Tag: magnetism and electromagnetism

Questions Related to magnetism and electromagnetism

Principle behind the working of electric generator?

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. When a conductor is moved in a magnetic field then voltage is induced in the conductor.

  2. When a conductor is moved in a magnetic field then current is induced in the conductor.

  3. both

  4. none

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Correct Option: B
Explanation:

Principle behind the working of electric generator When a conductor is moved in a magnetic field then current is induced in the conductor.
When a conductor moves in a magnetic field, an emf is induced across the conductor. This is the only basis on which each and every rotating electric generator works. Let us discuss the matter in a little bit details so that we can easily understand how does an electrical generator actually work. According to Faraday's law of electromagnetic induction, when a conductor links with a changing flux, it will have an induced emf across it. The value of induced emf across the conductor depends on the rate of change of flux linkage with the conductor. 

Mark the incorrect statement related to construction and working of a generator

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. a rectangular coil which is placed between two strong horse-shoe magnets.

  2. the direction of current is given according to the Flemings left hand rule.

  3. current produced by the generator is also called as alternating current.

  4. none

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Correct Option: D

Carbon brushes are not necessary in a dynamo, if the coil remains stationary and the magnet moves, because

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. Current is drawn from a stationary source

  2. Current is drawn from a moving source

  3. Direct current is produced

  4. It is not necessary to reverse the direction of the current.

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Correct Option: D
Explanation:

Carbon brushes are not necessary in a dynamo, if the coil remains stationary and the magnet moves, because It is not necessary to reverse the direction of the current.The brushes transfer the electricity from outside the motor to the spinning winding in the center of the motor. They undergo quite a lot of friction, and after a while wear out. Carbon is used as it is a reasonable conductor, and is soft enough to wear down instead of wearing down the "comutator" - the ring the brushes press against. Brushes are designed to be replaceable in large motors because of this.

A small AC dynamo is connected to a galvanometer and gently rotated, then the needle of the galvanometer,

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. Moves toward left.

  2. Moves towards right.

  3. Remains in the same position.

  4. Moves towards both right and left.

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Correct Option: D
Explanation:

A small AC dynamo is connected to a galvanometer and gently rotated, then the needle of the galvanometer moves towards both right and left The deflection of the needle of a galvanometer indicates a current flowing through the small stator winding (wound coil) inside the device. The more current flowing through the coil the greater the magnetic disturbance 

A domestic electrical appliance requires alternating current of 15 V. If 220 V of alternating current is supplied to the house, then the device that helps in the functioning of that electrical appliance is

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. Induction coil

  2. Step-up transformer

  3. AC dynamo

  4. Step-down transformer

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Correct Option: D
Explanation:

A step down transformer is one in which the secondary coil has lesser number of turns in secondary coil than in primary coil.  Due to this the ratio of voltages in the coils is

$\dfrac{V _s}{V _p}=\dfrac{N _s}{N _p}$ 
Hence the voltage across the secondary coil in the step down transformer is reduced.
Therefore such a transformer is used to apply a voltage of $15V$ across a device when alternating voltage of $220V$ is applied.

Dynamo produces:

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. Charge

  2. Electromotive force

  3. Electric field

  4. Magnetic field

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Correct Option: B
Explanation:

A dynamo is an electrical generator that produces direct current or (Electromotive force) with the use of a commutator.

A dynamo converts

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. mechanical energy into sound energy

  2. mechanical energy into electrical energy

  3. electrical energy into mechanical energy

  4. electrical energy into sound energy

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Correct Option: B
Explanation:

Dynamo is an electric generator where shaft is rotated mechanically and electric energy is produced. It converts the mechanical motion of the driven shaft into electrical motion,with the aid of a magnet.

When speed of a DC generator decreases, the armature current 

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. increases

  2. decreases

  3. does not change

  4. increases and decreases continuously

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Correct Option: A
Explanation:

The armature current will increase if the speed of the DC generator start decreasing due to an increase in voltage drop at armature resistance.

Dynamo is designed on the principle of 

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. Electromagnetic induction

  2. Self induction

  3. Mutual induction

  4. Pascal's law

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Correct Option: A

A six pole generator with fixed field excitation develops an emf of $100 { V } ,$ when operating at 1500$ { rm }$ At what speed must it rotate to develop $120 { V } ?$ 

physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

  1. $1200\mathrm { rpm }$

  2. $1800\mathrm { rpm }$

  3. $1500\mathrm { rpm }$

  4. $400\mathrm { rpm }$

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Correct Option: B