Electromagnetism Flashcards
The motor and generator effects, Induced potential, transformers and the National Grid
What happens when a current flows through a conducting wire?
A magnetic field is produced around the wire.
What does the strength of a magnetic field around a conducting wire depend on?
-the current through the wire
-the distance from the wire
What is a solenoid?
A coil of wire
What can be done to increase the strength of the magnetic field created by a current through a wire?
Shaping the wire to form a solenoid.
How do we describe the magnetic field inside a solenoid?
Strong and Uniform.
What does the magnetic field around a solenoid have a similar shape to?
The magnetic field of a bar magnet.
How can we increase the strength of the magnetic field of a solenoid?
Adding an iron core to form an electromagnet.
What is an electromagnet?
A solenoid with an iron core.
Explain how a solenoid arrangement can increase the magnetic effect of the current.
A solenoid is made of many coils of conducting wire.
The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field.
Describe how the magnetic effect of a current can be demonstrated.
-The magnetic field can deflect the needle of a magnetic compass
-Or you could use the ‘kicking
wire’
Describe the ‘right hand grip’ rule for finding the direction of a magnetic field.
-Grip the wire with your right hand
-Thumb in direction of current
-Fingers curled in the direction of the field
Explain why the (kicking) wire moves when the current is switched on.
-A current-carrying wire produces its own magnetic field
-When the wire is placed between the North and South poles of two bar magnets, the magnetic fields will interact
-A force is exerted on the wire
What does Fleming’s left hand rule represent?
The relative orientation of the force, the current in the conductor and the magnetic field.
Thumb = Thrust
First finger = Field
SeCond finger = Current
What are the factors that affect the size of the force on the conductor/wire? (motor effect)
-Strength of Magnetic Field (magnetic flux density)
-Strength of electric field
-Length of conductor
Equation for a conductor at right angles to a magnetic field and carrying a current:
Force = magnetic flux density x current x length
F = B I L
What is magnetic flux density?
A measure of how strong a magnet is.
1 Tesla is strong
Unit for magnetic flux density (B)?
Tesla, T
Describe the four ways to increase the strength of an electromagnet.(/solenoid)
-Increase the current
-Increase the number of turns in the solenoid, while keeping the length the same
-Reduce the length of the solenoid while keeping the number of turns the same
-Add a soft iron core
What is the shape of the magnetic field around a long, straight conductor?
A series of concentric (same centre) circles.
An electric current will always create what?
A magnetic field
What is the shape of a magnetic field around a circular coil?
Two elliptical fields in opposite directions with a single magnetic field through the centre of the coil.
The motor effect.
When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor exert a force on each other.
(this leads to movement).
What is the basis of an electric motor?
A coil of wire carrying a current in a magnetic field. This rotates.
Define potential difference (V)
The force driving the flow of electrons around a circuit.
Explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.
When a current is passed through the coil, the force on the left hand side acts upwards & the force on the right hand side acts downward, so the coil spins.
How do we keep the coil spinning in the same direction in an electric (DC) motor?
By changing the direction of the current every half turn using a split-ring commutator.
Briefly describe the function of loudspeakers and headphones.
They use the motor effect to convert variations in current (in electrical circuits) to pressure variations in sound waves.
Explain how a moving-coil loudspeaker works. (steps 1-4)
-An alternating current flows through the coil and creates a magnetic field (electromagnetism) in it.
-The magnetic field of the coil interacts with the magnetic field of the permanent magnet.
-The interacting magnetic fields will exert a force on the coil (motor effect), causing it to move back or forth along the permanent magnet.
-This causes the cone to change shape.
Explain how a moving-coil loudspeaker works. (steps 5-6)
-As the current is alternating, it will switch direction. This switches the direction of the force on the cone
-This means that the coil of wire and cone rapidly move back and forth, or ‘vibrate’
-The cone vibrations are so fast that they cause pressure variations in the air, or ‘sound waves’.
How do we make a higher pitched sound from a loudspeaker?
Increase the frequency of the a.c. through the coil.
This increases the frequency that the cone vibrates, meaning a higher pitch.
How do we make a louder sound from a loudspeaker?
Increase the size of the a.c. through the coil.
This increases the amplitude of the vibration, which increases the volume of the sound.
How do headphones work?
The same way loudspeakers do but on a smaller scale. x
What is the generator effect? -Move a coil/wire in a magnetic field and a current might flow
-If an electrical conductor moves relative to a magnetic field, or if there is a change in the magnetic field around a conductor..
-A potential difference is induced across the ends of the conductor
-If the conductor is part of a complete circuit, current is induced in the conductor
What does an induced current generate?
A magnetic field that opposes the original change (either the movement of the conductor or the change in the magnetic field).
What is the problem with the magnetic field produced by an induced current?
It opposes the original change.
This means repulsion when pushing a magnet into the coil and attraction when pulling a magnet out of the coil.
So these actions require more work.
What is work?
A force acting on an object which causes the object to move.
Name the factors that increase the size of the induced potential difference (/current) in the generator effect.
-Increase the strength of the magnetic field (strongere magnet)
-Increase the number of turns on the coil (=more cutting of field lines)
-Increase the speed of the movement
Name the factors that affect the direction of the induced potential difference (/current) in the generator effect.
-Reverse the poles
-Reverse the direction of the motion
Doubling the speed at which a magnet moves in and out of a coil increased the size of the induced potential difference by what?
4x
due to Ek = 1/2 m v²
You should be able to apply the principles of the generator effect in a given…
…context. x
How is the generator effect used?
-In an alternator to generate ac
-In a dynamo to generate dc
Explain how the generator affect is used in an alternator to generate alternating current.
-A coil rotates relative to a magnetic field
-This cuts across magnetic field lines
-This induces a potential difference in the coil, and a current if the circuit is closed
-Slip rings and brushes connect the coil to an external circuit (continuously)
If no work is done, what cannot happen?
Energy cannot be transferred to electrical energy.
What are slip rings and brushes made of? Why?
Slip rings - metal
Brushes - graphite
Both conduct electricity, and graphite has layers that can slide over each other which reduces friction.
Explain how the generator affect is used in a dynamo to generate direct current.
-A coil rotates relative to a magnetic field
-This cuts across magnetic field lines
-This induces a potential difference in the coil, and a current if the circuit is complete (&closed)
-A split ring commutator swaps the contacts between the external circuit and the coil every 1/2 turn so current always flows off the coil in the same direction
2 (+ve onto the +ve contact)
What do generators do?
Generate electricity from rotational motion.
How does a split ring commutator allow current to always flow off the coil in the same direction?
It swaps the contacts between the external circuit and the coil every 1/2 turn.
This means that the cutting of the field is always upwards on the left and downwards on the right.
Study the electromagnetism diagrams of pd generated in the coil against time. (kilonotes)
You need to be able to draw and interpret these!
In the generator effect, if the coil is moving perpendicular to the field, how many volts of pd are induced?
Maximum volts
In the generator effect, if the coil is moving parallel to the field, how many volts of pd are induced?
0V
be careful with parallel/perpendicular…it has to be in relation to something else and specify if talking about the coil itself or the motion.
Degrees in relation to voltages… (sinusoidal graph for alternator)
0° = 0V
90° = max volts
180° = 0V
270° = max volts but -ve
360° = 0V
Briefly describe the function of microphones.
Microphones use the generator effect to convert the pressure variations in sound waves into variations in current (in electrical circuits).
Explain how a moving- coil microphone works.
-Sound waves hit the diaphragm.
-This causes the diaphragm and coil of wire to move.
-As the wire is moving within the magnetic field of the permanent magnet, this induces a current (electromagnetic induction / generator effect).
What determines how much the diaphragm in a microphone vibrates?
-The frequency and amplitude of the sound waves.
-These also, therefore, determine the frequency and amplitude of the current.
What are the 3 main components of a transformer?
-Primary coil
-Iron core
-Secondary coil
Why is iron used in transformers?
It’s a soft magnetic material so easily magnetised and demagnetised.
What does the ratio of potential differences across the primary and secondary coil of a transformer depend on?
The ratio of the number of turns on each coil.
Symbol for number of turns on the primary and secondary coils?
Np and Ns
(subscript ps)
Symbol for potential difference of the primary coil (input pd)?
Vp
Symbol for potential difference of the secondary coil (output pd)?
Vs
What transformer does this describe: Vs > Vp
Step-up as the output voltage is greater than the input voltage.
What transformer does this describe: Vs < Vp
Step-down transformer as the output voltage is less than the input voltage.
What would be true IF all transformers were 100% efficient?
Electrical power output = Electrical power input
P=P
I x V = I x V
Is x Vs = Ip x Vp
Assuming the transformers are 100% efficient, what equation do we get to and how?
Is x Vs = Ip x Vp
If Electrical power output = Electrical power input, then P=P
and I x V = I x V
so Is x Vs = Ip x Vp
1st transformer equation?
Vp/Vs = Np/Ns
Where V= potential difference (p/s coils) and N = number of coils (p/s)
Vs x Is means what in context?
The power output (voltage from the secondary coil).
Vp x Ip means what in context?
The power input (voltage to the primary coil).
Unit for power input/output?
Watts, W.
Describe the path of electricity from the power station to your homes. [6]
-Electricity is generated at a power station
-A step-up transformer is used to increase voltage to 400 000V
-P=IV so current decreases
-This makes the transport of electricity through overhead cables more efficient (as less energy is wasted as heat)
-A step-down transformer is used to decrease voltage to a safe level of 230V for use in homes
What is the role of a step-up transformer?
To increase voltage, to increase the efficiency of the electricity transfer across pylons.
What is the role of step-down transformers?
To decrease voltage, to make electricity safer for use in homes.
1 sentence to explain how transformers work (the extreeeemeelyyy brief versionnn)
An alternating current in one coil induces a current in another.
Explain how transformers work.
-An alternating potential difference is applied across the primary coil, which causes an alternating current to flow
-This induces an alternating magnetic field around the primary coil
-The primary coil’s alternating magnetic field induces an alternating magnetic field in the iron core (magnetic material) making it an electromagnet.
-The iron core’s alternating magnetic field induces an alternating potential difference in the secondary coil
-If this is connected to an external circuit, an external current will flow from the secondary coil
What is the role of transformers?
To increase or decrease voltage.
If there are twice the number of turns in the secondary coil compared to the primary coil, what happens to voltage?
It doubles
At what voltage does electricity flow in the national grid?
400 000V
Why can’t electrical current conduct straight between the wires and iron core?
They are insulated in plastic.
How does the ratio of potential differences across the p/s coil in a transformer depend on the number of turns on each?
The voltage across each is directly proportional to the ratio of the number of turns on each coil.
𝑉𝑝/𝑉𝑠 = 𝑁𝑝/𝑁𝑠
You need to be able to relate the equations to…
…the advantages of power transmission at high potential differences.
What power equation should we use for this topic?
P=IV
Describe the relationship between current (I) and potential difference/voltage (V)
Inversely proportional.
Practice your…
transformer equations!
Describe the difference between an electromagnet and a permanent magnet.
A permanent magnet is always magnetic, whereas an electromagnet is only magnetic when there is a flow of current through the coil. They can be turned off.
