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 ac and dc ac vs dc electro-magnetism effects of electric current physics

Hotwire ammeters are used for measuring _____________ currents.

  1. A.C. only

  2. D.C only

  3. both A.C. and D.C

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

Hot wire instruments use the Joule’s law of thermal expansion. When a current (AC or DC) is passed through a special wire, the wire heats up due to $(I^2)\times R$ loss. This heating increases the length of the wire. This increase is recorded as the current. The calibrations indicates an aggregate value which goes with both AC and DC measurements. Over usage can tear the wire apart and render it un-usable. These also measure true RMS for an applied AC. 

Hence they are used for measuring both AC and DC.

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

Consider the following statements:
(a) The magnitude and direction of alternating current reverses periodically.
(b) Frequency of alternating current in india is 50 Hz.
(c) The frequency of direct current is zero.
Which of the above statements are correct?

  1. (b) and (c)

  2. (a) and (b)

  3. All of them

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

Electricity flows in two ways: either in an alternating current (AC) or in a direct current (DC).
Electricity or "current" is nothing but the movement of electrons through a conductor, like a wire. The difference between AC and DC lies in the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forward" and then going "backward." Alternating current is the best way to transmit electricity over large distances. 

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

Choose the correct option

  1. In Direct Current, Magnitude and direction of current is constant

  2. In Direct Current, Magnitude and direction of current reverses periodically

  3. In Alternating Current, Magnitude and direction of current is constant

  4. Direct current is periodic in nature
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Direct current is categorized by current of constant magnitude and direction. 

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

A neon lamp is connected to a voltage a.c. source. The voltage is gradually increased from zero volt. It is observed that the neon flashes at 50 V. The a.c. source is now replaced by a variable dc source and the experiment is repeated. The neon bulb will flash at

  1. 50V

  2. 70V

  3. 100V

  4. 35V
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The RMS (Root mean square) value tells us what equivalent DC voltage we would need to get the same power, for the neon bulb to glow.
In this case, the neon bulb glows at 50 V ac voltage. Therefore, for the bulb glow with dc voltage, $50\times \sqrt { 2 } \quad =\quad 50\times 1.414\quad =\quad 70\quad V$ dc voltage should be applied.
Hence, the neon bulb will flash at 70 V dc.

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

A low frequency AC current is passed through a galvanometer. What will be the behavior of the pointer needle?

  1. The needle remains stationary at zero reading

  2. The needle points to the rms value of ac

  3. The needle fluctuates about zero reading with a frequency equal to current frequency

  4. The needle fluctuates about zero reading with a frequency half of current frequency
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Needle of galvanometer shows magnitude and direction of current through it. In AC, both are continuously changing periodically and this behavior is observed in the needle. 

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

which is the correct option?

  1. Alternating current always flows in one direction but the direct current reverses its direction periodically

  2. Direct current always flows in one direction but the alternating current reverses its direction periodically

  3. Direct and alternating current always flows in one direction

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

Direct current never changes its direction it  have magnitude only but the alternating current is of the form: $I _0Sin \omega t$ which have magnitude and as well as direction.

So that it changes sign alternatively with time.

Multiple choice ac and dc ac vs dc electro-magnetism effects of electric current physics

Why is AC preferred over DC for transmission of electrical power

  1. AC to DC conversion and vice versa is very easy

  2. Transmission losses in AC are lower

  3. AC transmission is unaffected by random phase change

  4. Frequency of AC do not change
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transmission losses are given as $I^2R$. In AC, voltage transformation to higher(and also lower) values is easier at constant power. So, voltage is increased before transmission reducing current and hence reducing transmission losses.

Multiple choice physics alternating current power in ac circuits average power in ac circuit and power factor power in ac circuit

A voltage of peak value 283 V and varying frequency is applied to series LCR combination in which R = 3$\Omega$, L = 25 mH and C = 400$\mu$F. Then the frequency (in Hz) of the source at which maximum power is dissipated in the above is

  1. 51.5

  2. 50.7

  3. 51.1

  4. 50.3
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Here, $V _0 \, = \, 283 \, V, \, R \, = \, 3\Omega, \, L \, = \, 25 \, \times \, 10^{-3} \, H$
$C \, = \, 400 \, \mu F \, = \, 4 \, \times \, 10^{-4} F$
Maximum power is dissipated at resonance, for which 

$\nu \, = \, \dfrac{1}{2\pi \sqrt{LC}} \, = \, \dfrac{1 \, \times \, 7}{2 \, \times \, 22 \, \sqrt{25 \, \times \, 10^{-3} \, \times \, 4 \, \times \, 10^{-4}}}$

$= \, \dfrac{7 \, \times \, 10^3}{44\sqrt{10}} \, = \, 50.3 \, Hz$

Multiple choice physics alternating current power in ac circuits average power in ac circuit and power factor power in ac circuit

The power loss in an $AC$ circuit is $E _{rms}$ $I _{rms}$, when in the circuit there is only

  1. $C$
  2. $L$
  3. $R$
  4. $L,\ C$ and $R$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Inductors and capacitors bring a phase difference between the voltage and current in the circuit, hence changing the p.f. When only a resistance is present, $Poer\ factor= 1$.
The power loss in an AC circuit$ =E _{rms} I _{rms} Power\ factor $

Multiple choice physics alternating current power in ac circuits average power in ac circuit and power factor power in ac circuit

The self inductance of the motor of an electric fan is 10 H. In order to impart maximum powr of 50 Hz, it should be connected to a capacitance of

  1. $8\mu F$
  2. $4\mu F$
  3. $2\mu F$
  4. $1\mu F$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Maximum power ($ I^2 R )$ is obtained when $I$ is maximum ( $Z$ is minimum).

For $Z$ minimum, $X _L=X _C$, which yields
$C=\dfrac {1}{(2\pi n)^2L}=\dfrac {1}{4\pi^2\times 50\times 50\times 10}$

$\therefore C=0.1\times 10^{-5}F=1\ \mu F$

Multiple choice physics alternating current power in ac circuits average power in ac circuit and power factor power in ac circuit

The current which does not contribute to the power consumed in an AC circuit is called:

  1. Non-ideal current

  2. Wattless current

  3. Convectional current

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

Wattless current does not contribute to the mean rate of working of the circuit.
As, power factor $= \frac{\text{true power}}{\text{apparent power}}$
                             $=cos\phi$
                             $=\frac{R}{\sqrt{R^2+(X _L-X _C)^2}}$
$\therefore$ Power factor $=cos\phi = \frac{R}{Z}$
In a non-inductive circuit, $X _L=X _C$
$\therefore$ Power factor $=cos\phi = \frac{R}{\sqrt{R^2}}=\frac{R}{R}=1$
$\therefore \phi = 0^o$
This is the maximum value of power factor. Iris a pure inductor or an ideal capacitor 
$\phi = 90^o$
$\therefore$ Power factor $= cos \phi = cos 90^o= 0$. 
Average power consumed in a pure inductor orb ideal capacitor 
$P = E _V \cdot I _V cos\, 90^o = zero$.
Therefore, current through pure L or pure C; which consumes no power for its maintenance in the circuit is called ideal current or wattless current.

Multiple choice physics alternating current power in ac circuits average power in ac circuit and power factor power in ac circuit

The power loss is less in transmission lines, when :

  1. Voltage is less but current is more

  2. Both voltage and current are more

  3. Voltage is more but current is less

  4. Both voltage and current are less
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The power cables have some resistance. 

Power lost in the wires can be calculated as $P=I^2R$ with $R$ as the resistance of the wires and $I$ as the current that passes through them.
Power at the load is $P=VI$. 
From this one can see that if  voltage is increased by say $n$ times, then only $\dfrac{1}{n}$ the current is required to deliver the same power. However, if $\dfrac{1}{n}$ current is passed on the same wires, only $\dfrac{1}{n^2}$ of the power will be lost.