Electromagnetic effects Flashcards
(35 cards)
Explain - inducing an EMF
When a conductor (e.g. a metal wire) is moved through a magnetic field It cuts through the field lines which induces an EMF
What happens when a magnet is pushed into or taken out of a coil
As the magnet moved through the coil, the field lines cut through the turns of the coil This induces an EMF in the coil
What factors effect EMF
The size of induced EMF is proportional to the rate at which the field lines are cut If the field lines are cut at a faster rate, the EMF will increase
How can you cut field lines at a faster rate
Moving the wire or magnet faster Using a stronger magnet (increasing number of field line) Adding more turns to a coil
Explain - alternating current (a.c.)
A current that continuously changes direction Goes back and forth around a circuit
Explain - direct current (d.c.)
A current that is steady, constantly going the same way around a circuit from positive to negative
Example of alternating current
Mains electricity
Example of direct current
Electric cells (batteries)
Explain an A.C. generator
As the coil rotates it cuts through field lines which induces an EMF between the end of the coil which could then create a current
How can the size of the EMF in an A.C. generator be increased
Turning the coil faster Increased strength of magnetic field Adding more turns to the coil
What are the function of slip rings
Slip rings are attached to the ends of the coil These transfer the current to metal brushes whilst allowing the coil to rotate freely
How does the output of the EMF vary
The EMF is greatest when the coil is horizontal , as in this position it cuts through the field at the fastest rate The EMF is smallest when the coil is vertical, as in this position it does not cut through field lines
Definition - Transformer
An electrical device that can be used to increase or decrease the voltage of an alternating current It consists of two coils wrapped around a soft iron core
What are the four different types of transformers? What do each of them do?
Step up transformer - increases the voltage of a power source; this has more turns on the secondary coil than the primary coil Step down transformer - decreases the voltage of a power source; this has more turns on the primary coil than the secondary coil
How does a transformer work
When an alternating current is supplied to the primary coil, a changing magnetic force is the produced in the primary coil The field passes through a soft iron core and through he secondary coil The changing magnetic field in the secondary coil induces an EMF The EMF is also alternating and has the same frequency as the original current
State the two equations for transformers
1.) Input (primary) voltage/ Output (secondary) voltage = primary turns/ secondary turns
Vp / VP = Np/Ns
2.) Input power = output power
IpVp = IsVs
Point to learn on transformers
Although transformers can increase the voltage of a power source, due to the law of the conservation of energy, they cannot increase the power output
Hence, increasing voltage decreases current, and vice-versa
In what condition is electricity transmitted along overhead cables
At high voltages
What transformers are used to transmit electricity
Step-up transformer - to raise the potential difference before transmission
Step-down transformer - To step the potential difference back down to normal levels when it reaches its destination
How does high voltage reduce power loss
When electricity is transmitted over large distances, the current in the wire heats them, resulting in energy loss
By raising the voltage at which the electricity is transmitted at, the same amount of power can be transmitted using a much smaller current
This results in less heat being produced in the wire and hence less energy loss
What happens when there is a current in a wire?
A magnetic field is created around the wire
Magnetic field strength and direction
The direction of the force on the north pole of a magnet placed at that point
The strength and drection depend on the size and direction of the current:
If the current is increased, the field will get stronger
If the direction of the current is changed, the direction is field changed
The closer the field lines of a magnet are together, the stronger the magnetic field
The field lines around a wire get further apart the further they are away from the magnet
Explain the magnetic field around a coil
When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it
