Tag: magnetism and electromagnetism

Quizzes (6) Questions (43)

Questions Related to magnetism and electromagnetism

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

Principle behind the working of electric generator?

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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. 

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

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

  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.
Reveal answer Fill a bubble to check yourself
D Correct answer
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.

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

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

  1. Moves toward left.

  2. Moves towards right.

  3. Remains in the same position.

  4. Moves towards both right and left.
Reveal answer Fill a bubble to check yourself
D Correct answer
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 

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

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

  1. Induction coil

  2. Step-up transformer

  3. AC dynamo

  4. Step-down transformer
Reveal answer Fill a bubble to check yourself
D Correct answer
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.

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

A dynamo converts

  1. mechanical energy into sound energy

  2. mechanical energy into electrical energy

  3. electrical energy into mechanical energy

  4. electrical energy into sound energy
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

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 $120 { V } ?$ 

  1. $1200\mathrm { rpm }$

  2. $1800\mathrm { rpm }$

  3. $1500\mathrm { rpm }$

  4. $400\mathrm { rpm }$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

For a generator with fixed field excitation, the induced EMF is directly proportional to the speed of rotation (E = k * N). Given 100V at 1500 rpm, the constant k is 100/1500. To find the speed for 120V, we solve 120 = (100/1500) * N, which yields N = 1800 rpm.