AQA GCSE physics Electromagnetism P15 (Paper 2)
What happens when you pass a current through a wire?
A magnetic field is formed around the wire
What happens to a magnetic object that is placed in another magnetic field and what are the different scenarios in relation to polarity and distance?
A magnetic object placed in a magnetic field will experience a force induced by the first magnetic object. They will repel if they have the same polarity and attract if they have different polarity. The force will be the strongest nearer to the poles and weaker further from the poles.
What piece of equipment would you use and how would you use it to draw the field lines around a magnetic object?
A plotting object can be used. It always aligns itself along a magnetic field line. So it will point towards the south pole and away from the north pole. Also, the lines nearer to the magnet would be shorter.
What is a relay and why is it used?
A relay switches an electric machine on or off. It's used when a high voltage power supply and wire are involved, where physically pressing the switch would be dangerous. A relay externally switches on the machine without direct contact.
What is the structure of a circuit breaker?
A circuit breaker is made up of a switch held closed by a spring in series with an electromagnet.
What is the structure of an electric motor?
A simple motor has an iron armature coil connected to a battery by two graphite brushes. The brushes are pressed onto a metal split-ring commutator.
What is a solenoid and when would they be used?
A solenoid is a long coil of insulated wire. They have turns. They are used when a strong magnetic field needs to be produced once a current passes through them.
What is the definition of magnetic flux density?
The magnetic flux density is the strength of a magnetic field.
What is the structure of an alternating-current generator?
An alternator consists of an iron armature between two magnets. The armature is connected to two slip ring commutators which are connected to brushes. All of these are in a circuit with a voltmeter.
Describe an electromagnet and what happens when a current is passed through it.
An electromagnet is a solenoid in which the insulated wire is wrapped around an iron bar ( the core). When a current is passed through the solenoid, it magnetises the iron core so now it acts as a magnet.
What is the relationship between the position of the magnet and potential difference and what is the reason?
As the magnet enters and leaves the coil, a potential difference is induced. This is because when this happens, the magnetic field lines are being cut. While the magnet travels inside the coil, there is no potential difference induced because its running parallel to the field lines and therefore there is no cutting.
How does the ammeter behave as the magnet gets pushed into and out of the coil?
As the magnet is pushed into the coil, the arrow points in one direction. It then goes back to 0 as the magnet travels to the coil. It then goes to the other side when the magnet comes out. This is because the potential difference is induced in the reverse direction and therefore the current is the reverse direction.
What is the equation for working out the force, F on a conductor?
Force, F (newtons, N) = Magnetic flux density, B (tesla, T) x current, I (amperes, A) x length, l (metres, m)
What does a current coming out towards you look like?
Current coming out of the paper. The pointy part of the arrow.
What does current going in away from you look like?
Current going into the paper. Tail feathers at the end of an arrow.
Label your fingers to match Fleming's left-hand rule.
First finger is the direction of the magnetic field. The second finger is the direction of the current. The thumb is the direction of the force on the wire.
Why is graphite the material used for the brushes?
Graphite is used because it conducts electricity and is very slippery. It also causes very little friction when it is in contact with the rotating commutator.
How do the field lines in a solenoid differ from the field lines in a bar magnet even though they both have north and south poles?
In a bar magnet, the magnetic field lines go only around the outside, from the north to the south. Creating a semi -circle shape. However in a solenoid, the field lines can travel inside it, so it forms a complete loop.
How does a relay work?
In the relay circuit, a current passes through the electromagnet which generates a magnetic field. The switch in the high voltage circuit will be attracted to the electromagnet, which pulls it and completes the circuit. Current is shut off to the electromagnet which demagnetises it, the switch is no longer attracted and falls back, breaking the circuit.
What kind of objects can be magnetised?
Iron, steel, nickel, cobalt.
How does the movement of the magnet affect the potential difference and what is the reason?
Only when the magnet is moving, a potential difference is induced. When the magnet is still, there is not potential difference. The reason is that there is no cutting of the magnetic field lines, so no potential difference is induced.
What happens to two magnetic objects with opposite polarity (positive and negative)?
The magnets will attract each other
What's an example of permanent magnet and why is it like that?
Steel is a permanent magnet because once magnetised, it does not lose its magnetism easily.
If a south pole magnet is pushed out of a coil of magnet: what will be the direction of the current, what polarity will be induced on the side of the coil and how will the coil and magnet interact?
The direction of current will be anti-clockwise, the polarity induced on the side of the coil will be a north pole, therefore the magnet and coil will attract each other.
If a north pole magnet is pushed into a coil of magnet: what will be the direction of the current, what polarity will be induced on the side of the coil and how will the coil and magnet interact?
The direction of current will be anticlockwise, the polarity induced on the side of the coil will be a north pole, therefore the magnet and coil will repel each other.
If a north pole magnet is pushed out of a coil of magnet: what will be the direction of the current, what polarity will be induced on the side of the coil and how will the coil and magnet interact?
The direction of current will be clockwise, the polarity induced on the side of the coil will be a south pole, therefore the magnet and coil will attract each other.
If a south pole magnet is pushed into a coil of magnet: what will be the direction of the current, what polarity will be induced on the side of the coil and how will the coil and magnet interact?
The direction of current will be clockwise, the polarity induced on the side of the coil will be a south pole, therefore the magnet and coil will repel each other.
Using the right-hand corkscrew rule, work out the direction of the magnetic field lines when the current is travelling upwards
The direction of the magnetic field lines will be anti-clockwise
What is the structure of the electric bell?
The electric doorbell is a circuit that consists of a switch, a cell, an iron armature that is connected to a spring which is next to a bell and there is an electromagnet that is next to the iron armature
What do the magnetic field lines around the earth look like?
The field lines go from the magnetic south pole to the magnetic north pole. This is because there is a north pole at the south pole and there is a south pole at the north pole.
What do the magnetic field lines look like around a wire?
The field lines will be in concentric circles. They will start with small diameters nearer to the wire and will get bigger as you move further away from the wire.
What is the relationship between force and distance to the magnet and why?
The forces are strongest nearer to the magnet especially nearer to the moles. This is because the magnetic field lines are most concentrated in these areas. As you go further from the magnet, the forces are weaker because the magnetic field lines are further apart.
What is the structure of the relay?
The high voltage circuit consists of a switch pivoted on a spring. The relay consists of a circuit which has a low voltage supply and an electromagnet.
What do the magnetic field lines of a single bar magnet look like?
The magnet field lines go from the north pole to the south pole.
Why is the magnetic field in a solenoid stronger than a straight wire?
The magnetic field is stronger because the field lines inside the solenoid are parallel to the axis and are all in the same direction (uniform). Also, each turn has its own magnetic field, so they add up to a total field strength.
What will the magnetic field lines look like for two close magnets with like poles facing each other? (North and North)
The magnetic field lines coming from the opposite north poles will push each other up, causing the lines from both poles to be going upwards. There will be field lines going from the north pole to the south pole on the other side for each magnet.
Using the right-hand corkscrew rule, work out the direction of the magnetic field lines when the current is flowing downwards.
The magnetic field lines will be in the clockwise direction.
What happens to the magnetic field when you increase the current
The magnetic field strength is increased when you increase the current.
What happens to two magnetic objects that have the same polarity (positive and positive) or (negative and negative)?
The magnets will repel each other
What is the motor effect?
The motor effect is when a force acts on a current-carrying conductor in a magnetic field.
Using the right-hand grip rule, work out where the north pole would be if the current was travelling in the anti-clockwise direction in an electromagnet.
The north pole would be on the left side if the current was travelling in the anti-clockwise direction in an electromagnet.
Using the right-hand grip rule, work out where the north pole would be if the current was travelling in the clockwise direction in an electromagnet.
The north pole would be on the right if the current was travelling in the clockwise direction in an electromagnet.
What happens to an unmagnetised object when placed in a magnetic field and what is the terminology for that?
The object will be magnetised. The magnetic field will induce magnetism in the object.
When is the potential difference greatest in an AC generator?
The potential difference is greatest when the plane of the coil is parallel to the direction of the magnetic field, this is because the field lines are being cut.
What are two ways that can increase the size of the force acting on the current-carrying conductor?
The size of the force can be increased by increasing the current and using a stronger magnet.
What is the role of the split-ring commutator?
The split-ring commutator works by reversing the current around the coil every half-turn. So when the original position of one side of the armature experiences current flow into the page, then when it moves onto the other side the current is out of the page. The commutator also allows the wires to not get tangled.
What happens to the object when it has magnetism induced?
The unmagnetised object once magnetised will attract the other magnetic object. This is because the nearest pole to the already magnetised object will always have the opposite polarity. So if the magnetic object has a south pole, then it will induce a north pole on the other object.
What are the different kinds of poles on earth?
There are geographical poles and magnetic poles. Geographical poles are where the exact north and south direction is. However, the magnetic poles are where the magnetic field lines come from. They do not always stay in the same place because molten magnetic material moves in the mantle.
What will the magnetic field lines look like for two close magnets with opposite poles facing each other?
There will be field lines going from the north pole to the opposite south pole. There will also be lines going from the north poles to the south poles as usual/
What do the conditions need to be changed in order to reverse the force?
To reverse the force, the direction of the current or the magnetic field need to be reversed.
What happens when a wire crosses magnetic field lines and what is the term?
When a conductor crosses magnetic field lines, it induces a potential difference across the ends of the wire. This is called electromagnetic induction.
What is the generator effect?
When a conductor which connects to a circuit cuts through magnetic field lines which induces a potential difference and makes a current flow around the circuit.
What happens to the magnetic field when you reverse the current?
When the current is reversed, the direction the magnetic field lines travel in are reversed.
How does a circuit breaker work?
When the current is too large, the switch is pulled open by the electromagnet and it stays open until it is reset manually.
How does the electric bell work?
When the switch is closed, the circuit is complete. This allows current to flow to the the electromagnet, which creates a magnetic field. The iron armature gets attracted to the electromagnet and pulls towards it, in that process it hits the bell which creates the sound. The switch is then opened so that current stops flowing, the electromagnet is no longer magnetised and the iron is no longer attracted and returns back. Again, the switch closes and current flows again making the iron armature hit the bell. The process is repeated many times.
When is the force on the wire zero?
When the wire is parallel to the magnetic field lines is when the force is 0 because the lines are not being cut.
At what angle is the wire when the force is greatest?
When the wire is perpendicular to the magnetic field, the force is the greatest.
