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 electro-magnetism applications of electromagnets transformer transformers

A transformer:

  1. transforms energy

  2. transforms frequency

  3. transforms voltage

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

Answer is C.

Transformers are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of Electrical Power being transferred from one winding to another via the magnetic circuit. So the current is sent through step up transformers to increase the voltage and to push the power over long distances.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer is employed to ___________.

  1. convert A.C. into A.C.

  2. convert D.C. into A.C.

  3. obtain a suitable A.C. voltage

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

Answer is C.

Transformers are capable of either increasing or decreasing the voltage and current levels of their supply, without modifying its frequency, or the amount of Electrical Power being transferred from one winding to another via the magnetic circuit. So the current is sent through step up transformers to increase the voltage and to push the power over long distances.
Hence, transformer is employed to obtain a suitable A.C. voltage.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer is

  1. A device for stepping up D.C.

  2. A generator of current

  3. Device for converting direct current into alternating current

  4. A device for stepping up or down the voltage of A.C. supply
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

 A transformer is a device that is used to either raise or lower voltages and currents in an electrical circuit. In modern electrical distribution systems, transformers are used to boost voltage levels so as to decrease line losses during transmission

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a step-up transformer, the turns ratio is $5 : 1$. The primary is connected to a battery of emf $10 V$. The voltage across secondary is ________.

  1. $50 V$
  2. $100 V$
  3. $25 V$

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

Transformer works only for alternating current because there will be no flux linked with the coil of transformer as the voltage is constant.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer works on the principle of __________.

  1. self-induction

  2. electrical inertia

  3. magnetic effect of electric current

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

A transformer consists of a primary coil and the secondary coil. The current from the primary coil is induced into the secondary coil. Since there is the involvement of two coils mutually, it is termed as mutual induction.

A transformer works on the principle of mutual induction.

Option $(D)$ is correct.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer

  1. Converts AC to DC

  2. Converts DC to AC

  3. Increases or decreases (step up or step down) AC voltage

  4. Increases or decreases (step up or step down) DC voltage
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

C

Transformer is a static device which transforms electrical energy from one circuit to another and with the help of mutual inductor between two windings. The transformer may be step up or step down. A transformer that increases voltage from primary to secondary (more secondary windings turns than primary winding turns) is called step up transformer. Conversely a transformer designed to do just the opposite is called step down transformer. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Complete the given statement, in order for a transformer to work:

  1. the magnetic field must be changing.

  2. the magnetic field must not change.

  3. it must be connected to a direct current source.

  4. the primary coil and the secondary coil must have different numbers of turns.

  5. the number of turns in the secondary coil must be greater than the number of turns in the primary coil.
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

The transformer works on the principle of mutual induction. The voltage in one coil changes to change the magnetic field that it creates. This changing magnetic field causes a flux change in the secondary coil, which would create an induced emf in the coil. Thus it is necessary to have a changing magnetic field in a transformer to cause induction in secondary coil.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Fill the vacant spaces with appropriate terms. The _______ going into a transformer is equal to the _______ going out of the transformer.

  1. power, voltage

  2. power, power

  3. current, current

  4. voltage, voltage

  5. voltage, power
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Working of transformer is based on the conservation of power i.e no power is lost inside the transformer.

Thus total power going into a transformer is equal to the total power going out of the transformer.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a transformer, the immediate cause of the induced alternating current in the secondary coil is:

  1. a varying magnetic field

  2. a varying electric field

  3. the iron core of the transformer

  4. a motion of the primary coil
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Transformer works on the basis of electromagnetic induction, which is produced by changing magnetic flux. In a transformer both the coils are fixed and not moving thus, the change in magnetic flux is because of change in magnetic field.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer is used to:

  1. convert alternating current to direct current

  2. convert direct current to alternating current

  3. convert mechanical energy to electrical energy

  4. transform alternating current voltage
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In a transformer the change in the current in primary coil produces the change in the current in secondary coil because of electromagnetic induction. 

Thus, transformer changes the AC voltage.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The ratio of secondary to the primary tums in a transformer is $3:2.$ If the power out put be $P$, then input power, neglecting all losses, must be equal to:

  1. $5\ P$
  2. $1.5\ P$
  3. $P$
  4. $\dfrac {2}{5}\ P$`
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Power= $VI$

For secondary $P _2=V-2I _2=\cfrac { { T } _{ 2 } }{ { T } _{ 1 } } { V } _{ 1 }\times \cfrac { { T } _{ 1 } }{ { T } _{ 2 } } { I } _{ 1 }={ V } _{ 1 }{ I } _{ 1 }$
Hence, $P _2=V _1I=P$
Power is same in transforming primary and secondary.