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PAPER 2 : PHYSICS > P7 > Flashcards

P7 Flashcards

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1
Q

What are the ‘poles’ of a magnet?

A
  • the ends of the magnet, where the magnetic field is strongest
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2
Q

Describe the possible outcomes when two permanent magnets are brought together

A
  • the magnets will either attract or repel
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3
Q

2 magnets which would repel each other. What will you label each pole?

A
  • either N N or S S
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4
Q

Why is magnetism classed as a non-contact force?

A
  • the magnets do not have to touch to exert a force on each other
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5
Q

Which materials are magnetic?

A
  • iron (and steel), nickel and cobalt
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6
Q

What are the two categories of magnet?

A
  • permanent and induced (temporary)
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7
Q

What is a permanent magnet?

A
  • a magnet which produces its own magnetic field

- they stay magnetic

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8
Q

What is an induced (temporary) magnet?

A
  • materials which are only magnetic if they are put into the magnetic field of a permanent magnet
  • once they are removed from the field, they lose most, if not all, of their magnetism
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9
Q

What are the key points of permanent magnets?

A
  • they are always magnetic
  • they can attract object
  • they can repel an object
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10
Q

What are the key points induced magnets?

A
  • they aren’t always magnetic
  • they can attract object
  • they cannot repel an object
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11
Q

What is a compass?

A
  • a magnet which lines itself up with the Earth’s magnetic field
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12
Q

What does the behaviour of a compass give evidence for?

A
  • it shows the Earth has a magnetic core
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13
Q

What is meant by the term ‘magnetic field’?

A
  • the area around a magnet where a magnetic force can be felt
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14
Q

How can a magnetic field be shown?

A
  • by drawing lines
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15
Q

What do the arrows represent?

A
  • the direction of the magnetic field
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16
Q

How do you know where the magnetic field is strongest?

A
  • the field lines are closest together
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17
Q

What happens to the strength of a magnetic field as you move away from a magnet?

A
  • it gets weaker
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18
Q

Describe a method to show the magnetic field lines around a magnet using iron
filings

A
  • the magnet is put underneath a piece of paper and iron filings are sprinkled over the paper
  • the little pieces of iron line up with the direction of the field and the shape of the field can be seen
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19
Q

Where is the magnetic field around a bar magnet strongest?

A
  • magnetic field is strongest at the poles where the field lines are closest together
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20
Q

What are the two key points to note when drawing a magnetic field?

A
  • arrows point AWAY from the north pole, TOWARDS the south pole
  • field lines do not cross
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21
Q

Describe a method to show the magnetic field lines around a magnet using a
plotting compass

A
  • place the magnet on a piece of paper and draw around it, marking north and south poles
  • place the plotting compass by a pole of the magnet and make a dot at the tip of the compass needle
  • move the compass tail to the new dot and make another dot at the tip
  • repeat until the compass reaches the other pole of the magnet then draw a line through the dots and add arrows to show direction of field line (from north to south)
  • repeat for different starting positions at the poles
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22
Q

How would this diagram be different if it were two south poles?

A
  • the lines would have the exact same pattern, but the direction of the field would be reversed, so the arrows would point the other way
23
Q

Describe the magnetic effect of a current moving through a wire

A
  • when a current flows through a wire, a magnetic field is produced around the wire
24
Q

What does the strength of the magnetic field depend on?

A
  • the size of the current which is flowing
25
Q

In the right hand grip rule, what does the thumb represent?

A
  • the direction of the current
26
Q

What do the fingers represent?

A
  • the direction of the magnetic field
27
Q

What is a solenoid?

A
  • a long coil of wire
28
Q

Describe the magnetic field inside a solenoid

A
  • strong and uniform
29
Q

How can the strength of the magnetic field produced by the solenoid be increased?

A
  • more coils on the solenoid,
  • higher current (by using a larger potential difference)
  • or add an iron core to turn it into an electromagnet
30
Q

How can the direction of the magnetic field produced by the solenoid be reversed?

A
  • change the direction of the current
31
Q

What is an electromagnet?

A
  • is a solenoid with an iron core
32
Q

How can we turn a solenoid into an electromagnet?

A
  • add an iron core
33
Q

Why is using an electromagnet often better than using a permanent magnet?

A
  • it can be turned on and off, to pick up then release objects
  • the strength of the field can be varied
34
Q

How does the situation pictured above cause the wire to move?

A
  • when a wire with a current is put between the poles of a magnet, the magnetic field around the wire interacts with the magnetic field of the magnet and makes the wire and the magnet exert a force on each other
35
Q

What is the name of this effect?

A
  • motor effect
36
Q

When is the force zero?

A
  • if the wire is parallel to the magnetic field
37
Q

Describe the direction in which the force on the wire acts

A
  • the force acts perpendicular to the field and the current
38
Q

How can we make the force stronger? (2 ways)

A
  • stronger magnet

- larger current

39
Q

What are the three things that depend on the size of the force?

A
  • the magnetic flux density (B), measured in tesla (T) = this shows the size of the magnetic field
  • the size of the current (I) in the wire (amps, A)
  • the length of the conductor (l) which is inside the field (metres, m)
40
Q

What is the the equation to calculate the size of the force

A

F = BIL

force (N) = magnetic flux density (T) x current (A) x length (m)

41
Q

EXAMPLE 1:

An iron bar with a length of 30 cm is connected in a circuit with a current of 3A. The wire is placed in a field with a flux density of 0.72 T.

Calculate the force on the iron bar

A 
F = BIL
F = 0.3 x 3 x 0.72

F = 0.65 N (2sf)

42
Q

EXAMPLE 2:

A 2.0m length of wire is placed in a magnetic field with a strength of 0.25T. What is the force on the wire when the current is 1.2A?

A

answer = 0.60 N

43
Q

EXAMPLE 3:

A cable experiences a force of 15N when placed in a field of 0.80T. If the current in the cable is 4.8A, how much of the cable has been placed in the field?

A

answer = 3.90635

= 3.9 m

44
Q

EXAMPLE 4:

A 3.4m section of with experiences a force of 0.54kN when placed in a magnetic field of 0.34T. What is the current in the wire?

A

answer = 4.671280277

= 4.7 A

45
Q

EXAMPLE 5:

What is the resistance of the thermistor if the current in the circuit is 75 mA and the battery is supplying a potential difference of 9.0V?

A

R = V/I
75 mA = 0.075 A

R = 9.0/ 0.075
R = 120

= 120 Ω

46
Q

What are the key points of the conversion?

A
  • to convert from millitesla to tesla or milliamps to amps, you DIVIDE by 1000
  • to convert from kilonewtons to newtons, you multiply by 1000
  • to convert from milli units to units, you divide by 1000
  • to convert from kilo units to units, you multiply by 1000
  • to convert from centi units to units, you divide by 100
47
Q

Write a method to use Fleming’s Left Hand Rule

A

we use Fleming’s left-hand rule to work out the direction of the force

  • use your left hand
  • place thumb and first two fingers at right angles to each other
  • point your First finger in the direction of the Field
  • point your seCond finger in the direction of the Current
  • your thuMb points in the direction of the Motion (caused by a FORCE)
48
Q

What happens if the direction of either the current or the field is reversed?

A
  • if you reverse the direction of current then the direction of the field is revered and vice versa
49
Q

What happens if they are both reversed?

A
  • if they are both reversed (current & field) motion will move in the same direction
50
Q

Explain how an electric motor works

A
  • current flows = produces magnetic field in wire
  • this field interacts with field from magnets
  • force acts on coil = acts upwards on one side, downwards on other side
  • opposite forces spin coil
  • commutator reverses current every half turn
  • keeps coil spinning
51
Q

Why do you think students get confused? (HINT: Which

way does the force on the wire act?)

A
  • the force on the wire is acting UPWARDS

- students get confused as they don’t understand why this would mean the balance would show a reading

52
Q

So how come there is a force acting DOWNWARDS?

A
  • when the magnetic fields of the permanent magnets and the current carrying wire interact, TWO forces are produced (called a force pair)
  • in this case, the force on the WIRE acts UPWARDS, which means the force on the MAGNETS acts DOWNWARDS (causing the reading on the balance)
53
Q

EXAM QUESTION:

A student placed a permanent magnet on a top-pan balance

He clamped a straight piece of wire so that it was suspended in the magnetic field

When a current passed through the wire from A to B the reading on the balance increased.

Explain why

A
  • the current creates magnetic field in the wire
  • which interacts with the magnetic field from the permanent magnet
  • fleming’s left hand rule says the force on the wire is upwards
  • so the force on the permanent magnet is downwards

PAPER 2 : PHYSICS (11 decks)

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