P7: Magnetism & electromagnetism Flashcards Preview

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Flashcards in P7: Magnetism & electromagnetism Deck (56)
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1

Name 4 magnetic materials.

Iron, steel, nickel, cobalt.

2

What is a magnetic field?

A region where magnets or magnetic materials experience a non-contact force (either attractive or repulsive).

The field is generated by a magnet and becomes stronger with proximity to it.

3

Magnetic fields are drawn using magnetic field lines. Which direction do the arrows on these lines always point?

North to south (or the direction in which a north pole would experience a force).

4

How do magnetic field diagrams show the strength of a magnetic field?

The closer together the magnetic field lines are, the stronger the field.

5

Where on a magnet is its field strongest?

At the poles.

6

Like poles...

...repel.

7

Unlike poles...

...attract.

8

What does "magnetic flux density" mean?

Strength of a magnetic field.

9

What is the unit for magnetic flux density?

Tesla (T).

10

What are the 2 types of magnets?

Permanent and induced.

11

What are permanent magnets?

Magnets which constantly produce their own magnetic field.

12

What are induced magnets?

Magnetic materials which turn into magnets (with their own poles and magnetic field) when placed in a magnetic field. (When removed from the field, they quickly lose most/all of their magnetism).

13

Do induced magnets have poles in the same way that permanent magnets do?

Yes.

14

How does the force between two permanent magnets compare to the force between a permanent and induced magnet?

The force between two permanent magnets can be attractive or repulsive (depending on orientation), whereas the force between a permanent and induced magnet is always attractive.

15

How do compass needles point in the direction of the magnetic field they are in?

The needle contains a tiny bar magnet, whose north pole is attracted to the south pole of any magnet it's near, making the needle move.

16

Why do compasses point north (when not near a strong magnet)?

Compass needles contain a tiny bar magnet, whose north pole is attracted to the south pole of the earth's core. So the needle points to the North Pole.

(magnetic South pole = geographical North pole)

17

The force between a magnet and a magnetic material is always what?

Attractive.

18

How does the behaviour of compasses provide evidence that the core of the Earth is magnetic?

A magnetic compass contains a small bar magnet. The Earth has a magnetic field, and the compass needle points in the direction of this field.

19

Describe how to plot the magnetic field pattern of a magnet using a compass.

Move a compass around a magnet, tracing the needle's position on paper to plot field lines. The north pole of the compass' magnet points, along the field line, to the south pole of the other magnet.

20

What is created when a current flows through a wire?

A magnetic field around the wire.

21

When a current flows through a wire, a magnetic field is created around it. What is the shape of the field?

The field is made of concentric circles perpendicular to the wire (with the wire in the centre).

22

When a current flows through a wire, a magnetic field of concentric circles, perpendicular to the wire, is created around it. How could you find out the direction of the field?

Use the right-hand rule: point your thumb in the direction the current flows. Curl your fingers - the direction they curl is the direction of the magnetic field.

23

Changing the direction of the current flowing through a magnetic field also changes what?

The direction of the magnetic field around it.

24

What does the strength of a magnetic field around a current-carrying wire depend on?

The current and distance from the wire.

25

What is a solenoid?

A coil of insulated wire.

26

What are solenoids used for?

Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform.

27

Describe the shape of the magnetic field around a solenoid.

The magnetic field outside the core of a solenoid has a similar shape to that of a bar magnet. Inside the core, the field is strong and uniform.

28

What is an electromagnet? How do these compare to solenoids?

1) A solenoid with an iron core, and a magnet whose field can be turned on or off using an electric current. 2) Adding the iron core increases the strength of a solenoid's magnetic field.

29

How can you work out the direction of the magnetic field around a solenoid?

Use your left hand to curl your fingers in the direction which the current curls. Your thumb will point in the direction of the magnetic field lines above and below the solenoid.

30

How are electromagnets used in scrap yards?

They are used in cranes, and are turned on to pick up and move objects made from magnetic materials (iron, steel, etc), and turned off to drop the object.

31

How are electromagnets used as switches in circuits?

32

What is the motor effect?

A current-carrying wire in a magnetic field (and the magnet producing it) will experience a force perpendicular to both the direction of the current and the magnetic field.

This is because current-carrying wires produce a magnetic field, making them by definition magnets, and all magnets experience a force when in a magnetic field.

33

At what angle to the direction of a magnetic field does a current-carrying wire have to be in order to experience the full force resulting from the motor effect?

90º. For angles in between 0º and 90º, the wire experiences some force, and, for 0º, it experiences none.

34

How would you increase the magnitude of a force resulting from the motor effect?

  1. Increasing the magnetic flux density of the magnetic field that the wire is in
  2. Increasing the current passing through the wire
  3. The length of the conductor (wire)

35

In what circumstance can you use this (given) equation?

force on a current-carrying conductor = magnetic flux density x current x length of conductor

F = BIl 

Only when the direction of the current is at 90º to the direction of the magnetic field it's in.

36

force on a current-carrying conductor (90º to magnetic field) = magnetic flux density x current x length of conductor

F = BIl

How would you find the direction of this force?

Use Fleming's left hand rule (FBI):

 

37

Here is a simple electric motor. When there is a current in the coil, it rotates continuously. Explain why. (4 marks)

 

  1. The motor effect generates forces on either side of the coil, acting in opposite directions
  2. The coil is on an axle, so these forces cause the coil to rotate.
  3. The split-ring commutator allows the coil to keep rotating in the same direction.
  4. Every half turn, either side of the coil experiences a force in the opposite direction as before (or the same rotational direction) - so the coil rotates continuously.

38

Explain how loudspeakers/headphones (which are just tiny loudspeakers) use an alternating current to generate sound waves.

  1. The coil surrounds one pole of a permanent magnet, and is surrounded by the other.
  2. The motor effect causes a force to be exerted on the current-carrying coil, causing the cone to move.
  3. When the current reverses, the force acts in the opposite direction, so the cone moves in the opposite direction.
  4. Variations in current causes the air around the cone to vibrate, causing pressure variations that generate sound waves.
  5. Frequency of ac current = frequency of sound waves.

39

What is the generator effect?

If a conductor and magnetic field move in relation to each other, a potential difference is induced in the conductor.

If the conductor is part of a complete circuit, a current is also induced.

40

In what 2 ways could you induce a pd (by the generator effect)?

  1. Moving a conductor in a magnetic field.
  2. Moving a magnet in a coil of wire.

41

How would you increase the size of a pd (or current) induced by the generator effect?

You would need to change the rate at which the magnetic field is changing, by:

  1. Increasing the speed of movement (of the magnet or conductor).
  2. Increasing the magnetic flux density.

42

How could you generate an alternating current with a magnet?

You need a complete circuit. Either:

  1. Move the magnet backwards and forwards inside a coil of wire.
  2. Rotate the magnet inside a coil of wire.
  3. Continually reverse the polarity of the magnet.

43

How could you generate an alternating current by turning a magnet in a coil of wire?

  1. As the magnet turns, the magnetic field through the coil changes, inducing a pd (generator effect), so a current flows through the wire.
  2. Each time the magnet turns 180º, the direction of the magnetic field reverses, so the current in the coil reverses direction.
  3. As the magnet keeps rotating, an ac is produced.

44

Why does an induced current always oppose the change that makes it?

  1. The generator effect means a change in magnetic field generates a current in a wire.
  2. And, when a current flows through a wire, another magnetic field is generated around the wire.
  3. This latter magnetic field acts against the change that produced it - e.g. the movement of the wire / change in magnetic field.

45

Explain how microphones convert sound waves into electrical current.

Basically loudspeakers, in reverse:

  1. Sound waves hit the diaphragm, causing the coil to move.
  2. The coil is in the magnetic field - so a current is generated.
  3. The size of the current increases with the volume of the sound wave.
  4. So microphones convert the pressure variations of sound waves into electrical current.

46

Alternators generate __ current. Dynamos generate __ current.

  1. Alternating.
  2. Direct.

47

How do alternators generate alternating current?

• Generators either rotate a coil in a magnetic field, or a magnet in a coil.

• As the coil/magnet spins, a current is induced in the coil, which reverses direction every half turn.

• They have slip-rings and brushes which provide a continuous connection between the coil and ac meter.

• This means an alternating pd (and current) is produced.

48

How do dynamos generate direct current?

  1. Dynamos either rotate a coil in a magnetic field, or a magnet in a coil.
  2. As the coil/magnet spins, a current is induced in the coil, which reverses direction every half turn.
  3. They have split-ring commutators (rather than slip-rings and brushes), swapping the connection every half turn.
  4. This keeps the current flowing in the same direction - i.e. a direct current.

49

What is the function of transformers?

To change the magnitude of the pd of an alternating current.

50

What do step up and step down transformers do?

  1. Step up transformers increase the pd.
  2. Step down transformers decrease the pd.

51

Describe how step-up transformers increase the pd.

  1. Two coils of wire are joined with an iron core.
  2. When an alternating current/pd is applied across the primary coil, the iron core alternately magnetises and demagnetises.
  3. The changing magnetic field induces an alternating pd in the secondary coil - which is part of a complete circuit - so an ac is induced.
  4. Step-up transformers have more turns on the secondary coil than on the primary, meaning the pd is increased.

52

Explain how step-down transformers decrease the pd.

  1. Two coils of wire are joined with an iron core.
  2. When an alternating current/pd is applied across the primary coil, the iron core alternately magnetises and demagnetises.
  3. The changing magnetic field induces an alternating pd in the secondary coil - which is part of a complete circuit - so an ac is induced.
  4. Step-down transformers have fewer turns on the secondary coil than on the primary, meaning the pd is decreased.

53

Why are iron cores used in electromagnets and transformers?

Because iron is easily magnetised.

54

What can this (given) equation be used to find?

  1. Input pd
  2. Output pd
  3. # turns on primary coil
  4. # turns on secondary coil

55

What can this (given) equation be used to find?

  1. Power input/output (this will be the same - assume 100% efficient. P = VI)
  2. Input pd
  3. Output pd
  4. Input current
  5. Output current

56

In transformers, the ratio between the primary and secondary __ is equal to the ratio between the __.

In transformers, the ratio between the primary and secondary potential differences is equal to the ratio between the number of turns on the primary and secondary coils.