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 mutual inductance electromagnetic induction electromagnetic induction and alternating currents physics

Two coils P and Q are lying parallels and very close to each other. Coil P is connected to an AC source whereas Q is connected to a sensitive galvanometer. On pressing key K

  1. small variations are observed in the galvanometer for applied 50 Hz voltage

  2. deflections in the galvanometer can be observed for applied voltage of 1 Hz to 2 Hz.

  3. no deflection in the galvanometer will be observed

  4. constant deflection will be observed in the galvanometer for 50 Hz supply voltage
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Upon connecting to the AC source , a current starts to flow in the coil P. This current induces a current in the galvanometer coil Q due to mutual  inductance because of the flux linkages. This induced current causes a deflection in the galvanometer.

Multiple choice mutual inductance electromagnetic induction electromagnetic induction and alternating currents physics

The value of mutual inductance can be increased by

  1. decreasing N

  2. increasing N

  3. winding the coil on wooden frame

  4. winding the coil on china clay
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

$M = \mu N _1 N _2 \dfrac{ Area _{12}}{length _{12}} $


$ Area _{12} $ - area in common to both the coils where the flux links both of them together.
$length _{12}$ - length in common to both the coils where the flux links both of them together.
Therefore to increase the mutual inductance, the number of turns can be increased.  

Multiple choice physics static electricity properties of charges charge properties of charge

When resistance element of a heater fuses and then we reconnect it after removing a portion of it, the power of the heater will :

  1. decrease

  2. increase

  3. remain constant

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

When ressitance element of a heater fuses and then we reconnect it after removing a portion of it, the power of the heater will increases because the resistance of wire decreases due to decrease in length of it. 


Option B is correct.

Multiple choice chemistry changes - physical and chemical effects of oxidation reactions in daily life corrosion of metals electroplating and its application

During the charging of lead accumulator battery voltage varies from:

  1. $2.15$ V to $1.88$ V
  2. $1.88$ V to $2.0$ V
  3. $1.88$ V to $2.15$ V

  4. does not change
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

When the lead accumulator battery is charged, the voltage range is 1.88 V to 2.15 V as the concentration of sulphuric acid varies from 5% to 40%.


Hence, option C is correct.

Multiple choice physics magnetic effects of electric current electric generator ac generator effects of electric current

Suggest way/s in an a.c. generator to produce a higher e.m.f.

  1. By increasing the speed of rotation of coil

  2. By increasing the number of turns in the coil

  3. both A & B

  4. None of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation
C. both A & B.
In an a.c. generator to produce a higher emf-
1. By increasing the speed of rotation of the coil.
2. By increasing the number of turns in the coil.
Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

In an electric generator ,split rings are used for 

  1. Converting DC to AC

  2. Converting AC to DC

  3. Reducing power consumption

  4. To split the current
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Answer is B.

The electric generator is devised to convert mechanical power into electrical power. Permanent magnet (PM) current converts into electrical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly, the other field is produced by an electrical current flowing in the rectangular coil ABCD, through the brushes and split-ring commutators. These two fields result in a torque which tends to rotate the coil. As the coil turns, the current in the winding is commutated to produce a continuous torque output. 
The split ring commutator is made from 2 pieces of copper that are held apart in the center from touching each other. They look like a copper ring split in the middle. The effect of using split rings in a electric generator is that the direction of the current flowing in the coil is reversed and DC current is generated.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

If the speed of rotation of armature coil is increased in an AC generator

  1. Magnitude of current increases

  2. Frequency of current increases

  3. Both 1 and 2

  4. Magnitude of current increases, frequency decreases
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

If the speed of rotation of armature coil is increased in an AC generator

$\mathscr{E} = NAB\omega  \sin \theta$,Magnitude of current increases,Frequency of current increases The option C is correct.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

In an AC generator, the ends of the coil are connected to ____________.

  1. two slip rings

  2. the split ring commutator

  3. the external circuit directly

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

The slip rings allow the transfer of alternating e.m.f. induced in the rotating coil to the external circuit. Each ring is connected to one end of the coil wire and is electrically connected to the external circuit via the conductive carbon brushes.

So, In an AC generator, the ends of the coil are connected to two slip rings
Therefore, A is correct option

Multiple choice physics magnetism and electromagnetism electric generator ac generator magnetic effects of electric current

A domestic electrical appliance requires alternating current of 15 V. If 220 V of alternating current is supplied to the house, then the device that helps in the functioning of that electrical appliance is

  1. Induction coil

  2. Step-up transformer

  3. AC dynamo

  4. Step-down transformer
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

A step down transformer is one in which the secondary coil has lesser number of turns in secondary coil than in primary coil.  Due to this the ratio of voltages in the coils is

$\dfrac{V _s}{V _p}=\dfrac{N _s}{N _p}$ 
Hence the voltage across the secondary coil in the step down transformer is reduced.
Therefore such a transformer is used to apply a voltage of $15V$ across a device when alternating voltage of $220V$ is applied.

Multiple choice physics effects of electric current electric generator ac generator magnetic effects of electric current

In an a.c. generator the speed at which the coil rotates is doubled. How would this affect the frequency of output voltage ?

  1. Frequency of maximum output voltage is halved.

  2. Frequency of maximum output voltage is doubled.

  3. Frequency of maximum output voltage remains same.

  4. Insufficient data.
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation
In an a.c. generator, emf produced is,

    $e=NAB(2\pi v)sin(2\pi v)t$

          where $N=$ no. of turns

                      $A=$ cross- sectional area

                      $B=$ magnetic field.

A frequency of ac, $v=\dfrac{w}{2\pi}$

                    where $w=$ is the speed of rotation

From the above, Frequency of maximum output voltage is directly proportional to speed of rotation of coil.

B. Frequency of maximum output voltage is double.