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ELECTROMAGNETIC INDUCTION A-LEVEL PHYSICS

This Unit is about Electro-magnetic induction.

ELECTROMAGNETIC INDUCTION

Electromagnetic induction is the process of getting an emf by changing the number of magnetic field lines associated with the inductor. Electromagnetic induction forms the basis of working of power generation, dynamos, generators etc.

Laws of electromagnetic induction.

(1) Faraday’s law:

Whenever the magnetic flux liking a circuit changes, an emf is induced in the circuit. The magnitude of the induced emf is directly proportional to the rate of change of the magnetic flux linking the circuit.

Demonstration of Faraday’s experiments

Based on relative movement

INDUCTION

Observations:

(i)  Whenever there is relative motion between a coil and a magnet, the galvanometer shows a certain deflection. This indicates that current is induced in the coil.

(ii)  The deflection is temporary. It lasts so long as the relative motion between the coil and the magnet continues.

(iii)  The deflection increases with increase in relative motion.

(iv)  The direction of the deflection is reversed when either the pole of the magnet is reversed or the direction of motion of the magnet is reversed.

Based on changing magnetic field.

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As the switch K is closed, G shows a sudden temporary deflection showing that current is induced in a secondary coil. This is because the current in the primary coil increases from zero to a certain steady value, increasing the magnetic field and hence the number of field lines through the secondary.

When K remains closed, G indicates no deflection, no emf is induced when the magnetic field lines through the secondary remain constant. As K is opened, G shows a sudden temporary deflection in the opposite direction. This is because the decrease in the primary current causes the field lines though the secondary to decrease. Therefore G only deflects when the current in the primary is changing and hence the magnetic flux through the secondary is changing.

Lenz’s law:

The direction of the induced emf is in such a way as to oppose the change causing it.

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When the North Pole of the magnet is moved towards the coil, the current induced in the coil  flows in the direction indicated. And the galvanometer deflects in the clockwise direction.

End A of the coil becomes the North Pole. This implies that the fields created by the induced current in the coil opposes the field of the magnet (like poles repel).

When the magnet is moved away from the coil, the galvanometer deflects in the opposite direction, indicating that end A of  the coil becomes the South Pole.

Lenz’s law and conservation of energy

Lenz’s law is an example of conservation o energy. In order not to violate the principle of conservation of energy, the effects of the induced emf must oppose the motion of the magnet, so that the work done by the external agent in moving the magnet is the one converted to electrical energy.

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AC is a curved rod which can be rotated by a wheel ω round around the North Pole of a long magnet. Brushing contacts at X and Y connect the rod to the galvanometer and a series resistance, R.

When the wheel is turned, the rod AC cuts across the field, B of the magnet and an emf induced in it. If the wheel is turned steadily, the galvanometer gives a steady reading deflection, θ,  showing that a steady current is flowing around a circuit.

Keeping the circuit resistance constant, the rate at which the wheel is turned is varied and the time of revolutions per second is noted and recorded using a stop watch (clock). The number of revolutions per second (n) is obtained each time the wheel is turned. A graph of the deflection  of the galvanometer against n is plotted and it is a straight line through the origin.

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Emf induced in a moving conductor

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When PQ is moved  to the left with uniform velocity v, an induced current flows in the direction shown. Hence PQ becomes a current carrying conductor moving in the magnetic field.

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When a wire is moved across the magnetic filed, electrons in it are also moved, likewise across the filed. The force on electrons is at right angles to the plane containing the velocity of the wire, and magnetic flux density, B. hence it tends to drive electrons along the wire. When AC swept across B, the force on the electrons in it acts from A to C. Therefore, if the wire is not connected to a closed circuit, electrons pile up at C. end C hence becomes negative and A positive. End A is at higher potential than C.

Fleming’s right hand rule

It is used to determine the direction of the induced current. It states that: “When the thumb, first finger and second finger of the right hand are held mutually at right angles, with the thumb pointing in the direction of motion, the first finger in the direction of the magnetic filed, then the second finger points in the direction of the induced current”.

Emf induced between the center and the rim of a disc rotating about its axis in a uniform field.

Consider a disc of radius r, rotating about its axis at a distance.

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THIS VIDEO EXPLAINS MORE ABOUT ELECTRO-MAGNETIC INDUCTION

 

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