P7 Flashcards
What are the ‘poles’ of a magnet?
- the ends of the magnet, where the magnetic field is strongest
Describe the possible outcomes when two permanent magnets are brought together
- the magnets will either attract or repel
2 magnets which would repel each other. What will you label each pole?
- either N N or S S
Why is magnetism classed as a non-contact force?
- the magnets do not have to touch to exert a force on each other
Which materials are magnetic?
- iron (and steel), nickel and cobalt
What are the two categories of magnet?
- permanent and induced (temporary)
What is a permanent magnet?
- a magnet which produces its own magnetic field
- they stay magnetic
What is an induced (temporary) magnet?
- 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
What are the key points of permanent magnets?
- they are always magnetic
- they can attract object
- they can repel an object
What are the key points induced magnets?
- they aren’t always magnetic
- they can attract object
- they cannot repel an object
What is a compass?
- a magnet which lines itself up with the Earth’s magnetic field
What does the behaviour of a compass give evidence for?
- it shows the Earth has a magnetic core
What is meant by the term ‘magnetic field’?
- the area around a magnet where a magnetic force can be felt
How can a magnetic field be shown?
- by drawing lines
What do the arrows represent?
- the direction of the magnetic field
How do you know where the magnetic field is strongest?
- the field lines are closest together
What happens to the strength of a magnetic field as you move away from a magnet?
- it gets weaker
Describe a method to show the magnetic field lines around a magnet using iron
filings
- 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
Where is the magnetic field around a bar magnet strongest?
- magnetic field is strongest at the poles where the field lines are closest together
What are the two key points to note when drawing a magnetic field?
- arrows point AWAY from the north pole, TOWARDS the south pole
- field lines do not cross
Describe a method to show the magnetic field lines around a magnet using a
plotting compass
- 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
How would this diagram be different if it were two south poles?
- 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
Describe the magnetic effect of a current moving through a wire
- when a current flows through a wire, a magnetic field is produced around the wire
What does the strength of the magnetic field depend on?
- the size of the current which is flowing
In the right hand grip rule, what does the thumb represent?
- the direction of the current
What do the fingers represent?
- the direction of the magnetic field
What is a solenoid?
- a long coil of wire
Describe the magnetic field inside a solenoid
- strong and uniform
How can the strength of the magnetic field produced by the solenoid be increased?
- more coils on the solenoid,
- higher current (by using a larger potential difference)
- or add an iron core to turn it into an electromagnet
How can the direction of the magnetic field produced by the solenoid be reversed?
- change the direction of the current
What is an electromagnet?
- is a solenoid with an iron core
How can we turn a solenoid into an electromagnet?
- add an iron core
Why is using an electromagnet often better than using a permanent magnet?
- it can be turned on and off, to pick up then release objects
- the strength of the field can be varied
How does the situation pictured above cause the wire to move?
- 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
What is the name of this effect?
- motor effect
When is the force zero?
- if the wire is parallel to the magnetic field
Describe the direction in which the force on the wire acts
- the force acts perpendicular to the field and the current
How can we make the force stronger? (2 ways)
- stronger magnet
- larger current
What are the three things that depend on the size of the force?
- 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)
What is the the equation to calculate the size of the force
F = BIL
force (N) = magnetic flux density (T) x current (A) x length (m)
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
F = BIL F = 0.3 x 3 x 0.72
F = 0.65 N (2sf)
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?
answer = 0.60 N
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?
answer = 3.90635
= 3.9 m
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?
answer = 4.671280277
= 4.7 A
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?
R = V/I
75 mA = 0.075 A
R = 9.0/ 0.075 R = 120
= 120 Ω
What are the key points of the conversion?
- 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
Write a method to use Fleming’s Left Hand Rule
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)
What happens if the direction of either the current or the field is reversed?
- if you reverse the direction of current then the direction of the field is revered and vice versa
What happens if they are both reversed?
- if they are both reversed (current & field) motion will move in the same direction
Explain how an electric motor works
- 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
Why do you think students get confused? (HINT: Which
way does the force on the wire act?)
- 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
So how come there is a force acting DOWNWARDS?
- 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)
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
- 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
