Class 12 Physics Electromagnetic Induction Direction of Induced current

Direction of Induced current: Lenz law

Induced current opposes CHANGE in flux.

Example1: -

• Consider a region where magnetic field is acting in downward direction.
• Let magnetic field be 20T and it gets converted to 10T in the downward direction.
• As the magnetic field is changing therefore there is change in the flux which is equal to the decrease in the flux.
• As there is change in the flux there will be induced current.
• Flux of the induced current will try to oppose the decrease in the flux.
• Therefore the direction of induced flux will also be in the downward direction.
• Applying Fleming’s Right hand Thumb rule the direction of current will be given in the clockwise direction. Example2:-

• Consider a magnetic field of 20T in the upward direction.If the magnetic flux is reduced to 10T in the downward direction.
• Both magnitude and the direction are changing.
• As flux is changing therefore there will be induced current.
• The direction of the induced flux will be in the opposite direction. i.e. upward direction.
• The induced flux will oppose the change which is the downward direction.
• The direction of the current will be in the anticlockwise direction. Example3:-

• Consider a region having downward magnetic field and there is a rectangular loop which is entering the region of magnetic field.
• As a result flux increases, the induced flux will oppose the increase in the flux.i.e. it will decrease the flux.
• Therefore induced flux will be in the upward direction.
• As a result the induced current will be in the anti-clockwise direction. Example4:-

• Consider a region having magnetic field in the downward direction and a rectangular loop which is coming out of the magnetic field with some velocity v.
• As it is coming out therefore flux is decreasing, current will be induced because of change in flux.
• Induced flux will try to oppose the change i.e. it will try to increase the flux.
• Induced flux is in the downward direction as a result the direction of current will be clock wise. Problem:- Use Lenz’s law to determine the direction of induced current in thesituations described by Fig.: (a) A wire of irregular shape turning into a circular shape;

(b) A circular loop being deformed into a narrow straight wire.

Answer:- According to Lenz’s law, the direction of the induced Emf is such that it tends to produce a current that opposes the change in the magnetic flux that produced it.

(a) When the shape of the wire changes, the flux piercing through the unit surface area increases. As a result, the induced current produces an opposing flux. Hence, the induced current flows along adcb.

(b) When the shape of a circular loop is deformed into a narrow straight wire, the flux piercing the surface decreases. Hence, the induced current flows along

e= (4x3.14x50x0.1x10)/ (2x3.14x0.2)

e = 5x10-5V a’d’c’b’.

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