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Physics > 7. Electricity & Magnetism > Flashcards

7. Electricity & Magnetism Flashcards

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1
Q

What is a magnetic field?

A

A region of space where magnetic material can experience a force

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2
Q

What type of poles attract?

A

Unlike poles

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3
Q

What type of poles repel?

A

Like poles

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4
Q

What is inside a compass?

A

A tiny bar magnet

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5
Q

How does a compass show that the earth generates its own magnetic field?

A

When the compasses are not near a magnet they point north

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6
Q

Which direction do field lines point in?

A

North to South

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7
Q

What does it mean when field lines are closer together?

A

They are stronger

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8
Q

Do field lines ever touch or cross?

A

No

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9
Q

Where is the magnetic field strongest?

A

At the poles of the magnet

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10
Q

What do the direction of the field lines show?

A

The direction of the force that would act on a tiny N-pole placed at that point

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11
Q

What are magnetic materials made of?

A

Molecular magnets

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12
Q

What are molecular magnets?

A

Tiny magnets with N and S poles

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13
Q

What are domains?

A

Groups of molecular magnets

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14
Q

How are the molecular magnets when a material is unmagnetised?

A

The unmagnetised domains point in random directions

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15
Q

How are the molecular magnets when a material is magnetised?

A

Most molecular magnets are aligned

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16
Q

What does saturated mean when talking about a magnet?

A

When all molecular magnets are aligned

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17
Q

What happens when a magnet breaks?

A

Two magnets are simply made

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18
Q

Is iron a soft or hard magnetic material?

A

Soft

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19
Q

What are the properties of a soft magnetic material?

A
  • easy to magnetise

* will demagnetise easily (so useful in electromagnets)

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20
Q

What type of magnetic material is used for electromagnets?

A

Soft

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21
Q

Why is soft magnetic material used in electromagnets?

A

It is easy to magnetise and demagnetise

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22
Q

Is steel a hard or soft magnetic material?

A

Hard

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23
Q

What are the properties of a hard magnetic material?

A
  • harder to magnetise

* also hard to demagnetise

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24
Q

What is hard magnetic material used for?

A

Permanent magnets e.g. needles in compasses

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25
Q

What is an electromagnet?

A

A solenoid with an iron core inside

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26
Q

What is a solenoid?

A

A coil of wire

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27
Q

What happens when a current is passed along the wire in an electromagnet?

A

A magnetic field is created and as a result the iron core is magnetised

28
Q

What happens in an electromagnet when the current is switched off?

A

The iron bar loses it’s magnetism because it’s a temporary magnet

29
Q

What can the strength of the magnetic field in an electromagnet be increased by?

A
  • increasing the voltage or decreasing the resistance - to INCREASE THE CURRENT
  • increasing the number of turns on the coil
  • inserting an iron core
30
Q

Where are electromagnets used?

A
  • cranes

* in hospitals dealing with eye injuries - remove steel splinters

31
Q

How does an electric bell work?

A
  • bell-push completes circuit and current flows through electromagnet
  • iron is attracted towards electromagnet and hammer hits gong
  • movement breaks circuit so current stops and electromagnet is switched off
  • spring pulls iron back so contact is made and sequence repeats
32
Q

When will a wire NOT experience a force when placed in a magnetic field?

A

If it’s parallel to the direction of the field

33
Q

What does Fleming’s left hand rule show?

A

The direction of movement of the wire

34
Q

What does each finger represent in Fleming’s left hand rule?

A
  • index finger - direction of field from N to S
  • middle finger - direction of current from + to -
  • thumb - direction of movement
35
Q

How is the direction of the force in an electromagnet reversed?

A
  • reversing the direction of the current

* reversing the direction of the magnetic field

36
Q

How do loudspeakers work?

A
  • ac current is passed through coil which is attached to paper cone
  • current causes force on coil so paper cone moves
  • current reverses and force acts on cone in opposite direction
  • cone vibrates
37
Q

What is 1 tesla equal to?

A

1 newton per amp per metre

38
Q

What equation is used to calculate the force on a wire?

A

F=BIL

39
Q

What is the motor effect?

A

When a wire is placed in a magnetic field and experiences a force

40
Q

How does a d.c. motor work?

A

When a coil of wire is placed in a magnetic field and therefore the two sides of the coil experience a force due to the motor effect

41
Q

Why does the coil rotate in a d.c. motor?

A

The current of the opposite side of the coil is in the opposite direction, so the forces are in opposite directions

42
Q

Where are split ring commutators used?

A

In electric motors

43
Q

How do split ring commutators work?

A

Every half turn it breaks the current so the coil of wire keeps turning in the same direction

44
Q

What is an easy way to tell if a setup is a motor or a generator?

A

A motor will have a battery, a generator will not

45
Q

What is the generator effect?

A

The induction of a potential difference in a wire which is moving relative to a magnetic field

46
Q

How is a voltage induced in a wire using the generator effect?

A

• if a wire is moved downwards to cut through the magnetic field

47
Q

When is a voltage not induced using the generator effect?

A
  • if the wire is stationary

* if the wire is moved parallel to the field line

48
Q

How is the voltage induced increased using the generator effect?

A
  • move the wire faster

* use a stronger magnet

49
Q

How is the voltage induced reversed in a wire?

A
  • move the wire in the opposite direction

* reverse the magnetic field

50
Q

During electromagnetic induction, why do you have to do work when you push the magnet into the coil?

A

The electromagnet is repelling the magnet so energy is transferred

51
Q

During electromagnetic induction, why do you have to do work when you pull the magnet out of the coil?

A

The electromagnet is attracting the magnet so work is done against the force

52
Q

What happens when a magnet is moved into a coil of wire which is a part of a complete circuit?

A

A current is induced in the wire - the coil is now an electromagnet

53
Q

What is purpose of slip rings in an a.c. generator?

A

So the wire doesn’t twist as the coil rotates

54
Q

What is the energy transfer in an electric generator?

A

Kinetic to electrical

55
Q

What is the energy transfer in an electric motor?

A

Electrical to kinetic

56
Q

What is an a.c. generator also known as?

A

An alternator

57
Q

What is a d.c. generator also known as?

A

A dynamo

58
Q

Why does an alternator produce an a.c. output?

A
  • the two sides of the coil are always connected to the same side of the lamp
  • when the LHS of the coil moves past the top it reverses the direction of movement
  • so voltage and therefore current reverses
59
Q

Why does a dynamo produce a d.c. output?

A
  • the two sides the coil are connected to split rings so the two sides of the coil are not connected to the same side during a turn
  • every half turn the connections are reversed so when induced voltage is reversed, the current flowing is not reversed
60
Q

How does a microphone work?

A
  • a magnet is moved near a coil and a current is generated so the diaphragm is attracted to coil
  • diaphragm vibrates in response to incoming sound waves and coil vibrates
  • this induces a current in the coil (a.c.)
61
Q

What is a transformer made of?

A

A primary and secondary coil wrapped around an iron core

62
Q

Why is the iron core of a transformer laminated?

A

So no induced currents flow in the iron core

63
Q

What does it mean when an iron core is laminated?

A

It is made in layers of iron

64
Q

How does a transformer work?

A
  • a current flows in the primary coil and becomes an electromagnet
  • this magnetises the iron core which is linked to the secondary coil
  • a.c. voltage - magnetic field inside secondary coil will keep changing size and direction so a voltage is induced in the secondary coil
  • voltage will change direction and br an a.c. current
65
Q

What does the size of a secondary voltage in a transformer depend on?

A
  • primary voltage

* number of turns on each coil

66
Q

What is the equation to work out the voltage or number of turns in a transformer?

A

Vp/Vs = Np/Ns

Physics (11 decks)

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