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 circuits precaution with the electrical circuit precautions to be taken while dealing with current devices used to protect from shock of electric current

Which of the following is not a safety measure commonly used in electric circuits.

  1. An electric fuse is connected in parallel

  2. Proper earthing of all electric circuit is done

  3. Fuse protects the circuit from overloading

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

An electric fuse is connected in parallel. This is not a safety measure. Fuse is always connected in series.

Multiple choice physics electric circuits precaution with the electrical circuit precautions to be taken while dealing with current devices used to protect from shock of electric current

In domestic electrical circuit, switches are used in

  1. Neutral wire

  2. Earth wire

  3. Live wire

  4. All of these
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Switches are connected in live wire so that when the switch is in off position, no current flows through circuit. As a result there will not be any danger of electrical shock.

Multiple choice types of cells electricity physics

Primary cell are connected in parallel to

  1. Increase voltage

  2. Decrease capacity

  3. Decrease internal resistance

  4. Make electric current constant
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Equivalent resistance of parallel combination         $\dfrac{1}{R _{eq}}  = \dfrac{1}{R _1}+\dfrac{1}{R _2} + .......$


$\implies$ Equivalent resistance of the parallel combination is less than the minimum of the individual resistance used.

Hence primary cell are connected in parallel to decrease the internal resistance.

Multiple choice power work and power work, energy and power physics energy and its forms

A light bulb has the rating 100 W, 220 V. If the supply voltage is 110 V, then power consumed by the bulb is

  1. 50 W

  2. 75 W

  3. 25 W

  4. 20 W
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
$P =\dfrac{ V^2}R$  

If $V = 220$ V we have

$100 W =\dfrac{ 220^2}R$

$R = \dfrac{220^2}{100} Ω = 484 Ω$. This is the resistance of the bulb.

When $V = 110$ V, power consumed $=\dfrac{V^2}R= \dfrac{110^2}{484} = 25$ W.

So, 25 W power is consumed when it is operated on 110 V.


Multiple choice physics electromagnetic forces oersted experiment oersted's experiment magnetic field due to a straight current carrying conductor

Thermistor follows which law for small variations ________

  1. Charles law

  2. KVL

  3. KCL

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

For small changes in the values of current, voltage across a thermistor increases. It attains a peak value. Then the voltage across the thermistor decreases. As a result Ohms law is followed at small variations of current.

Multiple choice introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

the number of turn of primary and secondary coil of the transformer is 5 and 10 respectively ad the mutual inductance is 25 H. if the number f turns of the primary and secondary is made 10 and 5 , then the mutual inductance of the coils will be

  1. 6.25 H

  2. 12.5 H

  3. 25 H

  4. 50 H
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

$M=\mu _o\mu _r \cfrac {N _1N _2}{l}A$

$M \propto N _1N _2$
Since $N _1N _2=10 \times 5= 5 \times 10=50$ in both cases.
Mutual inductance will remain same.

Multiple choice introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

If a spark is produced on removing the load from an AC circuit then the element connected in the circuit is

  1. high resistance

  2. high capacitance

  3. high inductance

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

On removal of load from the circuit, the circuit suddenly becomes an open circuit.
Thus $ \dfrac{di}{dt} \rightarrow \infty $
For sparking, high voltage must appear across the open ends. This will happen only in case of an inductor as the voltage drop across the inductor is $ L\dfrac{di}{dt} $
Therefore, the circuit has high inductance.

Multiple choice introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

If length of a solenoid is increased then what change should be made on no. of turns to keep self inductance constant-

  1. increase

  2. remain same

  3. decrease

  4. none of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation
$N$=total number of turns in solenoid
$l$=length of solenoid
$R$=radius of cross section of solenoid
Magnetic field inside solenoid =$B=\mu _o \dfrac{N}{l}i$  where, $\dfrac{N}{l}$=number of turns per unit length

Now, emf induced= $E=NBA$

$\implies E=N\times \mu _o\dfrac{N}{l}i\times \pi R^2$

$\implies E=\dfrac{\mu _{o}N^2\pi R^2}{l}i=Li$
                                                                                    where $L$=self inductance
                                      
Thus, $L=\dfrac{\mu _o N^2 \pi R^2}{l}$

For constant $L$,    $N^2\propto l$

Hence, on increasing length of coil, number of turns should be increased to keep $L$ constant.

Answer-(A)
Multiple choice introduction to induction electromagnetic induction electromagnetic induction and alternating currents physics

Find the necessary inductance. if 110 V, 10 W rating bulb is to be used with 220 V A.C source having frequency 50 Hz.

  1. L=8.90 H

  2. L=6.75 H

  3. L=7.25 H

  4. L=6.5 H
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Given, $V=110v$ ,$P=10W$ ,$V _0=220 w$ ,$f=50Hz$

Current through series inductor,
Current through bulb=$\dfrac{P}{V}=\dfrac{10W}{110V}=0.09A$
Voltage across inductor,$V _{ind}=\sqrt{V _0^{2} -V^{2}}$=$\sqrt{220^{2}-110^2}=191V$
Reactance of inductor,$R=\dfrac{V _{ind}}{I}=\dfrac{191}{0.09}=2122.22$
Also,$R=2 \pi fL$ or $L$=$\dfrac{R}{2 \pi f}$=$\dfrac{2122.22}{2 \pi 50}$=$6.75H$

Multiple choice dielectric substances and polarization dielectrics and polarisation electrostatic potential and capacitance electrostatics physics

A parallel plate condenser of capacity $10\mu F$ is connected to an A.C. supply voltage $e = 4\sin \left( {100\pi t} \right)$. The maximum displacement current:

  1. $4\pi \mu A$
  2. $4\pi mA$
  3. $2.8\pi \mu A$
  4. $2\pi \mu A$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Displacement current, ${i _d} = {\varepsilon _ \circ }A\frac{{dE}}{{dt}}$
where, $A$ is area and $E$ is electric field
$\begin{array}{l}E = \frac{e}{d} = \frac{{4\sin (100\pi t)}}{d}\{i _d} = {\varepsilon _ \circ }A \times \frac{1}{d}\left[ {\frac{d}{{dt}}\left( {4\sin 100\pi t} \right)} \right]\ = \frac{{{\varepsilon _ \circ }A}}{d}.4(cos100\pi t) \times 100\pi \ = C.400\pi .cos100\pi t\{\left( {{i _d}} \right) _{\max }} = \left( {10 \times {{10}^{ - 6}}} \right) \times 400\pi A\ = 4\pi  \times {10^{ - 3}}A = 4\pi mA\end{array}$

Multiple choice physics electric current, potential difference and resistance electric potential and potential difference potential difference current in electric circuits

Where the switch is connected in a circuit ? 

  1. in the ground wire.

  2. in the live wire

  3. in the neutral wire

  4. in live and neutral both.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

A switch opens or closes an electrical circuit. If the circuit is open, no electricity flows. If the circuit is closed, electricity flows through the circuit. 
Example:
A light switch is commonly used. When you turn the switch on, the circuit closes and electricity flows through the light. When you turn the switch off, the circuit is opened and the flow of electricity stops, making the light go out. 
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 live wire is connected to the switch because, the live wire carries a voltage of 220 V and the passage of this voltage can be disabled by switching of the circuit.

Multiple choice physics electric current, potential difference and resistance electric potential and potential difference potential difference current in electric circuits

In an electric circuit, current starts from the-

  1. Positive terminal

  2. Either of the two terminals

  3. Negative terminal

  4. Depends upon the circuit
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The direction of an electric current is by convention the direction in which a positive charge would move. Thus, thecurrent in the external circuit is directed away from the positive terminal and toward the negative terminal of the battery. Electrons would actually move through the wires in the opposite direction.