6.2.1 - 6.2.4 Electromagnetic Induction Flashcards
When does Electromagnetic induction occour?
A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it
How does EM induction occour?
This is done by the conductor or coil cutting through the magnetic field lines of the magnetic field
This is often referred to as the generator effect and is the opposite to the motor effect
In the motor effect, there is already a current in the conductor which experiences a force
In the generator effect, there is no initial current in the conductor but one is induced (created) when it moves through a magnetic field
This is done by the conductor or coil cutting through the magnetic field lines of the magnetic field
How is a potential difference generated?
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
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
What are the 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
What is the direction of the induced potential difference determined by\\\\\\/
- The speed at which the wire, coil or magnet is moved:
Increasing the speed will increase the rate at which the magnetic field lines are cut
This will increase the induced potential difference
2. The number of turns on the coils in the wire:
Increasing the number of turns on the coils in the wire will increase the potential difference induced
This is because each coil will cut through the magnetic field lines and the total potential difference induced will be the result of all of the coils cutting the magnetic field lines
3. The size of the coils:
Increasing the area of the coils will increase the potential difference induced
This is because there will be more wire to cut through the magnetic field lines
What can the generator effect be used to do?
Generate a.c in an generator
Generate d.c in a dynamo
What is an alternator?
A simple alternator is a type of generator that converts mechanical energy to electrical energy in the form of alternating current
A rectangular coil that is forced to spin in a uniform magnetic field
The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings
The slip rings and brushes provide a continuous connection between the coil and the meter
When the coil turns in one direction:
The pointer deflects first one way, then the opposite way, and then back again
This is because the coil cuts through the magnetic field lines and an alternating potential difference, and therefore current, is induced in the coil
An alternating current may also be produced when a magnet rotates within a stationary coil
Both methods operate on the principle that p.d. is induced when a coil experiences a changing external magnetic field
The induced potential difference and the current alternate because they repeatedly change direction
What is a dynamos?
A dynamo is a direct-current generator
A simple dynamo is the same as an alternator except that the dynamo has a split-ring commutator instead of two separate slip rings
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)
What is a 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
What is the structure of a transformer?
A primary coil
A secondary coil
An iron core
Iron is used because it is easily magnetised
How does a transformer work?
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
What can transformers do?
A transformer can change the size of an alternating voltage
They also have a number of other roles, such as:
To increase the potential difference of electricity before it is transmitted across the national grid
To lower the high voltage electricity used in power lines to the lower voltages used in houses
Used in adapters to lower mains voltage to the lower voltages used by many electronic devices
What is a step up transformer?
A step-up transformer increases the potential difference of a power source.
A step-up transformer has more turns on the secondary coil than on the primary coil
What is a step down transformer?
A step-down transformer decreases the potential difference of a power source.
A step-down transformer has fewer turns on the secondary coil than on the primary coil
How does transformers work in electricity transmissions?
When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss
The electrical energy is transferred at high voltages from power stations
It is then transferred at lower voltages in each locality for domestic uses
The voltage must be stepped up by a step-up transformer
These are placed after the power station
For the domestic use of electricity, the voltage must be much lower
This is done by stepping down by the voltage using a step-down transformer
These are placed before buildings
