Physics

Electrical Machines and Circuits

754 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 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.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A step-up transformer increases the AC voltage. The statement is 

  1. true

  2. false

  3. ambiguous

  4. data insufficient

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A step up transformer increases the voltage This transformer has more windings on the "secondary" side of the transformer, with fewer windings of thicker wire on the primary or input coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

State whether true or false:

Transformers can change high voltage to low voltage.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A transformer takes in electricity at a higher voltage and lets it run through lots of coils wounded around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core, there is an output wire with fewer coils. The magnetism changing back and forth creates a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The transformer is used in ................. current circuits.

  1. alternating

  2. directing

  3. no

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

A transformer consists of two coils - the primary coil and the secondary coil both wound on a soft iron core. If an ac voltage is applied to the primary coil then this will produce a changing magnetic field in the iron core. This induces an alternating voltage in the secondary coil according to electromagnetic induction.
A voltage in the secondary coil occurs only if the magnetic field is changing. Therefore, transformers will only work with ac current only. The transformer is used in alternating current circuits.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformers are used

  1. in DC circuit 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

Transformers work on the principle of voltage induction by the linkage of changing magnetic flux between between the two cores. Changing magnetic flux requires changing/alternating current. 
Transformers are used only in AC circuits. In DC circuits, no alternating/changing magnetic flux is produced to induce voltage in secondary from the primary terminals of the transformers.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

How will you  relate the number of turns in the primary coil of a step-down transformer  to that in secondary coil ?

  1. has large number of turns

  2. is very heavily insulated

  3. is very thin

  4. all the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A step down transformer has more turns of wire on the primary coil and less turns of wire on the secondary coil. This makes a smaller induced voltage in the secondary coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a step down transformer, the number of turns in the primary is always :

  1. greater than the number of turns in the secondary

  2. less than the number of turns in the secondary

  3. equal to the number of turns in the secondary

  4. either greater than or less than the number of turns in the secondary
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In step down transformers, voltage in secondary coil decreases. and we know that 


$\dfrac { { E } _{ s } }{ { E } _{ p } } =\dfrac { { n } _{ s } }{ { n } _{ p } } $

So, number of turns decreases in secondary coil with respect to primary coil.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer changes the voltage

  1. without changing the current and frequency

  2. without changing the current but changes the

    frequency

  3. without changing the frequency but changes the

    current

  4. without changing the frequency as well as the

    current
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A transformers changes an alternating potential difference from one value to another of greater or smaller value using the principle of mutual induction. so, in transformer current and voltage changes but frequency remains same.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Statement A: In a transformer, a large alternating current at low voltage can be transformed into a small alternating current at high voltage
Statement B: Energy in current carrying coil is stored in the form of magnetic field.

  1. A is true but B is false

  2. Both A and B are true

  3. A is false but B is true

  4. Both A and B are false
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformer is used to increase voltage and decrease current in the secondary coil with respect to primary coil or vice versa.
Energy in the coil is always stored in the form of magnetic field.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformers are used in

  1. d.c circuits only

  2. a.c. circuits only

  3. Both a.c and d.c circuits

  4. Integrated circuits
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformers are used only in A.C. Circuits because D.C. current could not produce changing flux which in turn not able to produce induced emf in the secondary coil of the transformer but A.C. Current produces changing flux.