P7: Electromagnetic effects

Magnetic effect of an electric current

A current within a wire creates circular magnetic fields perpendicular to the direction to the flow of the current
The magnitude of the magnetic field is higher when closer to the wire
The direction of the magnetic field can be found by doing a thumbs up and down gesture
When the current is going upwards, use your right hand thumb to indicate the direction of the current with a thumbs up gesture. The rest of your fingers indicate the circular direction of the magnetic field (which should be an anticlockwise direction)
When the current is going in another direction, simply just position your thumb to face the direction of the current and see the direction your fingers are facing. Imagine your fingers creating a circular loop around you thumb. This would be the direction of the magnetic field
Solenoids (long spiral coils of wire) act as a magnet. Each coil has a magnetic field which combines togethr to make a big magnetic field
Reversing the direction of the current will also reverse the direction of the magnetic field

D.C motor

A DC motor normally has a coil carrying a current in a magnetic field
The current in the coil causes a turning effect. This effect can be increased by...

This motor works by its magnetic field from the current, which acts with magnets
The magnets will cause one part of the coil to be pushed up, and the other to be pushed down, causing the coil to spin
There is also a commutator (a device to reverse the flow of electric current).
Due to the reversing direction of the current, the coil continues to spin through being pushed up and down constantly

The magnetic fields around a wire with an electric current

Electromagnetic induction

An electromotive force can be induced in a conductor when it passes through an electromagnetic field
The magnitude of the electromotive force depends on the rate the conductor cuts the magnetic field lines of the electromagnetic field
This magnitude cab be increased by

A.C generator

Alternating current is a current that changes its direction
Direct current is a current with one direction
An A.C generator has a spinning coil that produces electricity
The coil is rotating mechanically, and when it does, it cuts through the field lines and induces an electromagnetic force
Slip rings at the ends of the coil transfered the current to metal brushes

Transformer

A transformer is a device that changes the voltage of an alternating current
A transformer normally consists of two coiled pieces of wire around soft iron
A transformer works by producing a change in magnetic field in a primary coil first when an alternating current reaches it
The alternating current then reaches the secondary coil of wire through the soft iron
The secondary coil experiences a change in its field, and causes an alternating electromotive force to be formed in the soft iron core

Transformers that increase the voltage of a power source are said to be step-up transformers. They have more coil turns in their secondary coil than their first
Transformers that decrease the voltage of the power source are said to be step-down transformers. They have less coil turns in the secondary coil than the first
The outage voltage depends on the input voltage in addition to the amount of coil turns
The output voltage can be worked out by the following formula: Input (primary) voltage ÷ Output (secondary) voltage = Primary turns ÷ Secondary Turns OR (V_p /V_s) = (N_p/N_s)
Voltage increases when current decreases in a transformer, and the other way round
This is displayed through the equation I_pV_p = I_sV_s OR current of primary coil x voltage of primary coil= current of secondary coil x voltage of secondary coil
When electricity is transmitted in cables, high voltages are involved
A step-up transformer raises the voltage before these transmissions while the step-down transformer tries to keep the voltages at normal levels
This is because at a high voltage, there is more heating and energy loss over long distances of transmission than lower currents that could transfer the same amount of electricity with less heating and energy loss