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 electric current, potential difference and resistance electric potential and potential difference potential difference current in electric circuits

In a DC circuit when a switch is closed to operate the circuit, the electrons that cause the current:

  1. travel from the negative plate of the battery to the positive plate

  2. travel from the positive plate of the battery to the negative plate

  3. travel from one resistance to the next resistance

  4. travel from the negative plate of the battery to the positive plate inside the battery

  5. travel from one atom in the circuit to the next atom
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In a DC circuit , when switch is closed , the free electrons in wire are drifted from negative plate of the battery to positive plate.

Multiple choice physics semiconductors band theory of solids, a brief introduction electron energies in solids energy bands

A transformer has 500 turns in its primary and 1000 turns in its secondary winding.The primary voltage is 200 V and the load in the secondary is 100 ohm.Calculate the current in the primary,assuming it to be a ideal transformer.

  1. 25 A

  2. 45 A

  3. 8A

  4. 22 A
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In usual rotation we have -

$\Rightarrow \dfrac{V _s}{V _p}=\dfrac{N _s}{N _p}$
$\therefore V _S=V _p\times \dfrac{N _s}{N _p}$
          $=200v\times \dfrac{1000}{500}$
          $=400V$
There is load resistance of $100\Omega$ in the secondary circuit. Therefore current in the secondary 
$\Rightarrow i _s=\dfrac{V _s}{R _s}=\dfrac{400V}{100\Omega}=4A$
In an ideal transformer the output power and input power are equal that is 
$\Rightarrow V _s\times i _s=V _p\times i _p$
$\therefore i _p=i _s \times \dfrac{V _s}{V _p}=4A\times \dfrac{400V}{200V}$
                            $=4A\times 2$
                            $=8A$
Hence, the answer is $8A.$

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

What is the meaning of the statement "the current rating of a fuse is $5 A$" ?

  1. Maximum $10 A$ pass through it after it will melt.
  2. Maximum $5 A$ pass through it after it will melt.
  3. Maximum $2 A$ pass through it after it will melt.
  4. Maximum $1 A$ pass through it after it will melt.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Fuses are safety devices that are to be built into our electrical system. If there were no fuses and we operated too many appliances on a single circuit, the cable carrying the power for that circuit would get extremely hot, short circuit, and possibly start a fire. To prevent electrical overloads, fuses are designed to trip or blow, stopping the flow of current to the overloaded cable.
Fuse is a piece of wire of a material with a very low melting point. When a high current flows through the circuit due to overloading or short circuit, the wires gets heated and melts. As a result, the circuit is broken and current stops flowing. 
The fuse must always be connected to the mains and it must be of correct current rating. For example, a 15-ampere fuse should trip when the current through it exceeds 15 amperes. A 20-ampere fuse should blow when the current through it exceeds 20 amps.
Hence, the statement 'the current rating of a fuse is 5 A' means that the fuse should trip when the current through it exceeds 5 amperes. So correct option is B.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

Complete the following sentences :

A fuse is connected in .......... with the .......... wire.

  1. series, neutal

  2. series, earth

  3. series, live

  4. parallel, live
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In our homes, we receive supply of electric power through a main supply (mains), either supported through overhead electric poles or by underground cables.The live wire and neutral wire, coming from the electric pole, enter a box fitted just outside our house which has a main fuse. The fuse is connected in series with the live wire. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential. The fuse has a high rating of about 50 amperes. Thus it prevents any damage such as fire to the entire electrical wiring entering the house due to short-circuit or overloading.
The two wires then enter the electricity meter which records the electrical power consumed by us in kilowatt-hour (kWh).
Hence, A fuse is connected in series with the live wire.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

The main fuse is connected in :

  1. live wire.

  2. neutral wire.

  3. both the live and earth wires.

  4. both earth and neutral wire.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The live wires coming out from the output terminals of kWh meter has another fuse in it called the main fuse. 
The fuse is connected in series with the live wire before it enters the household circuit. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential. The fuse has a high rating of about 50 amperes. Thus it prevents any damage such as fire to the entire electrical wiring entering the house due to short-circuit or overloading.Hence correct option is A.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

Where the main fuse in a house circuit is connected ?

  1. In live wire before the ring system.

  2. In ground wire.

  3. In neutral wire.

  4. In between ground and live.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The live wires coming out from the output terminals of kWh meter has another fuse in it called the main fuse. 
The fuse is connected in series with the live wire before it enters the household circuit. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential. The fuse has a high rating of about 50 amperes. Thus it prevents any damage such as fire to the entire electrical wiring entering the house due to short-circuit or overloading.Hence correct option is A.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

Two identical fuses are rated at 10 A. If they are joined
(A) in parallel, the combination acts as a fuse of rating 20 A
(B) in parallel, the combination acts as a fuse of rating 5 A
(C) in series, the combination acts as a fuse of rating 10 A
(D) in series, the combination acts as a fuse of rating 20 A
Select the correct options

  1. A, B

  2. A, C

  3. B, D

  4. B, C, D
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The rating of fuse is ,if the current is greater than the rated current then the fuse blows.
So if the rating is 10A, then it is the maximum current that passes through fuse.
If two such fuses are connected in parallel , the total current divides into two components where each can be maximum of 10 A. The maximum total current can be 20 A.
If they are connected in series,same current flows through each of them. The maximum current allowed would be the minimum rated current among the two fuses.So here the current rating will be 10A.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

The cable of an electric fan becomes so worn that the live wire makes electrical contact with the metal case The case is earthed The plug to the fan contains a 5 A fuse There is a current of 4 A when the fan works normally What will happen in this case?

  1. The current will run to earth and the fuse will not be affected

  2. The metal case will become very hot

  3. The metal case will become live and dangerous

  4. The fuse will melt and switch off the circuit
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Answer is D.

When the live wire (high potential) is in contact with the earthed metal case (zero potential) and since there is a potential difference, current flows. As there is little to no resistance in the completed path, the current flow is very large (larger than 4 A melting the fuse). The fuse melts due to high current surge and switches off the circuit.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

On Diwali Mr.Antriksh wants to light his house with a number of $10 W (220 V)$ bulbs in parallel. If the house circuit has a fuse of $5 A$ then what is the maximum number of such bulbs he can use if he is not using any other electrical appliance in the circuit.

  1. $10$
  2. $11$
  3. $100$
  4. $110$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Answer is D.

The power dissipated is given as P=VI.
In this case, the power of each the bulb is 10 W which uses 220 V supply.
So, the amount of current used is I=P/V = 10/220 = 0.045 A. 
The fuse has a power rating of 5 A. Hence, 5 A / 0.045 A = 111 such bulbs can be connected in parallel with security.
Hence, Mr. Antriksh can use 110 bulbs without any problem. 

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

A car headlamp of 48 W works on the car battery of 12 V. The correct fuse for the circuit is 

  1. 5A

  2. 2A

  3. 3A

  4. 15A
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.

Fuses are safety devices that are to be built into our electrical system. If there were no fuses and we operated too many appliances on a single circuit, the cable carrying the power for that circuit would get extremely hot, short circuit, and possibly start a fire. To prevent electrical overloads, fuses are designed to trip or blow, stopping the flow of current to the overloaded cable.
The fuse must always be connected to the mains and it must be of correct value. For example, a 15-ampere fuse should trip when the current through it exceeds 15 amperes. A 20-ampere fuse should blow when the current through it exceeds 20 amps.
In this case, the car headlamp is 48 W and the voltage is 12 V.
That is, P = VI, I = P/V = 48/12 = 4 A.
Hence, the fuse of 5 A should be connected to the circuit.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

The circuit breaker device which can be used in place of a fuse is called

  1. CBD

  2. MCB

  3. MCD

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

Answer is B.

Fuse boxes contain safety devices called fuses or circuit breakers - MCB (Miniature Circuit Breaker) which are designed to prevent an excess of electric current entering your house by only allowing a certain maximum current to flow in. When the electrical current is too large, an electromagnet attracts a spring loaded switch which breaks the circuit and cuts the power supply. The switch must be reset before any current can flow again.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

In an electric circuit, a fuse is connected :

  1. in the live wire

  2. in the neutral wire

  3. in the earth wire

  4. Any of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.

The live wire and neutral wire, coming from the electric pole, enter a box fitted just outside our house which has a main fuse F1. The fuse is connected in series with the live wire. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential.

Multiple choice physics electric current and its effects electric fuse electrical fuse and mcb fuse

Fuses are connected in ________ to the live wire:

  1. parallel

  2. series

  3. either way

  4. None
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The live wire and neutral wire, coming from the electric pole, enter a box fitted just outside our house which has a main fuse F1. The fuse is connected in series with the live wire. This is done so because it is only the live wire which has a high potential of 220 volts unlike the neutral wire which carries zero potential.