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 electro-magnetism applications of electromagnets transformer transformers

Electrical energy is transmitted over large distance at high alternating voltages Which of the following statements is (are) correct?

  1. for a given power level there is a lower current

  2. lower current implies less power loss

  3. transmission lines can be made thinner

  4. It is easy to reduce the voltage at the receiving end using step-down transformers
Reveal answer Fill a bubble to check yourself
A,B,D Correct answer
Explanation

Power = Voltage x Current
As voltages are high so currents are lower. This is done to reduce the resistive losses.
Resistance Loss = ${ { I } _{ rms }^{ 2 } }R$
So, lower current implies less power loss. 
As resistance is inversely proportional to their diameter; hence decreasing their diameter by x will increase resistance by $x^2$ and hence increasing power loss hence not done.
As the voltage required for domestic and commercial uses are pretty less then they can be easily brought down by step-down transformers. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Tum ratio in a step up transformer is 1 : 2 if a Lechlanche cell of 1.5 V is connected across the input, then the voltage across the output will be 

  1. 0.1 V

  2. 1.5 V

  3. 0.75 V

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

A transformer can not step up a d.c. input so output potential here will be zero. No potential will be induced in the secondary coil.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Large transformers, when used for some time, become very hot and are cooled by circulating oil. The heating of the transformer is due to

  1. The heating effect of current alone

  2. Hysteresis loss alone

  3. Both the heating effect of current and hysteresis loss

  4. Intense sunlight at noon
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation 

Heat loss in a magnetic circuit is due to two reasons one is hysteresis and  other is due to ohmic loss due to eddy current .Laminations and circulatingoil are some of methods to avoid heating. Hysteresis loss is entirelymaterial property while eddy current loss depends on geometry of core and amount of current.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A certain transformer has a turns ratio of 1 and a 0.85 coefficient of coupling. When 2 V ac is applied to the primary, the secondary voltage is

  1. 1.7 V

  2. 0.85 V

  3. 1 V

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

Coefficient coupling  $\eta = 0.85$
Input voltage in primary coil   $V _p = 2 \ V$
Turn ratio  $\dfrac{N _s}{N _p} = 1$
Thus output voltage in secondary coil   $V _s = \eta \dfrac{N _s}{N _p}V _p$
$\implies \ V _s = 0.85\times 1\times 2 = 1.7 \ V$

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.