Physics

Electrical Machines and Circuits

720 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 kirchhoff's law circuit problems kirchoff's law and problems on it equivalent resistance in series and parallel connection

If the magnitude of voltage in a circuit changes with time between zero to peak value and direction changes periodically then the voltage is called 

  1. Alternative voltage

  2. direct voltage

  3. constant voltage

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

An alternating voltage is defined as a voltage whose magnitude changes continuously with time between zero and a peak value while periodically reversing its direction. Direct and constant voltages do not periodically change direction.

Multiple choice properties of an ideal op-amp electronics physics the operational amplifier (op-amp)

The supply voltage to an opamp is +-15volts. The opamp is connected as an inverting amplifier of gain -20. The output voltage for an input voltage of 20mV is:

  1. -400 mV
    • 400 mV

  3. -200 mV

  4. +200 mV
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Voltage gain and output voltage are related by the formula $A _v=\dfrac{output \ voltage}{input \ voltage}$. Substituting the value of input voltage, we get, output voltage = - 20 x 20mV = -400 mV

Multiple choice properties of an ideal op-amp electronics physics the operational amplifier (op-amp)

An ideal Opamp is an ideal

  1. Current controlled current source

  2. current controlled voltage source

  3. Voltage controlled voltage source

  4. Voltage controlled current source
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The ideal opamp voltage is maintained constant. It is controlled by the input voltage. Thus the output of an opamp is controlled by the source (input) voltage

Multiple choice properties of an ideal op-amp electronics physics the operational amplifier (op-amp)

An opamp has a supply voltage of 9V AND -9V at its power terminals. What is the maximum output voltage one can obtain by varying the voltage across the inverting and non inverting terminals 

  1. Greater than 9V

  2. 9V

  3. 18V

  4. Can be infinity depending on the voltage gain of the opamp
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

maximum voltage that can obtained will be equal to the supply voltage w.r.t ground and beyond which the amplifier will saturate

Multiple choice properties of an ideal op-amp electronics physics the operational amplifier (op-amp)

As the supply voltage increases, the voltage gain of the circuit _____ and the power consumption _____.

  1. increases, increases

  2. increases, decreases

  3. decreases, decreases

  4. decreases, increases
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Increasing the supply voltage to an op-amp generally allows for a larger output swing (increasing gain potential) and increases the quiescent power consumption of the device.

Multiple choice properties of an ideal op-amp electronics physics the operational amplifier (op-amp)

Current cannot flow to ground through

  1. A mechanical ground

  2. An ac ground

  3. A virtual ground

  4. An ordinary ground
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
  1. Virtual ground refers to a circuit elements that is not connected to the ground directly, it is held at a reference voltage. This reference voltage need not to be same voltage as ground either.
    2. While in all other ground it may be mechanical A.C or an ordinary ground it act as a reference point against which other voltage can be measured.
    3. The key point  is that the current flow from one point of the circuit through ground then back into the circuit.
    So, current cannot flow to ground through a virtual ground.
Multiple choice physics effects of electric current joule's law of heating thermal effect of electric current heating effect of electric current

Which of the following terms does not represent electrical power in a circuit?

  1. $I^2R$
  2. $IR^2$
  3. $VI$
  4. $V^2/R$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$P=VI\quad .............(i)$

Hence, option (c) represents power.
$V=IR\quad .............(ii)$

From (i) and (ii),
$P=I^2R$
$P=V^2/R$
Hence, option (a) and (d )represents power too.
Option (b) does not represent power.

Multiple choice physics effects of electric current joule's law of heating thermal effect of electric current heating effect of electric current

The power dissipated as heat in a conductor of resistance $R$ due to current $I$ through it is called ________.

  1. Joule's loss
  2. $I^2R$ loss
  3. Either $(1)$ or $(2)$
  4. Neither $(1)$ nor $(2)$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

The loss in power in electric circuit due to heat produced in conductors due to their resistance is called $Joule's \hspace{2mm} heat$.


Heat loss is given by $I^2R$.

Answer-(C)

Multiple choice physics effects of electric current joule's law of heating thermal effect of electric current heating effect of electric current

For a constant power source across a resistor, heat generated in a resistor:

  1. increases with increase in resistance

  2. increases with increase in applied potential

  3. increases with decrease in resistance

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

Heat generated across the resistor is given by

$Q=I^2Rt = VIt$
Since, power supplied is constant,
$VI = \text{constant}$
$\implies Q = constant$ 

Multiple choice capacitance of an isolated spherical conductor capacitance of isolated bodies capacitance physics

A coil, a capacitor and an A. C. source of rms voltage 24 V are connected in series. By varying the frequency of the source, a maximum rms current of 6 A is observed. If the coil is connected to a battery of emf 12 V and internal resistance $4\Omega$, the current through it will be    

  1. 2.4 A

  2. 1.8 A

  3. 1.5 A

  4. 1.2 A
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

$\begin{array}{l} { E _{ rms } }=24V \ r=4\Omega ,\, \, \, { I _{ rms } }=6A \ R=\frac { E }{ I } =\frac { { 24 } }{ 6 } =4\Omega  \ Internal\, { { Re } }sis\tan  ce=4\Omega  \ Hence,\, net\, resis\tan  ce=4+4=8\Omega  \ \therefore Current=\frac { { 12 } }{ 8 } =1.5A \  \end{array}$

Hence, the option $C$ is the correct answer.

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

Transformer is used for

  1. increasing the voltage.

  2. decreasing the voltage.

  3. both A and B

  4. neither A nor B
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

In A.C, voltage can be increased or decreased with the help of a transformer. But in D.C, this cannot be done. Voltage may be increased using a step up transformer and may be decreased using a step down transformer.

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

An A.C. source is rated $220V,50Hz$. The average voltage is calculated in a time interval of $0.01s$, it-

  1. must be zero

  2. may be zero

  3. is never zero
  4. is $\frac { 20 }{ \sqrt { 2 } } $ volt.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The frequency of the AC source is 50 Hz, meaning the time period is T = 1/50 = 0.02 s. The given time interval is 0.01 s, which is exactly half of the time period (a half cycle). The average of a sinusoidal voltage over a full cycle is zero, but over a half cycle starting from zero, the average voltage is non-zero (2V_0/pi). However, depending on the exact phase of the 0.01s interval, the average can be zero (e.g., from peak to peak). Thus, it may be zero.

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.