topic 7 - magnetism Flashcards
At which part of a magnet are the magnetic forces strongest?
The poles of the magnet
What occurs when two magnets are brought close together?
They exert a force on each other
What is a non-contact force?
A force that can act at a distance and does not touch any object
What is the difference between a permanent magnet and an induced magnet?
- a permanent magnet produces its own magnetic field
- An induced magnet becomes magnetic when placed in a magnetic field
What type of force does induced magnetism always cause?
A force of attraction
What happens when an induced magnet is removed from the magnetic field
- An induced magnet loses most/all of its magnetism quickly
what is a magnetic field?
a region around a magnet where a force acts on another magnet or a magnetic material
give four examples of magnetic materials.
- iron
- steel
- cobalt
- nickel
What can always be said about the force between a magnet and a magnetic material?
- It is always attractive
How does distance affect the strength of a magnetic field?
As the distance increases, the magnetic field strength decreases
In what direction does a magnetic field point?
- from the north seeking pole to the south seeking pole of a magnet
How does a magnetic compass work?
- A magnetic compass contains a small bar magnet
- The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field
How can you use a compass to plot the magnetic field pattern of a magnet?
- place the magnet on a piece of paper
- draw around the magnet
- mark the north and south poles of the magnet
- make a dot at the tip of the compass needle
- move the compass tail to the new dot
- make a dot at the tip
- repeat until the compass reaches the other pole of the magnet
- draw a line through the dots
- add arrow direction of field line
- repeat for different starting positions at the poles
what happens when a current flows through a conducting wire?
- a magnetic field is produced around the wire
what does the strength of the magnetic field depend on?
- the current through the wire
- the distance from the wire
What is a solenoid?
A coil of wire which when current passes through creates a strong magnetic field
Describe the magnetic field found inside a solenoid
Strong and uniform
What is an electromagnet?
- a solenoid with an added iron core
- adding the iron core increases the strength of the magnetic field
If the direction of current in a wire placed in a uniform magnetic field is reversed, what happens to the force?
- the direction of the force is reversed
What criteria must be met for the equation linking force, magnetic flux density, current and length to hold?
The conductor must be at right-angles to the magnetic field it is placed in
What is the unit used for magnetic flux density?
T, Tesla
What is the function of the relay
- the relay is used to switch an electrical machine on or off
What is the function of the relay
- the relay is used to switch an electrical machine on or off
Describe how a relay works
- A small current through the coil produces a magnetic field around the electromagnet
- This attracts the iron arm
- This closes the gap between the contacts and turns on the machine as the current can now flow
Describe how an electric bell works
- When the switch is closed, current flows through the circuit
- a magnetic field is now produced by the electromagnet
- the iron armature is now attracted towards the magnetic field and is pulled onto the electromagnet
- this opens the make-and-break switch, and the electromagnet is switched off. Because of this, the armature springs back and the make-and-break switch closes again, so the whole cycle repeats itself
Describe how scrap vehicles work
- Scrap vehicles are lifted in a scrap yard using powerful electromagnets attached to cranes
- the steel frame of a vehicle sticks to the electromagnet when current passes through the coil of electromagnet
- When the current is switched off, the vehicle frame falls off the electromagnet
Describe how a circuit breaker works
- circuit breakers act as resettable fuses
- they are automatically operated electrical switches that detect the faults
- this can disconnect the circuit faster
what is the motor effect?
the result of two interacting magnetic fields. One is produced around the wire due to the current flowing through it, The second is the magnetic field into which the wire is placed, e.g, between two magnets
What rule is used to determine the force experienced due to the motor effect?
Fleming’s left-hand rule
What is the equation for force?
force = magnetic flux density x current x length
what criteria must be met for the equation linking force, magnetic flux density, current and length to hold?
- the conductor must be at right-angles to the magnetic field it is placed in
How does an electric motor work?
- the current creates a magnetic field around the coil
- this interacts with the field of the magnets, exerting forces on the side of the coils, pushing the side in opposite directions causing the coil to spin
- the commutator reverses the current every half turn, which reverses the forces, keeping the coil spinning
What happens if the conductor is parallel to the magnetic field?
it will not experience a force
Explain why a force acts on a current-carrying wire when placed in a magnetic field
- the magnetic field around the wire now interacts with the magnetic field between the magnets
- this means that the wire now experiences a force
Describe how a loudspeaker works
- a current in the coil creates a magnetic field
- the magnetic field interacts with the permanent magnet generating a force, which pushes the cone out
- the current is made to flow in the opposite direction
- the direction of the magnetic field reverses
- the magnetic force on the cone now pulls it back in
- repeatedly alternating the current direction makes the cone vibrate in and out
- the cone vibrations cause pressure variations in the air, which are sound waves
How are the frequency of the sound and the frequency of the ac related?
the higher the frequency of AC, the higher the pitch sound
How is the level of current and sound related?
the higher the current, the louder the sound
How can you increase the size of the induced potential difference?
- use a magnet with a stronger magnetic field
- move the magnet faster
- increase the turns in the coil
What happens to the induced potential difference when the magnet is held still inside the coil?
- the magnet held inside the coil will not induce a potential difference
- this is because there is no change in the magnetic field around the wire
Describe the purpose of the speaker
To convert electrical energy to sound energy
What is the generator effect?
- the effect of inducing a potential difference using a magnetic field
How can we switch the direction of the current?
- the direction of the current switches when the direction of movement switches
- we can also switch the direction of the induced current if we switch the poles of the magnet
Describe the relationship between the coil of wire and the magnet during the generator effect
- when we move a magnet into a coil of wire, a current is induced in the wire
- this induced current creates its own magnetic field
- this magnetic field opposes the movement of the magnet
What is an alternator?
A coil of wire rotating in a magnetic field
Describe the graph of an alternator
- the maximum potential difference is when the coil is horizontal
- when the coil is vertical, the potential difference falls to zero. this is because the wire is not cutting through the magnetic field lines
- as the coil continues moving around, we get a potential difference again
- when the potential difference has reversed direction, the two sides of the coil are moving in a different direction
How can we increase the size of the alternating current?
- increasing the strength of the magnetic field
- increasing the number of turns on the coil
What is the effect of increasing the rotation speed of the coil
this increases both the size and the frequency of the alternating current
What is the function of a dynamo?
A dynamo produces a direct curent
Describe the graph of a dynamo
- because the coil is cutting through the magnetic field lines, a potential difference and current are induced
- when the coil is vertical it is moving parallel to the magnetic field so the potential difference is zero
- Now the coil has moved around
- Because both sides of the coil are still connected to the same sides of the split-ring commutator, the direction of the potential difference and current do not reverse when the coil rotates
- there are two peaks for one rotation of the coil, this is because each side of the coil passes through the magnetic field twice during each cycle of rotation, once passing down through the field and once passing up
Suggest the purpose of slip rings
- the slip rings allow the ring to rotate without the circuit getting tangled
- it provides a continuous connection between the coil and the transformer
What is the function of microphones?
microphones use the generator effect to convert the pressure variations in soundwaves into variations in current in electrical circuits
How does a moving-coil microphone work?
- pressure variations in the sound waves cause the coil to vibrate
- the permanent magnets magnetic field cuts through the vibrating coil as the coil experiences an alternating magnetic field, inducing an alternating potential difference
- a complete loop is formed, inducing an alternating current
What does a basic transformer consist of?
A primary coil and a secondary coil wound on an iron core
Why is iron used as a core for a transformer?
it is easily magnetised
Explain how a transformer works
- insulation prevents current from flowing through the iron core
- alternating current in the primary coil produces a changing magnetic field in the secondary coil
- this induces an alternating p.d. across the secondary coil
- and hence alternating current in the secondary coil
Why must the current flowing through the primary coil of a transformer be alternating?
- For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing
- For the magnetic field to be changing, the current in the primary coil must be alternating
What can you say about the electrical input and electrical power output of a transformer if it’s assumed to be 100%?
The electrical power input is equal to the electrical power output
Describe the path of electricity from the power station to your homes
- electricity is generated in the power station
- the electricity passes through a step-up transformer which increases the p.d to 400,000V
- this is because less energy is lost in the power cables when the potential difference is very high
- the electricity passes through step-down transformers which reduce the potential difference to around 230V
- this is then passed to homes
