7.3 Induced potential Flashcards
Generator effect
The generator effect is the opposite of the motor effect.
Instead of using electricity to create motion, motion is being used to create electricity.
The generator effect is defined as:
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, a current is induced in the conductor.
The generator effect occurs when a wire cuts through the magnetic fields lines.
Generating potential difference
A potential difference will be induced in the conductor if there is relative movement between the conductor and the magnetic field.
Moving the electrical conductor in a fixed magnetic field.
When a conductor (such as a wire) is moved through a magnetic field, the wire cuts through the fields lines.
This induces a potential difference in the wire.
Moving the magnetic field relative to a fixed conductor.
As the magnet moved through the coil, the field lines cut through the turns on the coil.
This induces a potential difference in the coil.
When the magnet enters the coil, the field lines cut through the turns, inducing a potential difference.
A sensitive voltmeter can be used to measure the size of the induced potential difference.
If the conductor is part of a complete circuit then a current is induced in the conductor.
This can be detected by an ammeter.
Factors Affecting the Induced Potential Difference
The size of the induced potential difference is determined by:
The speed at which the wire, coil or magnet is moved.
The number of turns on the coils of wire.
The size of the coils.
The strength of the magnetic field.
Alternators
An alternator, or a.c. generator, is a device which converts energy from motion into an electrical output.
An alternating potential difference (p.d.) is generated which causes an alternating current to flow.
A simple alternator consists of:
a rotating coil of wire between the poles of a permanent magnet.
slip rings and brushes connected to an external circuit.
Components of an alternator
permanent magnet - to provide a uniform magnetic field.
rotating coil - to cut the magnetic field as it rotates and allow an induced current to flow.
slip rings - to allow the alternating current to flow between the coil and the external circuit.
carbon brushes - to provide a good electrical connection between the coil and the external circuit.
Operation of an alternator
A rectangular coil rotates in a uniform magnetic field.
The coil is connected to an external circuit via slip rings and brushes.
The induced p.d. in the coil can be measured by adding a galvanometer (centre-zero meter) to the external circuit.
A p.d. is induced in the coil as it cuts the magnetic field.
The pointer defects first one way, then the opposite way, and then back again.
This indicates the size and direction of the p.d. is constantly changing.
As a result of the alternating p.d., an alternating current is also produced as the coil rotates.
This continues as long as the coil keeps turning in the same direction.
Motion of an alternator
The size and direction of the induced p.d. (and current) depend on the orientation of the coil with the field.
A maximum p.d. is induced when:
the position of the coil is horizontal.
the motion of the coil is perpendicular to the field.
This is because the greatest number of lines are cut when the coil is moving perpendicular to the field.
No p.d. is induced when:
the position of the coil is vertical.
the motion of the coil is parallel to the field.
This is because no lines are cut when the coil is moving parallel to the field.
Dynamos
A dynamo, or d.c. motor, is a device which converts an electrical input into motion.
A simple dynamo contains the same components as an alternator except instead of slip rings, it uses a split-ring commutator.
Components of a dynamo
permanent magnet - to provide a uniform magnetic field.
rotating coil - to provide the rotation as the current flows through it.
split ring commutator - to allow the connection between the coil and the external circuit to change every half turn.
carbon brushes - to provide a good electrical connection between the coil and the external circuit.
Operator of a dynamo
As the coil rotates, it cuts through the field lines.
This induces a potential difference between the end of the coil.
The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction.
This happens each time the coil is perpendicular to the magnetic field lines
Therefore, the induced potential difference does not reverse its direction as it does in the alternator.
Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative).
This means the current is always positive (or always negative).
Factors affecting alternator or dynamo output
The magnitude of the induced p.d. can be increased by:
increasing the frequency of rotation of the coil.
increasing the number of turns on the coil.
increasing the strength of the magnet.
inserting a soft iron core into the coil.
Moving coil microphone
Microphones convert sound waves into electrical current.
A moving coil microphone works using the principles of the generator effect.
When sound waves reach the microphone, the pressure variations cause the diaphragm to vibrate.
This in turn causes the coil to move back and forth, through the magnetic field produced by the magnet.
As it does so, the coil cuts through the field lines.
This induces a potential difference in the coil.
The potential difference will be alternating because the coil is continually changing direction due to the vibrations of the diaphragm.
Transformer
A transformer is a device used to change the value of an alternating potential difference or current.
This is achieved using the generator effect.
A basic transformer consists of:
A primary coil
A secondary coil
An iron core
Iron is used because it is easily magnetised.
How a transformer works
An alternating current is supplied to the primary coil.
The current is continually changing direction.
This means it will produce a changing magnetic field around the primary coil.
The iron core is easily magnetised, so the changing magnetic field passes through it.
As a result, there is now a changing magnetic field inside the secondary coil
This changing field cuts through the secondary coil and induces a potential difference.
As the magnetic field is continually changing the potential difference induced will be alternating.
The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil.
If the secondary coil is part of a complete circuit it will cause an alternating current to flow.
Transformer equation
The output potential difference (voltage) of a transformer depends on:
the number of turns on the primary and secondary coils.
the input potential difference (voltage).
It can be calculated using the equation:
Vp / Vs = Np / Ns
Vp = potential difference (voltage) across the primary coil in volts (V)
Vs = potential difference (voltage) across the secondary coil in volts (V)
Np = number of turns on primary coil
Ns = number of turns on secondary coil
Step up transformer
A step-up transformer increases the potential difference of a power source (Vs > Vp).
A step-up transformer has more turns on the secondary coil than on the primary coil (Ns > Np).
Step down transformer
A step-down transformer decreases the potential difference of a power source (Vs < Vp)
A step-down transformer has fewer turns on the secondary coil than on the primary coil (Ns < Np)
Ideal transformer equation
An ideal transformer would be 100% efficient.
Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output.
If a transformer is 100% efficient:
Input power = Output power
The equation to calculate electrical power is:
P = V × I
P = power in Watts (W)
V = potential difference in volts (V)
I = current in amps (A)
Therefore, if a transformer is 100% efficient then:
Vp × Ip = Vs × Is
Vp = potential difference across primary coil in volts (V)
Ip = current through primary coil in amps (A)
Vs = potential difference across secondary coil in volts (V)
Is = current through secondary coil in amps (A)
AC and high voltage transmission
Transformers have a number of roles:
They are used to increase the potential difference of electricity before it is transmitted across the national grid.
They are used to lower the high voltage electricity used in power lines to the lower voltages used in houses.
They are used in adapters to lower mains voltage to the lower voltages used by many electronic devices.
Advantages of high voltage transmission
When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss.
To transmit the same amount of power as the input power the potential difference at which the electricity is transmitted should be increased.
This will result in a smaller current being transmitted through the power lines.
This is because P = IV, so if V increases, I must decrease to transmit the same power.
A smaller current flowing through the power lines results in less heat being produced in the wire.
This will reduce the energy loss in the power lines.
