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 electro-magnetism applications of electromagnets transformer transformers

The primary winding of a power transformer should always be

  1. open

  2. shorted

  3. switched

  4. fused
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When the secondary load is overloaded, it draws maximum current in the primary side due to which damaging the primary side as well as appliances. Therefore primary winding of power transformer is fused. Suppose peak voltage of power source is varying then current will also fluctuate so also to limit the current it is fused.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a transformer the number of turns in the primary coil is 200 and secondary coil is 600. The ratio between the electric current in the primary and secondary respectively is

  1. 1 : 4

  2. 4 : 1

  3. 1 : 3

  4. 3 : 1
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a transformer, we know that 
$\frac{V _2}{V _1}=\frac{N _2}{N _1}$
where $V _1$ and $V _2$ are voltage across primary and secondary coils respectively and $N _1$ and $N _2$ are respective number of turns. 

Also, the transformer doesn't add or take away energy, so the power in
the primary and secondary is equal:
$V _1I _1=V _2I _2$
Therefore, $\frac{I _1}{I _2}=\frac{N _2}{N _1}$ 
$\frac{I _1}{I _2}=\frac{600}{200}=\frac{3}{1}$ 
Hence, desired ratio is 3:1. (D)

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformer works on.

  1. AC only

  2. DC only

  3. Both AC and DC

  4. AC more effectively than DC
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Transformer works on the principle of Electromagnetic induction i.e an emf is induced in the coil if the magnetic flux linked with the coil changes. Magnetic flux in the coil can change only in case of AC voltage supply. In case of DC voltage supply, magnetic flux in the secondary coil remains constant and thus no emf is induced in the secondary coil and so transformer cannot work in case of DC supply.
That's why, transformer can work on AC only.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer is employed to

  1. convert A.C. into D.C.

  2. covert D.C into A.C

  3. obtain a suitable A.C voltage

  4. obtain a suitable D.C voltage
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

C

A transformer is a device that is used to either raise or lower voltage, and currents in an electric circuit. Thereby, we can suitably obtain the voltage and current.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Which of  the following has its working based on electromagnetic induction -

  1. Ammeter

  2. Voltmeter

  3. Transformer

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

C

Electromagnetic Induction is the production of an emf across an electrical conductor in aa changing magnetic field in a transformer the alternating current through the primary winding produces a continually changing flux and die to this alternating flux on emf is induced in the secondary winding.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Read the following statements and answer whether the given statement is true or false.


A transformer is an electrical device that works on the principle of self-induction

  1. True

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

False

A transformer consists of two electrically isolated coils and operates on Faraday’s principle of mutual induction, in which an EMF is induced in the transformer secondary coil due to the magnetic flux generated by the voltages and currents flowing in the primary coil working. In self induction there is only one coil and emf is induced in that coil due to the change in flux linked with that coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer works on both A.C. and D.C.

  1. True

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

False

Transformer does not work on DC. Direct current doesn’t have a variable magnetic field induced if fed through the primary winding of a transformer. Thus, only a constant magnetic field will act on secondary and shall not induce any voltage across the terminals of the same.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Why are transformers used in the transmission of electric power?

  1. Because they can reduce transmission losses

  2. Because they can conserve electrical energy

  3. Because they speed up transmission

  4. Because they help to distribute electrical energy efficiently
Reveal answer Fill a bubble to check yourself
A,D Correct answer
Explanation

Transformer used in the transmission of electrical power because they help to distribute electrical energy efficiently and can reduce transmission loss.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Which of the following statement about the action of a transformer is correct?

  1. Alternating current always flows in the secondary coil when an alternating voltage is applied to the primary coil.

  2. The current in the secondary coil is always larger than the current in the primary coil.

  3. An e.m.f. is induced in the secondary coil when an alternating voltage is applied to the primary coil.

  4. An e.m.f. is induced in the secondary coil when a steady direct current flows in the primary coil.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Alternating current does not flow in the secondary coil unless electrical load is connected to its terminals.
In step-up transformer current in secondary coil is less than the current in primary coil.
No e.m.f. is induced in the secondary coil if the primary coil is connected to a d.c. supply.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The current drawn by the primary of a transformer, which steps down 200 V to 20 V to operate a device of resistance 20 $\Omega$ is: (Assume the efficiency of the transformer to be 80 %)

  1. 0.125 A

  2. 0.225 A

  3. 0.325 A

  4. 0.425 A
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
$V _{primary} =200\ V$
$V _{secondary}=20\ V$
$R=20\ \Omega$
Power secondary $=\dfrac {(V _{secondary}^2)}{R}$

                               $=20\ W$

Power primary $=\dfrac {20}{\dfrac {80}{100}}=25\ w\quad (80\%\ efficiency)$

Power primary $=(I _{primary}) (V _{primary})$
$25=I _P \times 200$
$I _P=0.125\ A$
Option $(A)$
Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a step up transformer the turn ratio is 1:2. A Leclanche cell (emf = 1.5 V) is connected across the primary. The voltage across the secondary is

  1. 3.0 V

  2. 1.5 V

  3. 0.75 V

  4. 0
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

For the step up transformer -

$\dfrac { { V } _{ 1 } }{ { V } _{ 2 } } =\dfrac { { n } _{ 1 } }{ { n } _{ 2 } } $

$\dfrac { 1.5 }{ { V } _{ 2 } } =\dfrac { 1 }{ 2 } $

$\Rightarrow { V } _{ 2 }=3V$.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer works on the principle of

  1. self induction

  2. electrical inertia

  3. mutual induction

  4. magnetic effect of the electrical current
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Transformer is based on the principle of electromagnetic induction.
There are two coils named as primary and secondary wound on a soft iron core.
When AC is applied to the primary coi, the resulting current will the production of
continously varying magnetic flux.
Due to mutual induction, induced EMF is produced in secondary coil which in return results in current.  
Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformer is used to

  1. convert ac to dc voltage

  2. convert dc to ac voltage

  3. obtain desired dc power

  4. obtain desired ac voltage and current
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
Transformer converts some value of voltage/current into some other desired value based upon the principles of mutual induction.
Transformer is based on the principle of electromagnetic induction. There are two coils named as primary and secondary wound on a soft iron core. When AC is applied to the primary coil, the resulting current will the production of continuously varying magnetic flux. Due to mutual induction, induced EMF is produced in secondary coil which in return results in current.