Electromagnetism

Electromagnetism:The connection develops from the fact that

an electric current (the flow of electrons in a metal) produces a magnetic field

Faraday shows that a changing electric field produces a magnetic field and, vice-versus, a changing magnetic field produces an electric current

Maxwell completes the theory with a full mathematical description of the relationship between electric and magnetic fields = electromagnetism

Electric Motor

MElectricalEnergy

MechanicalEnergy

DC MotorDC Motor

The electric motor

Electric current flowing around the coil of the electric motor produces oppositely directed forces on each side of the coil.

These forces cause the coil to rotate.

Every half revolution the split ring commutator causes the current in the coil to reverse otherwise the coil would stop in the vertical position.

Model Electric MotorBeakman MotorBeakman Motor

What do you need?What do you need?1. Electric Energy2. Coil3. Magnetic Field

1. Electric Energy2. Coil3. Magnetic Field

Electric GeneratorG

MechanicalEnergy

ElectricalEnergy

Stationary magnets - rotating magnets - electromagnetsStationary magnets - rotating magnets - electromagnets

Electromagnetic inductionIf an electrical conductor cuts through magnetic field lines, a voltage is induced across the ends of the conductor.

If the wire is part of a complete circuit, a current is induced in the wire.

This is called electromagnetic induction and is sometimes called the generator effect.

If a magnet is moved into a coil of wire, a voltage is induced across the ends of the coil.

If the direction of motion, or the polarity of the magnet, is reversed, then the direction of the induced voltage and the induced current are also reversed.

Electromagnetic induction also occurs if the magnetic field is stationary and the coil is moved.

The transformerA transformer is a device that is used to change one alternating voltage level to another.

circuit symbol

Structure of a transformerA transformer consists of at least two coils of wire wrapped around a laminated iron core.

laminated iron core

PRIMARY VOLTAGE Vp

PRIMARY COIL of Np

turns

SECONDARY COIL of Ns turns

SECONDARY VOLTAGE Vs

The transformer equationThe voltages or potential differences across the primary and secondary coils of a transformer are related by the equation:

primary voltage = primary turnssecondary voltage secondary turns

Vp = Np

Vs Ns

Question 1Calculate the secondary voltage of a transformer that has a primary coil of 1200 turns and a secondary of 150 turns if the primary is supplied with 230V.

Vp = Np

Vs Ns

230 / Vs = 1200 / 150230 / Vs = 8230 = 8 x Vs

230 / 8 = Vs

Secondary voltage = 28.8 V

Question 2Calculate the number of turns required for the primary coil of a transformer if secondary has 400 turns and the primary voltage is stepped up from 12V to a secondary voltage of 48V.

Vp = Np

Vs Ns

12 / 48 = Np / 4000.25 = Np / 4000.25 x 400 = Np

Primary has 100 turns

Step-up transformersIn a step-up transformer the voltage across the secondary coil is greater than the voltage across the primary coil.

The secondary turns must be greater than the primary turns.

Use: To increase the voltage output from a power station from 25 kV (25 000 V) to up to 400 kV.

Step-down transformersIn a step-down transformer the voltage across the secondary coil is smaller than the voltage across the primary coil.

The secondary turns must be smaller than the primary turns.

Use: To decrease the voltage output from the mains supply from 230V to 18V to power and recharge a lap-top computer.

Exercises:1. A transformer has a primary voltage of

480 volts and a secondary voltage of 120 volts. If the primary windings have 700 turns, how many turns are in the secondary windings? (Voltage and number of turns are directly proportional.)