. They always point from north to south.

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4
Q

What does it mean if magnetic field lines are closer together?

A

The magnetic field is stronger.

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5
Q

What is magnetic flux density?

A

The strength of a magnetic field, measured in teslas (T).

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6
Q

How can you see magnetic field patterns?

A

You can use iron filings, which align themselves to field lines, or a compass.

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7
Q

How can you use a compass to plot magnetic field patterns?

A

Put a magnet on paper and place the compass next to it. The needle will point in the direction of the field line at the position.

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8
Q

Why is using a compass better than iron filings for plotting magnetic fields?

A

Drawing the field will mean that the field lines are there after the magnet is removed. A drawing also showing the direction of field lines.

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9
Q

How are compasses evidence that the Earth has a magnetic north and south pole?

A

Compasses always point North (which is the south magnetic pole) when they aren’t near a magnet. This is evidence that Earth has a magnetic north and south pole and therefore a magnetic core.

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10
Q

What is induced magnetism?

A

When a magnet is brought near a magnetic material, the material acts a magnet. This is induced by the original magnet.

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11
Q

What is the difference between permanent and induced magnets?

A

Induced magnets usually lose their magnet once the magnet is moved away.

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12
Q

What does an electric produce in a material?

A

A magnetic field - the stronger the current, the stronger the field

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13
Q

What is a magnetic field around a straight current-carrying wire made up of?

A

Concentric circles with the wire in the centre.

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14
Q

How can you find the direction of the magnetic field around a straight wire?

A

The direction of the current is your thumb and the direction of the field lines is the direction of your fingers.

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15
Q

What is the magnetic field in the centre of a flat circular coil of wire?

A

There are concentric ellipses (stretched circles) of magnetic field lines around the coil.

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16
Q

What is a solenoid and what does it do?

A

Lots of coils of wire joined together. They have a stronger magnetic effect - the field inside a solenoid is strong and uniform.

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17
Q

What ways are there to increase the magnetic effect of an electromagnet?

A

Increase the current
Increase the number of turns (but maintain the length)
Decrease the length (but maintain the number of turns)
Add an iron core

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18
Q

What happens when a current-carrying condcutor goes between magnetic poles?

A

The two magnetic fields affect one another.

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19
Q

What angle does a wire have to be between two magnetic fields to experience the full force?

A

90°

20
Q

How can you find the direction of the force on a current-carrying conductor?

A

Fleming’s left hand rule.
First finger - Field
Second finger - Current
Thumb - Force

21
Q

What is another way of finding the direction of force on a current-carrying conductor?

A

Apply a current to a set of rails inside a horseshoe magnet. A bar is placed on the rails, which generates a force and rolls the bar along the rails.

22
Q

How can you find the force on a conductor carrying a current, using magnetic flux density, current and length?

A

Force on a conductor carrying a current (N) = Magnetic flux density (T) * Current (A) * Length (m)

23
Q

How does an electric motor work?

A

A current-carrying coil sits between two poles of a magnet.
The current flows in different directions on either side of the coil, and each side of the coil is perpendicular to the magnetic field, so each side experiences force in different directions.
The coil rotates.

24
Q

What does the split-ring commutator do in an electric motor?

A

It swaps the contacts every half turn to keep an electric motor rotating in the same direction.

25
Q

How can you change the direction of an electric motor?

A

By reversing the current or swapping the magnetic poles.

26
Q

How can you speed up an electric motor?

A

Increase magnetic flux density (field strength)

27
Q

What is a loudspeaker made of?

A

A coil of wire surrounded by one magnet. Another magnet is inside the coil.

28
Q

How does a loudspeaker work?

A

Alternating current electrical signals are fed to the coil of wire, which is wrapped around the based of a cone.
The interaction between the field and the current in the coil forces the coil to move in one direction. As it’s AC, the current changes direction, and the coil moves back and forth.

29
Q

What is electromagnetic induction?

A

When a voltage is induced across a conductor which is experiencing a change in its external magnetic field.

30
Q

When does a conductor experience change in its external magnetic field?

A

When it passes through magnetic field lines. If the conductor is part of a complete circuit, the induced voltage will result in a current in the circuit.

31
Q

What does a current do during electromagnetic induction?

A

It produces its own magnetic field. This field opposes the original change in the external magnetic field.

32
Q

What are the two situations in which electromagnetic induction occurs?

A

An electrical conductor and a magnetic field move relative to each other.
The magnetic field through an electrical conductor changes.

33
Q

What are generators?

A

The opposite of motors - they use the relative motion of a conductor and magnetic field to induce a voltage and current.

34
Q

How do you increase the current in a generator?

A

The current induced will be greater if there are more turns on the coil, the magnetic flux density is increased or the speed of rotation is increased.

35
Q

What are two types of generators?

A

Dynamos - direct current

36
Q

How does an alternator work with a magnet?

A

This produces an alternating current if the coil is part of a complete circuit.

37
Q

How does an alternator work without a magnet?

A

Slip rings at the end of the coil remain in contact with brushes that are connected to the rest of the circuit. This means that they contacts don’t swap every half turn, producing AC.

38
Q

How does a dynamo work?

A

A split ring commutator swaps the connection every half turn to keep the current flowing in the same direction, producing DC.

39
Q

How does a microphone work?

A

Sound waves cause the diaphragm to move back off and forth. As it moves, the coil of wire moves, inducing a voltage at the ends of the coil.

40
Q

What do transformers do?

A

They change the size of the voltage of an alternating current.

41
Q

How do the coils work in a transformer?

A

They all have two coils, the primary and secondary coils, joined by an iron core. When an alternating voltage is applied across the primary coil, it produces an alternating magnetic field.

42
Q

How does the alternating magnetic field in a transformer affect the second coil?

A

It induces a voltage in the secondary coil.

43
Q

How do you calculate power in a transformer, with voltage and current?

A

Power (W) = Voltage (V) * Current (C)

44
Q

How is power transferred in a transformer?

A

Power in primary coil = Power in secondary coil

45
Q

What is a step-down transformer?

A

It steps the voltage down. They have more turns on the primary coil than the secondary.

46
Q

What is a step-up transformer?

A

It steps the voltage up. They have more turns on the secondary coil than the primary coil.

47
Q

What is the relationship between the voltage on both coils and the turns on both coils?

A

Voltage across primary coil ÷ Voltage across secondary coil = Number of turns in primary coil ÷ Number of turns in secondary coil

Physixs (8 decks)

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