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GCSE Physics > Magnetism > Flashcards

Magnetism Flashcards

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1
Q

What is a magnet?

A

Any material or object that produces a magnetic field.

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2
Q

All magnets have two ends, what are these called?

At what point of the magnet is the place where the magnetic force the strongest?

A

The two ends are poles (opposite poles attract and like poles repel)

The poles of the magnet are the place where the magnetic field is strongest.

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3
Q

Describe what happens when two magnets are brought close to each other:

A
  • They exert a force on each other; a force of attraction or repulsion.

^This force of attraction/repulsion is a non-contact force because the magnets don’t need to be touching for the force to be experienced by them.

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4
Q

What is a permanent magnet?

A

A magnet that produces its own magnetic field.

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5
Q

State an example of a permanent magnet:

A

A bar magnet.

So if we bring two permanent magnets close to each other, they can attract or repel each other depending on the direction of the poles.

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6
Q

What is an induced magnet?

A

An object that becomes a magnet ONLY when it is placed in a magnetic field.

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7
Q

True or false, induced magnets can cause a force of repulsion.

A

FALSE! Induced magnets always cause and experience a force of ATTRACTION ONLY - NEVER REPULSION.

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8
Q

If we have a permanent magnet producing a magnetic field, what happens if we place a magnetic material in this field?

A
  • The magnetic field from the permanent magnet causes the material to become and induced magnet.
  • The induced magnet now experiences a force of attraction with the permanent magnet.

^The moment the permanent magnet is removed, the induce magnet loses its magnetism so is no longer a magnet. Bc the magnetic field has been removed.

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9
Q

What term do we use to describe a magnetic material that gains and loses magnetism slowly?

A

A magnetically HARD material (think of it like hard to gain and lose magnetism so loses it slowly)

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10
Q

What term do we use to describe a magnetic material that gains and loses magnetism quickly?

A

A magnetically SOFT material

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11
Q

State the 4 types of magnetic materials:

A

Nickel, Iron, Cobalt, Steel.

N.I.C.S

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12
Q

What is meant by a magnetic field?

A

The region around a magnet where the magnetic force acts on another magnet or on another magnetic material.

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13
Q

True or false, induced magnets are always attracted to the permanent magnet.

A

True, when a magnet comes close to a magnetic material, the force exerted on the magnetic material will always be one of attraction.

So induced magnets are always attracted to permanent magnets regardless of the poles.

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14
Q

Magnetic fields: Draw one

RULES:

A
  • The direction of magnetic fields always point from North to South.
  • The strength of the magnetic field decreases as the distance from the magnetic field increases.
  • Magnetic field lines must NEVER touch!!!
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15
Q

How can we determine the direction of a magnetic field?

A

Using plotting compasses.

^The plotting compass contains a small bar magnet- if we place the compass near the bar magnet then we can draw the magnetic field.
^On the bar magnet, the compass needle points in the direction of the Earth’s magnetic field when there is no magnet nearby.

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16
Q

True or false, magnetic field lines determine the strength and the direction of an object?

A

True.

  • The strength is determined by the spacing of the lines.
  • The direction
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17
Q

The direction at any point on a magnetic field is given by the what?

A

By the direction at which a magnetic force will act on another North Pole placed on that point.

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18
Q

How can we use a plotting compass to determine the shape and the direction of a magnetic field?

A
  • Place the compass near the north pole of the bar magnet and draw a dot here.
  • Draw a cross at the North Pole of the compass.
  • Move the compass so that the South Pole of the compass is on the previously drawn cross.
  • Draw another cross at the North Pole of the compass.
  • Continue doing this until we have a complete magnetic field line and we can connect all the dots with a line.
  • Show the direction of the field line with an arrow, from the N pole to the S pole.
  • Repeat this process starting at different points of the bar magnet.
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19
Q

When we hold a bar magnet away from any magnet, the needle always points in what direction?

What does this tell us?

A

The North - South direction.

That the Earth has its own magnetic field due to its core.

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20
Q

True or false, when a current flows through a wire, a magnetic field is produced about that wire?

How can we prove this?

A

True.

We can prove this by using a compass.

When the current is switched off the compass points from N-S.
When the current is now switched on the compass needle deflects in the direction of the magnetic field.

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21
Q

What does the strength of a magnetic field produced by a wire depend on?

What two factors?

A
  • current size in the wire; a larger current will produce a stronger magnetic field.
  • The distance from the wire.
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22
Q

If we change the direction of the current by reversing the direction of the conventional current, what else is also change?

A

The direction of the magnetic field.

The conventional current direction always aligns with the direction of the magnetic field.

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23
Q

How can we work out the direction of a magnetic field produced by a wire?

A

The right hand grip rule.

The thumb shows the direction of the conventional current, up or down.

The direction of your fingers shows the direction of the magnetic field :)

  • Practice!
24
Q

How can we increase the strength of a magnetic field due to a current flowing through a wire?

What happens when we turn on the current of a solenoid?

A

By coiling the wire.

This coiled wire is called a solenoid.

We get a strong and uniform magnetic field inside the solenoid.

This magnetic field is similar to that of a bar magnet.

25
Q

What are the 3 ways we can increase the magnetic field strength produced by a solenoid?

A
  • By increasing the current flowing through the wire.
  • By increasing the number of turns of the wire.
  • By placing a soft iron core in the centre of the solenoid.
26
Q

What is a solenoid that contains an iron core called?

A

An electromagnet.

27
Q

Draw the magnetic field of a solenoid.

A
28
Q

Explain how an electromagnetic device can be used to turn a high voltage circuit of and on.

In a relay switch.

A

The electromagnetic device is the low voltage circuit that we can use to turn on a high voltage circuit that is too unsafe to turn on otherwise..

  • The low voltage circuit with the electromagnetic device is switched on.
  • There is now a current that flows thru the electromagnetic circuit.
  • This produces a magnetic field around the electromagnet
  • The magnetic field attracts the iron block above the electromagnet, in the high voltage circuit.
  • This causes the contacts to close which switches on the high voltage circuit.
29
Q

How can we use an electromagnetic device in a doorbell?

A
  • The switch of the doorbell is closed when the buzzer is pressed.
  • This causes a current to flow through the circuit and therefore the electromagnet.
  • A magnetic field is produced by and around the electromagnet which causes the iron contact to be attracted and move towards the electromagnet.
  • This causes the clapper to also move and hit the door bell.
  • At the same time the circuit is now broken so there no longer a current flowing anymore.
  • So there is no more magnetic field and no more attraction between the iron contact and the electromagnet, so it springs back to its original position.
30
Q

What is the motor effect?

A

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other and this force results in the motor effect.

This force pushes the conductor out of the magnetic field.

^The two magnetic fields interact.
^This interaction results in a force acting on the wire which will eventually push it out of the field.
^To experience the full force, the wire has to be at exactly 90 degrees to the full force.

31
Q

An exam question may ask you to work out the direction of the force due to the motor effect, how would you do this?

A

Using Fleming’s LEFT hand rule.

First finger = direction of the mag. field

Second finger = direction of current.

Thumb = direction of the force (motion of force)

32
Q

How do we calculate the size of the force produced by the motor effect?

A

F = BIL

Force = Magnetic flux density x Current x length

Force, F = Newtons
Magnetic flux density, B = Tesla
Current, I = Amperes
Length, l = metres.

33
Q

What is magnetic flux density measured in?

Magnetic flux density is the same as saying?

A

T, Tesla

Magnetic field strength.

34
Q

What are the factors which affect the size of a force?

A

(think of BIL)

  • Magnetic flux density
  • Current size&raquo_space;don’t just say current
  • length of the wire
35
Q

Give an example of where the motor effect may be useful

A

In electric motors.

36
Q

Explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.

A
  • A loop of wire carrying a current is placed in a magnetic field.
  • The wire experiences a force on both the left and right sides of it and these forces act in opposite directions.
  • Force on LHS acts upwards and force on RHS acts downwards.
  • So now there is a moment on the LHS and a moment on the RHS side.
  • This causes the loop to rotate in the clockwise direction.
  • When the loop reaches 90 degrees it will stop rotating.
  • To solve this issue we use a split-ring commutator to switch the direction of the current each time the loop passes 90 degrees so the loop continues to rotate clockwise.
37
Q

How does a split-ring commutator allow for a motor to keep rotating?

A

By reversing the direction of the current every 180 degrees.

38
Q

Explain how a moving coil loudspeaker works to convert electrical signals into sound?

A
  • The current passing through the coil generates a magnetic field which interacts with the magnetic field of the permanent magnet.
  • This interaction produces a resultant force which causes the cone to move.
  • Now when the current is reversed the direction of the RF acting on the cone reverses too.
  • This causes the cone to move in and out which generates sound waves.
39
Q

Changing the FREQUENCY of the AC supply:

A

Changes the frequency at which the cone vibrates and thus the PITCH.

Higher frequency of AC = more vibrations so higher pitch.

40
Q

If we increase the size of the current:

A

We increase the amplitude of the vibrations and the volume of the sound produced.

41
Q

When is potential difference induced in an electrical conductor/wire?

A
  • When it is over THROUGH a magnetic field.

^A potential difference is induced at the ends of the conductor.
^Each time the conductor stops moving through the wire, the p.d is lost.
^Each time the wire move through the magnetic field, the potential difference reverses, so may be - + up then + - down.

42
Q

When is a current induced in an electrical circuit that is part of a wire and this is being passed through a magnetic field:

What is this known as?

A
  • If the conductor is part of an electrical circuit and this is being moved through the magnetic field, an electrical current is induced.

This is known as the generator effect.

^The direction of current switches when the direction of movement switches.
^If the movement stops then the current also stops.

43
Q

Two conditions that may cause the generator effect:

A
  • If a conductor, part of an electrical circuit is moved through a magnetic field.
  • If the magnetic field around an object changes

^Generator effect is only seen when:

The wire passes thru the magnetic field. If a wire moves along then we will not get an induced p.d or a current.

44
Q

What does the size of the induced p.d/induced current depend on? x3

A
  • The strength of the magnetic field
  • How fast the wire is being moved through the magnetic field
  • The shape of the wire, ie if it is in a coil induced p.d increases.

^The greater the number of turns on a coil, the greater the induced p.d and current.

45
Q

When we move a magnet into a coil of wire, a current is induced in the wire.

What can we say about the induced current?

A

The induced current creates its own magnetic field which opposes the movement of the magnet.

So when we insert a North Pole magnet into the coil, that end of the coil also becomes a North Pole and repels the magnet, making it harder for the magnet to get in.

When pull the magnet out, that end of the coil becomes a South Pole which attracts the magnet making it harder to pull out.

By doing this work is done, energy is transferred from the movement of the magnet into the movement of the current.

46
Q

How does a moving coil microphone work?

A
  • The coil of wire is attached to the diaphragm.
  • When sound waves hit the diaphragm they cause it to vibrate which causes the coil of wire to move in and out of the magnetic field.
  • This induces a p.d across the ends of the wire which changes as the wire moves in and out of the mag. field.
  • The frequency of the changing p.d is the same as the frequency of the sound waves.
  • So the changing pattern in the pd is input into an amplifier and then a moving coil loud speaker.
47
Q

A basic transformer consists of:

A
  • A primary coil and a secondary coil wrapped around an iron core.
48
Q

Why is iron used as the core of a transformer?

A
  • Because it is easily magnetised.
49
Q

How does a transformer work?

A
  • The primary coil of a transformer is connected to an AC
  • As the current from the AC flows thru the primary coil it generates a changing magnetic field.
  • This magnetic field is transmitted along the iron core to the secondary coil.
  • When the changing magnetic field passes through the secondary coil it induces a p.d
50
Q

The input pd causes and alternating current.

Explain why there is an alternating current in the output when the transformer is connected to the circuit.

3 marks

A

A changing magnetic field around the primary coil is is caused by the AC the coil is connected to.

This change in the direction of the mag field occurs now through out the iron core.

This now induces an alternating potential difference about the secondary coil and therefore an alternating current around it.

51
Q

How do step up transformers increase the potential difference of an electricity supply?

A
  • The secondary coil has more coils than the primary coil so the p.d is stepped up.
  • Vs > Vp
52
Q

How do step down transformers decrease the potential difference of an electricity supply?

A
  • The secondary coil has less coils than the primary so the p.d is stepped down.
  • Vs < Vp
53
Q

If transformers are 100% efficient, what can we say about the electrical power output in relation to the electrical power input.

A

They are equal.

54
Q

Transformer equation:

A

1) Vp/Vs = Np/Ns

Vp = pd in primary coil
Vs = pd in secondary coil
Np = number of coils in primary coil
Ns = number of coils in the secondary coil

(use this equation and rearrange to work out what u need to work out)

VpIp = VsIs

Vp = pd in primary coil
Ip = current in pc.
Vs = pd in secondary coil
Is = current in sc

^This equation shows that power in a transformer must always be conserved.
^This only applies if the transformer is 100% efficient.

55
Q

Describe the advantages of using
power transmission at high potential differences.

A
  • Electrical power is transmitted from power stations to homes through high voltage cables.
  • Power = IV, if we want to use transmit a large amount of power we can use a large current or a large pd.
  • Using a large current means a lot of the power will be wasted as heat.
  • So instead of using a large current we use a large pd bc LESS POWER IS BEING LOST AS HEAT.

GCSE Physics (4 decks)

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