Magnetism and Electromagnetism Flashcards
What is Fleming’s left hand rule?
Motion (Manchester)
Field (football)
Current (club)
Why does a motor spin?
1) current in coil
2) creates magnetic field around the wires of the coil
3) interaction of this field with that of the permanent magnets
4) there is a force on the wire of the coil
5) left hand rule
6) force up on one side and down on the other side
How can you make a motor spin in the opposite direction?
1) reverse supply polarity
2) reverse current direction
3) swap magnets over
How can you make a motor spin more slowly?
1) reduce current
2) reduce voltage
3) increase resistance of circuit
4) weaker magnetic field
5) less turns on coil
Describe an investigation to investigate the shape of the magnetic field pattern of a bar magnet
1) place plotting compass at end of the magnet
2) mark the direction of compass
3) move compass to a new point along the line and mark its direction again
4) link the directions together to form a field line
5) start a new line from a different point
Which way do magnetic field lines go?
North to South
Why is a voltage induced a coil of wire is turned in a magnetic field?
The coil cuts through the magnetic field lines of the magnet
What happens to a coil if there is no split ring commutator?
1) turns clockwise
2) oscillates
3) comes to rest at 90 degrees to field
Explain why a force acts on a wire in a magnetic field when the switch is closed
1) closing switch makes current flow through wire
2) current flowing creates a magnetic field around the wire
3) interacts with the permanent magnetic field of the two magnets
4) moves up and down due to Flemings left hand rule
Why are thick wires used as opposed to thin wires?
1) less heat loss
2) thin wire might melt
3) reduce resistance
Suggest how curved magnets and a piece of iron would improve the performance of the electric motor
1) force increases by stronger field
2) radial magnetic field rather than a uniform one
3) coil remains in the field for a longer time
How does a magnet passing through a coil at a higher speed affect the size of the induced voltage/current?
1) greater voltage/current
2) more magnetic field lines cutting coil per second (rate of change of flux is greater)
What is a uniform magnetic field?
One were the lines are parallel to each other
How do you produce a uniform magnetic field?
1) two permanent magnets
2) north facing south
3) magnets close together but not touching
Explain why a data logged records a varying voltage when a magnet on a spring enters a coil
1) voltage is induced
2) because field lines are cut
3) voltage changes direction when the magnet does
4) magnet slows down causing a decrease in amplitude
Explain how an alternating current generates a sound wave in a speaker
1) alternating current continuously changes direction
2) current in coil produces alternating magnetic field
3) producing force on the coil
4) reversing direction of current reverses direction of the force
5) hence coil vibrates
6) cone vibrates air particles causing sound waves
What materials are magnetic?
1) iron
2) cobalt
3) nickel
What happens to a magnetically hard material once it has been magnetised?
It stays magnetised as the domains are locked in position
What happens to a magnetically soft material once it has been magnetised?
It will lose its magnetism as the domains become jumbled up again once the magnetising influence has been removed
How are domains (groups of atoms) arranges in an unmagnetised object?
Randomly so their magnetic field can cancel out
How are domains (groups of atoms) arranges in an magnetised object?
The domains line up so their magnetic fields add together
What are magnetically hard materials used to make?
Permanent magnets
What are magnetically soft materials used to make?
Electromagnets or the cores of transformers
Describe the stages of an electric bell ringing
1) switch closes so current flows in the coil creating an electromagnetic field
2) soft iron armature is affected to the electromagnet
3) clanger hits the bell and contact breaks at adjustable screw
4) electromagnetic switches off meaning armature springs back… etc.
What is the magnetic field line around a straight wire?
Concentric circles (use right hand grasp to work out direction)
What happens to charged particles in a magnetic field?
It experiences a force as long as it’s motion is not parallel to the field
What does a transformer do?
Changes the size of an alternating voltage by having different numbers of turns on the input and output sides
What is the structure of a transformer?
Two wires wrapped around a magnetically soft iron core
How are transformers useful?
Step down transformers reduce the voltage as electricity is transported otherwise too much energy would be lost as heat and then step up transformers increase the voltage again cause too large a current would be dangerous around people
what is a step up transformer?
a transformer that increases the voltage
what is a step down transformer?
a transformer that decreases the voltage
the number of turns on the primary coil of a step down transformer is more or less than the number on the secondary coil?
more
the number of turns on the primary coil of a step up transformer is more or less than the number on the secondary coil?
less
what is the relationship between the primary and secondary voltage and primary and secondary current in a transformer?
Vp = Is
Vs Ip
How does a transformer becoming warm affect the output?
1) energy/power loss
2) efficiency does not equal 100%
3) less available energy/power
4) resistance increasing with temperature
what is the relationship between turns on the primary and secondary coil and primary and secondary voltage?
Np = Vp
Ns Vs
why could an electromagnetic material be made from iron?
1) iron is a soft magnetic material
2) so it loses its magnetism easily
describe the construction of an electromagnet
1) current carrying insulated wire
2) wrapped into coil
3) wrapped on an iron core
if a door is held open by an iron plate attracted to an electromagnetic material closes what has happened?
1) current reduces
2) magnetic field of the electromagnet reduces
3) magnetic force holding the iron plate to the magnet is no longer present
describe the electromagnetic field around a wire in a coil
1) at least one straight, vertical central field line
2) field lines circling the wire/two peripheral field loops
If two identical coils are suspended vertically next to each other explain why the two coild move together when a D.C. passes and apart when the current in one of the coils is reversed
1) magnetic fields interact
2) idea of magnetic attraction or repulsion
3) reversing current reverses direction of magnetic field
4) like poles repel, unlike poles attract
how would changing from a direct current to an alternating current affect the magnetic field of a soft magnetic material in a coil?
1) field in coil switches polarity
2) field in rods weaker
explain why the core of transformers is made of a soft magnetic material, such as iron
1) soft material loses magnetism quickly/easily;
2) the magnetic field in the core alternates/changes;
explain how a transformer works
1) it steps up or steps down the voltage
2) current in primary coil produces magnetic field
3) the current is changing
4) causing a changing magnetic field in the core
5) the core strengthens the magnetic field
6) field lines interact with the secondary coil
7) which induces a voltage in the secondary coils
8) transformer won’t work with steady d.c
Explain how the transmission of electrical power is made more efficient by using step-up and step-down transformers
1) increased voltage with step up transformer
2) therefore reduced current;
3) current linked to heating
4) therefore less energy/power is lost in transmission
5) reference to P=I^2 x R
6) example of an efficiency enhancing detail of cables (e.g. good conductor, low resistance, large diameter)
7) example of an efficiency enhancing detail of transformer construction (e.g. low resistance coils, coils wrapped on top of each other, soft iron core)
8) step down transformer reduces voltage
how do you calculate the power lost intransmission?
P = I^2 x R
A.K.A: power = current squared x resistance
give examples of an efficiency enhancing detail of cables
1) good conductor
2) low resistance
3) large diameter
give examples of an efficiency enhancing detail of transformer construction
1) low resistance coils
2) coils wrapped on top of each other
3) soft iron core
Describe the structure of a step-down transformer
1) primary AND secondary coils
2) soft iron core
3) primary/input coil has more turns;
explain why a d.c. produced from a generator must be changed to a.c. before it is transmitted
1) Transformers change the voltage/current
2) Transformers use alternating current
