Physics

Electrical Machines and Circuits

720 Questions

Electrical machines and circuits involve the principles of alternating and direct current, transformers, diodes, and power consumption. These concepts are essential for engineering and technical exams. Review these practice questions to test your knowledge of circuit analysis.

Transformers and impedanceAlternating and direct currentCircuit components and diodesPower consumption calculations

Electrical Machines and Circuits Questions

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

A dynamo is used to supply electric current to a small electric bulb, If the number of turns in the coil of this dynamo is doubled, then the voltage in the bulb.

  1. Will decrease

  2. Remains unchanged

  3. Will be doubled

  4. Will increase by a factor between 1 and 2
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the number of turns in the coil increases, the strength of the voltage induced in the coil will also increase as the number of turns and induced voltage are directly proportional to each other.
Therefore, if the number of turns in the coil of this dynamo is doubled, then the voltage in the bulb will also be doubled.

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

What is a commutator?

  1. A device that reverses the direction of flow of current through a circuit is called a commutator.

  2. A device that flows in the direction of flow of current through a circuit is called a commutator.

  3. A device that reverses the direction of flow of voltage through a circuit is called a commutator.

  4. All
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

Fleming's left and right hand rule are used in

  1. DC motor and AC motor

  2. DC generator and AC motor

  3. DC motor and DC generator

  4. both rules are same, any one can be used
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

DC motor is a device which coverts electrical energy into mechanical energy. It employs Fleming's left hand rule.
DC generator converts mechanical energy into electrical energy in from of DC. It employs Fleming's right hand rule.

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

If a loop in a basic dc generator suddenly begins rotating at a faster speed, the induced voltage

  1. remains unchanged

  2. reverses polarity

  3. increases

  4. decreases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Induced voltage in the rotating loop is given by   $\mathcal{E} = NBAw \ \sin wt$
where $N$ is the number of turns in the loop, $A$ is the area of the loop, $B$ is the magnetic field and $w$ is the angular speed of rotation of loop.
$\implies \ \mathcal{E}\propto w$
So, the induced voltage increases on increasing the speed of rotation of loop.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

Which of the following statement is correct?

  1. AC generator generates a higher voltage

  2. DC generator generates a higher voltage

  3. AC generator has a permanent magnet whereas a DC generator has an electromagnet

  4. There is a split-ring commutator in a DC generator but not in an AC generator
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

The voltage generated by a generator , whether AC or DC , depends upon the rate of change of flux linked with coil .

Both type of generators usually have permanent magnets .
There is a split- ring commutator in a DC generator but an AC generator has slip - rings .
Option D is correct .

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

The current produced in a generator armature is AC because

  1. the magnetic field reverses at intervals

  2. The current in the field coils is AC

  3. the rotation of the armature causes the field through it to reverse

  4. the commutator feeds current into it in opposite directions every half cycle
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

After every half rotation, the direction of e.m.f reverses and hence current changes its direction after a regular interval.

Hence, Answer-(C)

Multiple choice physics simple electric circuits bulbs bulb bulb connected to an electric cell

In the bulb there is a -

  1. Long wire

  2. Short wire

  3. Thin wire

  4. Wide wire
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

as we know $Resistance =Resistivity\dfrac{Length}{Area}$

therefore, for making resistance more the radius of the wire should be less to make area less and make resistance more. as resistance is inversely proportional to area. therefore to make bulb we need thin wire. which is capable of withstanding high temperature and have high resistance.

Multiple choice physics simple electric circuits bulbs bulb bulb connected to an electric cell

The filament of a bulb is usually a:

  1. thin wire with many coils

  2. thick wire with many coils

  3. thin straight wire

  4. thick straight wire
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

 When electric current goes through the filament, the filament glows. It may also be known as the electron emitting element in a vacuum tube. To make the bulb produce more light,the filament is usually made of coils of thin wire, also known as the coiled coil in order to get more emmision.

Generally, Filaments in incandescent light bulbs are made of tungsten as its melting point is very high.
Therefore, A is correct option.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformer are used

  1. in $DC$ circuits only
  2. in $AC$ circuits only
  3. in both $DC$ and $AC$ circuits
  4. neither in $DC$ nor in $AC$ circuits.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformer is an electrical device that requires time varying current to link two or more electrical coils through electromagnetic induction and thus transfer energy between the magnetically coupled circuits. Therefore transformer are used only in AC circuits

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Why does the light of a car dim when the starter is operated?

  1. The starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery.

  2. When starter motor is switched on, it increases the resistance of the filament of the bulb in the car light.

  3. When the starter motor is switched on, it reduces the emf of the battery.

  4. The starter motor produces fumes, which absorb a part of the light of the car lamp.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Since, the starter motor draws a large current resulting in a large potential drop in the battery, which reduced the thermal voltage of the battery hence, the light of a car are dim when the starter is operated.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Name the device used to transform 12 V a.c. to 200 V a.c. Name the principle on which it works.

  1. Step down transformer, magnetic induction

  2. Step down transformer, electromagnetic induction

  3. Step up transformer, magnetic induction

  4. Step up transformer, electromagnetic induction
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit.
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
Therefore, to convert 12V a.c. to 200V a.c., a step up transformer is used.
This transformer works in the principle of Electromagnetic induction which states that rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil.
Say we have one winding which is supplied by an alternating electrical source. The alternating current through the winding produces a continually changing flux or alternating flux that surrounds the winding. If any other winding is brought nearer to the previous one, obviously some portion of this flux will link with the second. As this flux is continually changing in its amplitude and direction, there must be a change in flux linkage in the second winding or coil. According to Faraday's law of electromagnetic induction, there must be an EMF induced in the second. If the circuit of the later winding is closed, there must be an current flowing through it. This is the simplest form of electrical power transformer and this is the most basic of working principle of transformer.