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 power transmission household electricity household circuits electricity and magnetism physics

Power transmission is carried out at ........................ and  ..........................?

  1. high voltage, high current

  2. low voltage, high current

  3. high voltage, low current

  4. low voltage, low current
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

One of the key concerns in the transmission of electricity is the power loss in transmission lines, dissipated as heat due to the resistance of the conductors.
The power loss ${P} _{loss} = { I }^{ 2 }R={ (\dfrac { P }{ V } ) }^{ 2 }R(\because p=VI)$
Since $P $and $R $ are fixed conditions, less power will be lost if high voltages V are used.
The transmission line then uses a transformer to step down the voltage.
For a given amount of power, a higher voltage reduces the current and thus the resistive losses occurs in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the$ {I}^{2}{R}$ losses by a factor of 100, provided the same sized conductors are used in both cases. 

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

A town situated $20km$ away from a power house at $440V$, requires $600KW$ of electric power at $220V$. The resistance of transmission line carrying power is $0.4\Omega$ per km. The town gets power from the line through a $3000V-220V$ step down transformer at a substation in the town. Which of the following is /are correct

  1. The loss in the form of heat is $640KW$
  2. The loss in the form of heat is $1240KW$
  3. Plant should supply $1340kW$
  4. Plant should supply $640kW$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Total length of wire = $20 \times 2 = 40km$


Resistance of wire $R = 40 \times 0.4 = 16\Omega $ ,


${I _{\max }} = \dfrac{P}{V} = \dfrac{{600kW}}{{3000V}} = 200A$

Power loss in the form of heat $ = {I^2} _{rms} \times R = {\left( {200} \right)^2} \times 16 = 640kW$

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

Which is not a correct advantage of parallel combination in domestic circuit?

  1. Same voltage of the mains is available for all electrical appliances

  2. Switching on or off one switch has no effect on the other

  3. If one appliance stops working, other appliances keep in working

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

All options A, B, and C are correct advantages of a parallel circuit in a domestic setting. Since the question asks for a 'not correct' advantage and none of the listed options are incorrect, 'none' is the correct choice.

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

Power is transmitted from a power house on high voltage ac because :

  1. The rate of transmission is faster at high voltage

  2. It is more economical due to less power wastage

  3. The life of the current carrying wire is prolonged

  4. A precaution-against the theft of transmission line
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The higher the voltage V, the lower the current I and therefore much lower power loss.Transmitting electricity at high voltage reduces the fraction of energy lost to resistance

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

Transmission lines transmit a voltage of $V$ volt to our houses from power stations, then the power $P$ supplied by them is proportional to:

  1. $\cfrac{1}{V}$
  2. $V$
  3. ${V}^{2}$
  4. $\cfrac{1}{{V}^{2}}$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Power supplied is equal to to the voltage multiplied by the current flowing through the wire.

  $\therefore$     $P = VI$
$\implies  P  \propto  V$

Multiple choice power transmission household electricity household circuits electricity and magnetism physics

From a power station, the power is transmitted at a very high voltage because :

  1. It is generated only at high voltage

  2. It is cheaper to produce electricity at high voltage

  3. Electricity at high volatge is less dangerous

  4. There is less loss of energy in transmission at high voltage
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

When power is transmitted from power station to different different substation then one important factor is kept in mind that power loss in pathway during transmission over large distances should be minimum. 

we know that P = V I 
so for a particular power that is to be transmitted we can choose a combination of voltage and current so that loss is minimum as much possible.
if we do current high and voltage minimum then due to high current , heat loss will be more and power will get lost , hence voltage is kept high and current is kept low as much possible so that current can not produce high vibrations and collisions  in free electrons insides the wire making a lot of loss of power.
so by keeping voltage to high value the loss is minimum as compared to other cases.

Multiple choice physics magnetic effects of electric current direct current types of current movement of charge

What is the voltage of the electricity that is generally supplied to a house ?

  1. $400 V$
  2. $200 V$
  3. $220 V$
  4. $260 V$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Worldwide, many different mains power systems are found for the operation of household and light commercial electrical appliances and lighting. The different systems are primarily characterized by the following.

  • Voltage
  • Frequency
  • Plugs and sockets (receptacles or outlets)
  • Earthing system (grounding)
  • Protection against overcurrent damage (e.g., due to short circuit), electric shock, and fire hazards.
  • Parameter tolerances.
All these parameters vary among regions. The voltages are generally in the range 100 to 240 V. The two commonly used frequencies are 50 and 60 Hz. Single-phase or three-phase power is most commonly used today.
In India, a voltage of nearly 220 volts and a frequency of 50 cycles per second is used.
Hence correct option is option C

Multiple choice physics magnetic effects of electric current direct current types of current movement of charge

What will be the reading of a hot wire voltmeter if it is connected across the terminals of a generator whose voltage wave form is represented by 
$V = 200\sin \omega t + 100\sin 3\omega t + 50\sin 5\omega t$?

  1. 100V

  2. 162 V

  3. 150 V

  4. 200 V
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The RMS value of a complex waveform is the square root of the sum of the squares of the RMS values of its individual harmonic components. RMS = sqrt((200/sqrt(2))^2 + (100/sqrt(2))^2 + (50/sqrt(2))^2) = sqrt(20000 + 5000 + 1250) = sqrt(26250) approx 162V.

Multiple choice safety precautions to be taken while handling electricity electric charge and electric current physics ohm's law current electricity

Which of the following statements is correct?

  1. A wire with a green insulation is usually the live wire

  2. MCB works on the heating effect of current

  3. The earth wire should be connected to the body of an appliance

  4. A fuse should always be placed in the neutral wire ofa mains circuit
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
Earth wire or Earthing connection in the electrical wiring at home is connected to a large capacity storage of metallic or salted materials.  These materials drain electric charge if it is present on the wire and make the charge/voltage potential on the wire to 0.

     Usually the metallic bodies of the electric appliances are connected to the Earthing in the 3-way plug connections.  If there is a surge in the electric power supply or if there is a short circuit some where, then the high voltage and the excess current flow in to the Earthing connection.  So the appliance is secure from damage.  

    If the electric wiring in the house or in the appliance is faulty and as a result, the metallic body of the appliance carries electricity/charge on it.  Then the high voltage or leaked charges flow in to the earth.  This saves us from electric shocks when we touch the appliances.
Multiple choice safety precautions to be taken while handling electricity electric charge and electric current physics ohm's law current electricity

In case of household circuit, earth wire is connected to

  1. Input terminals of $kwh$ meter
  2. Body of the $kwh$ meter

  3. Both

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

Inside the house, the live and neutral wires are connected to the input terminals of $kwh$ meter, whereas earth wire is connected to body of $kwh$ meter.

Multiple choice safety precautions to be taken while handling electricity electric charge and electric current physics ohm's law current electricity

If the insulation of live wire got damages and the live wire comes in contact with the metallic body of the appliance. When such electric appliance is connected to the mains, if not connected to the earth wire will also be raised to

  1. $440 V$
  2. $110 V$
  3. $220 V$
  4. $600 V$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Under these conditions if someone by chance happens to touch it then the person will get an electric shock.