Form 4 physics

ELECTROMAGNETIC INDUCTION.

SPECIFIC OBJECTIVES

  1. Perform and describe simple experiments to illustrate electromagnetic induction.
  2. State the factors affecting the magnitude and the direction of induced current.
  3. State the laws of electromagnetic induction.
  4. Describe simple experiments to illustrate mutual induction.
  5. Explain the working of alternation current generator and direct current generator.
  6. Explain the working of a transformer.
  7. Explain the application of electromagnetic induction.
  8. Solve numerical problems involving transformers.

                                     ELECTROMAGNETIC INDUCTION

  • Is the production of an c.m.f in a conductor when there is relative motion between the conductor and a magnet such that the conductor cuts the magnetic field or causes a change in the magnetic field.-
  • Is a process where anc.m.f is induced in a conductor when there is relative motion between the conductor and magnet such that the conductor cuts the magnetic field or causes a change in the magnetic field.

EXP : TO SHOW ELECTROMAGNETIC INDUCTION.

      Requirement

  1. A coil of several turns
  2. A magnet
  3. Galvanometer
  4. Connecting wires.

Procedure;

  1. Connect the galvanometer in series with the coil.
  2. Move the north pole of the magnet towards the coil and note what happens.
  3. Pull the magnet away from the coil.

Observations;

  1. When the magnet is pushed inside the coil, the galvanometer deflects away from the magnet that is to the right.
  2. When the magnet is left stationary inside the coil , there was no deflection on the galvanometer.

Explanation;

  1. When the north pole is pushed inside the magnetic fields changes inducing current in the coil, which flows through the coil such that the end near the magnet becomes a north pole. Like poles repel and the force of repulsion tries to stop the magnet being pushed in and the galvanometer deflects.
  2. When the magnet is stationary no change in magnetic field. No current induced hence no deflection.
  3. When the north pole is withdrawn an induced current flows through the coil such that the end near the magnet becomes south pole. Unlike poles attract and the force of attraction tries to stop the north pole of the magnet to be withdrawn.
  4. A force has to be used to move the magnet in and out of the coil. This is the force which drives the current in the circuit.

Factors affecting the magnitude of induced e. m. f

  1. The speed of the conductor cutting the magnetic field.
  • The faster the conductor cuts the magnetic field of the faster the magnetic is withdrawn from the coil, the greater the induced e. m. f.
  1. The number of turns in coil
  • The higher the number of turns the higher the induced e. m. f since in every turn there is a current induced.
  1. The strength of the magnetic field
  • The stronger the magnetic field the higher the induced e. m. f
  1. The angle at which the conductor cuts the magnetic field
  • The conductor should cut the magnetic field at a right angle and not parallel.

Laws of electromagnetic induction.

  • There are two laws of electromagnetic induction.
  1. Faraday’s law.
  • States that the magnitude of induced e.m.f is directly proportional to the rate of change of magnetic flux linkage or to the rate at which the conductor cuts the magnetic flux.
  1. Lenz’s law.
  • States that the direction of induced e.m.f is such that the current which it causes to flow produces a magnetic effect that opposes the change producing it.
  • States that the induced current flows always in such a direction as to oppose the change which is giving rise to it.
  • This is an example of conservation of energy.
  • The induced current set up a force on the magnet which the mover of the magnet must overcome. Work is then done in overcoming this force and provides the electrical energy of the current.

                                    Fleming’s left hand rule.

  • It states that if the thumb and the first two fingers of the right hand are held mutually at right angles to each other with the first finger pointing in the direction of magnetic field, the thumb pointing the direction of motion then the second finger points the direction of induced current.

                                    Application of electromagnetic induction.

  1. Generators.
  2. Mutual induction (transformer, induction coil, ignition coil).
  3. Moving coil microphone.

                                           Generators.

  • They change mechanical energy of rotating coil to electrical energy.
  • There are two types of generators.
  1. Alternating current generator (a.c).
  • Produces an alternating e.m.f and alternating current.
  • It has a number of turns of wires rotating in a powerful magnetic field.
  • The coil can be rotated by either falling water, steam or wind.

A simple a.c generator (dynamo).

  • Consists of;
  1. A rectangular coil of wire connected to slip rings.
  2. Permanent magnet which produce a uniform magnetic field.
  • Carbon or copper brushes which presses against the slip rings.
  • Carbon (graphite) is commonly used because it is slippery and act as a lubricant.

                                    How it works.

The coil is made to rotate in clockwise direction in the magnetic field. AB moves up while CD moves down and cuts the magnetic field. Maximum e.m.f is induced in the coil which causes the current to flow from AB and CD. No e.m.f is induced in BC and AD. No current is induced as they are in the same direction of the magnetic field or are parallel to the magnetic field.

Brush B2 is the positive terminal while B1 is negative. As the coil rotates from horizontal to the vertical position, the angle at which the size of the coil cuts the magnetic field reduces from 90 to zero. The induced e.m.f reduces from maximum to zero. When the coil rotates past the vertical position, the side AB moves downwards and CD moves upwards. The induced e.m.f changes direction in the coil AB and CD hence current also changes direction and the brushes B2 is now negative and B1 is positive. The induced e.m.f is directly proportional to the angle of inclination to the coil to the vertical.

The coil rotates in magnetic field at a frequency of 50Hz.

The induced e.m.f and current also changes direction 50 times per second hence a.c 50Hz

  1. DC generator.

A simple DC generator consists of;

  1. A coil of one turn.
  2. Split rings or commutator.
  3. Carbon or copper brushes.
  4. Permanent magnet.
  5. Output resistor.

How it works.

The coil is made to rotate in clockwise direction. An e.m.f is induced in the coil as it rotates from horizontal to vertical position in the parts ab and cd. The current flows through the resistor R and decreases from maximum to zero when the coil is in minimum position. At vertical position, the brushes are not in contact with the rings. When the vertical position is passed, the split rings exchange brushes and the current in ab and cd changes direction but the direction of current through the internal resistor remains the same. B1 is negative and B2 is positive. The sides bc and ab do not cut the magnetic field hence no current is induced in them.

The generator produces a d.c direct voltage and a direct e.m.f through it reduces to zero every half of rotation.

In both generators, ac and dc, the induced e.m.f can be increased by

  1. Increasing the rotation of the coil.
  2. Increasing the number of turns of the coil.
  3. Increasing the strength of the magnet or magnetic field.
  4. Winding the coil on a laminated soft iron core.

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