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 and its effects electric fuse electrical fuse and mcb fuse

A fuse is connected to the

  1. Live wire

  2. Neutral wire

  3. Earth wire

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Fuses are always connected in series with the components to be protected from overcurrent, so that when the fuse blows (opens) it will open the entire circuit and stop current through the components. A fuse connected in one branch of a parallel circuit, of course, would not affect current through any of the other branches.

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

If a fuse is rated as 8 A, and current more than 8 A passes through it then

  1. it would melt

  2. circuit gets broken

  3. both

  4. none
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The maximum current which can flow through a fuse without melting it is called its rating.for example a fuse rated at 8A,can stand current upto 8 A. If current higher than 8 A flows through the fuse, it would melt and circuit gets broken the rating of a fuse depends upon the thickness of the fuse wire (because length of the wire is fixed). The thicker fuse has higher rating. So, the thickness of the fuse wire in 8 A circuit is more than that of the fuse wire in 5 A circuit.

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

Mark the correct reason why a fuse is always connected in series

  1. When fuse melts, it breaks down the entire home circuit and no current flows

  2. In series connection the current for the entire house pass through the fuse

  3. Both

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

A fuse is always connected in series because 

when fuse melts, it breaks down the entire home circuit and no current flows
$\rightarrow$ In series connection the current for the entire house pass through the fuse 

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

Which of the following is the function of MCB in household circuit?

  1. This switches automatically turn off when current exceeds safe limit

  2. This switches automatically turn on when current exceeds safe limit

  3. Saves the electrical appliances from getting damaged due to excess current

  4. Saves from shortcircuiting
Reveal answer Fill a bubble to check yourself
A,C,D Correct answer
Explanation

An MCB functions by interrupting the continuity of electrical flow through the circuit once a fault is detected. In simple terms MCB is a switch which automatically turns off when the current flowing through it passes the maximum allowable limit.

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected.

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

Choose the correct option 

  1. MCB is much more sensitive to over current than fuse

  2. Fuse is much more sensitive to over current than MCB

  3. Handling MCB is more electrically safe than fuse

  4. Handling fuse is more electrically safe than MCB
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

Nowadays MCB in low voltage electrical network is commonly used instead of fuse. The fuse may not sense the overcurrent but the miniature circuit breaker does it in a more reliable way. MCB is much more sensitive to over current than fuse. Handling a MCB is electrically safer than a fuse.

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

Electric fuses is a _____________ to electrical circuits.

  1. safety device which prevents damages

  2. device which causes damage and fire

  3. simple device which pass current

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A fuse is an electrical safety device (a component) that removes electrical current from an electrical circuit when the current in the electrical circuit is too high.

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

  1. in parallel, the combination acts as a fuse of rating 20 A

  2. in parallel, the combination acts as a fuse of rating 5 A

  3. in series , the combination acts as a fuse of rating 10 A

  4. in series, the combination acts as a fuse of rating 20 A
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

When two identical fuses are joined in parallel, the total current capacity is the sum of the individual ratings because the current splits between them. Thus, 10 A + 10 A = 20 A.

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

Two identical fuses are rated at $10\space A$. If they are joined

  1. in parallel, the combination acts as a fuse of rating $20\space A$
  2. in parallel, the combination acts as a fuse of rating $5\space A$
  3. in series, the combination acts as a fuse of rating $10\space A$
  4. in series, the combination acts as a fuse of rating $20\space A$
Reveal answer Fill a bubble to check yourself
A,C Correct answer
Explanation

A fuse is a type of low resistance resistor that acts as a device to provide over-current protection, of either the load or source circuit.
Where several fuses are connected in series, it is desirable to blow (clear) only the fuse (or other over-current device) electrically closest to the fault. Hence, when two fuses are in series the current rating is simply the current rating of each fuse i.e. in series, the combination acts as a fuse of rating 10 A
When two fuses are in parallel the current rating is simply twice the current rating of each fuse, hence, in parallel, the combination acts as a fuse of rating 20 A.

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

An electric bulb rated 220 V - 100 W will fuse if it consumes 131 W. What voltage fluctuations can it withstand?

  1. up to 230 V

  2. up to 241 V

  3. up to 225 V

  4. up to 252 V
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

We know that: $P = \dfrac{V^{2}}{R}$
Using the fact that the bulb is $220V - 100W$
$ 100 = \dfrac{220^{2}}{R}$

$R = \dfrac{48400}{100} = 484\ ohm$

Given that it will fuse on use of power of $131W$.
Substituting this:
$131 = \dfrac{V^{2}}{484}$

$V^{2} = 131 \times 484 = 63,404$

$\Rightarrow V = 251.8V$   Which is the highest voltage it can experience without fusing.

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

An 1800 W toaster, a 1.3 kW frying pan are plugged into the same $20 A-120V$ line then

  1. fuse will not blow

  2. fuse will blow

  3. supply will spark

  4. fuse is just saved
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$I _{toaster}=\dfrac{1800}{120}= 15 A,$


$I _{frying pan}=\dfrac{1300}{120}= 10.83 A$

$I _{net}= 25.83 A > 20 A$ 

because applied current is more than possible value of current which is given in ques. Hence fuse will blow

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

What we used to protect the electric circuits from over loading and short circuit ?

  1. capacitance

  2. inductance

  3. fuse wire

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

A  fuse is the device to protect the electric circuits from over loading and short circuit.
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. 
Hence, the fuse element is made of zinc, copper, silver, aluminum, or alloys to provide stable and predictable characteristics.
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. Hence correct option is C.

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

 What is the current through an electrical appliance of rating 5 kW, 200 V and can you use a fuse which is rated 8 A ? 

  1. 25 A, yes

  2. 2.5 A, yes

  3. 25 A, no

  4. 2.5 A, no
Reveal answer Fill a bubble to check yourself
C 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.
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, let us consider a device of power 5 kW, that is, 5000 W and 200 V.

The current through the appliance is given as $I=P/V.$

That is, $I=\dfrac { P }{ V } =\dfrac { 5000 }{ 200 } =25A$.

The fuse of 8 A cannot be used because the current through the appliance is 
greater the fuse rating. When the 25 A current flows through the fuse it will 
blow off and it will not be able to fulfill it purpose.

Hence, a fuse is rated 8 A cannot be used with this appliance.