Abdullah Ahmed

Electromagnetic induction

Electromagnetic induction is the electricity (emf) produced in the conductor when it is exposed to a magnetic field which varies. The greater the change of magnetic field, the grater the emf produced. Also the more number of coils present in the wire, the greater the emf produced. When an emf is produced in the conductor, it can be observed flowing in a closed loop. This can be demonstrated by using a galvanometer, magnet and a conductor such as a coil of wire. Passing the magnet over a coil, the galvanometer should detect this and display it on the meter. If the magnet is not moving over the coil, the galvanometer should show a zero on the meter. This proves that the magnet needs to move for there to be emf.

Electromagnetic induction was discovered by Michael Faraday. The mathematical formula which describes electromagnetic induction is known as Faradays law of Induction.

Faradays law of induction describes the process of electromagnetic induction. Faradays law of induction states that the induced electromagnetic motive force is equal to the rate of change of magnetic flux which is in contact with the circuit the following formula is assuming that the circuit is closed:

ε=−δ ϕBδt

Where ε is the electromotive force (emf).

Where ϕB is the magnetic flux.

Where t is the time.

If the circuit is not closed, a more general formula can be used. This formula is valid in all situations rather than only closed:

ε=−Nδ ϕBδt

Where ε is the electromotive force (emf).

Where ϕB is the magnetic flux.

Where t is the time.

Where N is the number of coils in the wire.

1

Abdullah Ahmed

The turning movement of the coil in a magnetic field produces the emf. The voltage which is generated is sinusoidal. Fardays law describes this.

Lenz’s law describes the direction of the electromotive force. Lenz’s law states that the direction of the induced current always opposes the change which produced it.

2

Abdullah Ahmed

When the motor turns, it generates an emf which goes the opposite direction. According to Lenz’s law, the back emf will oppose the change that produced it. This means that the back emf will balance the input voltage, and a small amount of current will flow through. On the other hand, if the motor is not carrying a heavy load, then the back emf will be significantly lower. Therefore the electrical power produced is converted to mechanical power to turn the load.

D2

Theoretically, the movement of a conductor through a magnetic field in a linear manner makes it easy for it to produce electricity. However, moving a wire constantly through a magnetic field practically poses a number of problems. Therefore the conductor is then shaped into a loop, so it can be easily rotated inside the permanent magnetic field. The magnet is polarised on one side and south on the other side. Each end of the loop is connected to a copper ring. According to faradays laws, the emf generated would be directly proportional to the number of loops of the wire.

As the loop rotates, inside the magnetic field, the emf rises to a peak, and falls to zero, and then to another peak in the opposite direction, then back to zero again. Therefore an alternating voltage and current is produced and observed.

3

Abdullah Ahmed

The above graph shows how the magnitude and direction of voltage generated by the rotating loop varies depending in the alignment of the loop.

Electric motors

The input of a generator is in the form of mechanical energy, and the output is electrical energy. However, an electric motor has an input of electrical energy and an output of mechanical energy. In the electric motor, the conductor is rotating in a magnetic field. When the single loop is placed between the north and south permanent magnet, the loop can now freely rotate. When a current is now passed through, the magnetic field which was generated, now goes against the magnetic field of the magnet, which allows the loop to start rotating. When the loop rotates 180 degrees, the existing force geos away as the magnetic fields are no longer against each other.

For continuous rotation of the loop, the current needs to be reversed through the loop so that the magnetic field are once again in opposition. A device called a commutator is used to accomplish the process of reversing the current. Instead of two rings, like the generator, the commutator only has one. When a voltage is initially supplied, the current only goes into one direction. When the conductor, rotates, so does the commutator. Therefore when the rotation has passed 180 degrees, the commutator reverses, and the electrical connection is also reversed. Therefore the current also reverses, which means the magnetic opposition is now restored, therefore the loop continues to move. Sometimes the commutator is split into many segments to improve the efficiency, because each sector has its individual conducting loop.

A DC machine has an outer stationary structure which is known as the stator. This consists of a ring which is connected to the magnetic poles and is surrounded by a field winding.

A DC machine also has an inner component called the armature which has a steel cylinder which is surrounded by a commutator.

4

Abdullah Ahmed

Reference

http://en.wikipedia.org/wiki/Electromagnetic_induction

http://www.electronics-tutorials.ws/electromagnetism/electromagnetic-induction.html

http://home.howstuffworks.com/induction-cooktops2.htm

http://physics.about.com/od/physicsetoh/g/induction.htm

https://www.nde-ed.org/EducationResources/HighSchool/Electricity/electroinduction.htm

http://electrical4u.com/faraday-law-of-electromagnetic-induction/

5

Abdullah Ahmed

6