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CLASS 12 – PHYSICS-- Chapter 4: ELECTROMAGNETISM INDUCTION

This is a discussion on CLASS 12 – PHYSICS-- Chapter 4: ELECTROMAGNETISM INDUCTION within the 12th forums, part of the Classes category; CLASS 12 – PHYSICS Chapter 4: ELECTROMAGNETISM INDUCTION Short answer question: Q1. Does the induced emf in a circuit depend ...

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    CLASS 12 – PHYSICS-- Chapter 4: ELECTROMAGNETISM INDUCTION


    CLASS 12 – PHYSICS

    Chapter 4: ELECTROMAGNETISM INDUCTION
    Short answer question:

    Q1. Does the induced emf in a circuit depend on the resistance of the circuit? Does the induced current depend on the resistance of the circuit?

    Ans. No, the induced emf in a circuit does not depends upon the resistance of the circuit but only depends upon the rate of change of magnetic flux and the number of turns on the coil.
    Yes the induced current depends upon the resistance of the circuit. Smaller the resistance of the circuit, greater the induced current.

    Q2. A square loop of wire is moving through a uniform magnetic field. The normal to the loop is oriented parallel to the magnitude field. Is an emf induced in the loop? Give a reason for your answer.

    Ans. The induced emf in the coil is given by
    E = BωAsinθ
    Where θ is angle between B and A
    When the area A is parallel to magnetic field then θ = 0°
    E = BωAsinθ but sinθ = 0
    For this position of the coil, the emf is the coil will be zero.

    Q3. A light metallic ring is released from above into a vertical bar magnet, does the current flow clockwise or anti-clockwise?

    Ans. When the metallic ring is released from above the north pole of the bar magnet, the motion of this ring will change the magnetic flux through the ring due to which an induced emf produces in ring which causes induced current. By Lenz’s law the direction of this induced current is such that it opposes the motion of the ring. The North Pole must produce an opposite pole i.e. South Pole at the upper face of the ring which will cause clockwise flow to induced current in the ring when viewed from above.

    Q4. When the switch in the circuit is closed a current is established in the coil and the metal ring jumps upwards, explain why? What would happen to the ring if the battery polarity were reversed?

    Ans. When the switch is closed in the circuit, the electric current flow through the coil. This current will produce magnetic field in the coil, the value of which changes from zero to maximum, this change of magnetic field in the coil will change the magnetic flux through the ring, which will produce an induced current in it. This induced current produce its own magnetic field which is opposite to the magnetic field produced its own magnetic field which is opposite to the magnetic field produced in the coil. Both the field repels each other and as the ring is free to move, therefore it move upward due to force of repulsion. The change in polarity will not have any effect on ring and will still move upward.

    Q5. A metal rod moves over conducting rails perpendicular to a magnetic field. There is motional emf generated in the rod. Does a stationary voltmeter connected?

    Ans. The motion of the rod over a conducting rail in magnetic field will produce motional emf in the rod this emf will produce current, the voltmeter in both position, will show due to the passage of current through the rod and also through the rails.

    Q6. How would you position a flat loop of wire in a changing magnetic field so that there is no emf induced in the loop?

    Ans. The emf induced din loop in changing magnetic field is given by
    E = BAωsinθ
    For the emf in the coil to be zero, then the angle θ should be zero.
    When θ = 0° E = ABωsinθ but sinθ = 0
    E = 0
    For this position the plane of the loop should be perpendicular to the magnetic field so that the angle between B and A is 0°.

    Q8. Can an electric motor be used to drive an electric generator with the output from the generator long being used to operate the motor?

    Ans. No, it is against the law of conservation of energy. The input power is not equal to out power. When the electric motor runs a generator, the electrical output is less than the input. The combination of motor and generator can never produce more energy out, than its energy input.

    Q9. Can a step-up transformer increase the power level?

    Ans. No the transformer cannot step up power for an ideal transform
    Input power = out power
    But in actual transformer there is a loss of energy therefore the output power is always less than the input power. Hence the transformer cannot increase the power level.

    Q10. When the primary of a transformer is connected to A.C. main the current in it

    (a) Is very small if the secondary circuit is open. But
    (b) Increase when the secondary circuit is closed. Explain these facts.
    Ans.
    a) When the secondary circuit is open, then the output is zero. As the output power is always less than the input power so a very so a very small current is drawn by the primary coil of transformer from A.C main.
    b) When the secondary circuit is closed, the output power increase due which transformer will draw large current from the A.C mains to increase the input to the primary coil. So, large current is required in primary coil to equalize the input power with the output power in secondary coil.


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