Tag: applications of electromagnets

Questions Related to applications of electromagnets

A 220 - volt input is supplied to a Transformer. The output circuit draws a current of 2.0 ampere at 440 volts. If the efficiency of the transformer is 80%, the current drawn by the primary windings of the transformer is

physics electro-magnetism applications of electromagnets transformer transformers

  1. 5.0 ampere

  2. 3.6 ampere

  3. 2.8 ampere

  4. 2.5 ampere

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Correct Option: A
Explanation:

$\mu =\frac { { V } _{ s }{ I } _{ s } }{ { V } _{ p }{ I } _{ p } } =0.8\Rightarrow { I } _{ p }=\frac { { V } _{ s }{ I } _{ s } }{ { V } _{ p }\mu  } =\frac { 440\times 2 }{ 220\times .8 } =5A$

Electrical energy is transmitted over large distance at high alternating voltages Which of the following statements is (are) correct?

physics electro-magnetism applications of electromagnets transformer transformers

  1. for a given power level there is a lower current

  2. lower current implies less power loss

  3. transmission lines can be made thinner

  4. It is easy to reduce the voltage at the receiving end using step-down transformers

Show answer
Correct Option: A,B,D
Explanation:

Power = Voltage x Current
As voltages are high so currents are lower. This is done to reduce the resistive losses.
Resistance Loss = ${ { I } _{ rms }^{ 2 } }R$
So, lower current implies less power loss. 
As resistance is inversely proportional to their diameter; hence decreasing their diameter by x will increase resistance by $x^2$ and hence increasing power loss hence not done.
As the voltage required for domestic and commercial uses are pretty less then they can be easily brought down by step-down transformers. 

A direct current (DC) motor is connected to a battery by means of two leads. The motor moves for slightly lessthan half the cycle and comes to halt. Which of the following components is missing?

physics electro-magnetism applications of electromagnets transformer transformers

  1. one of the brushes

  2. commutator

  3. slip ring

  4. one of the two magnets

Show answer
Correct Option: C
Explanation:

Answer is C.

Slip-ring commutator merely maintains a connection between the moving rotor and the stationary stator.
Slip rings are useful for DC motors where the current has to change direction every half revolution whereas split rings are used to make constant current connection to a commutator, which is what we want for an AC motor.
Hence, if the slip rings are not present, then the armature stops its rotation.

Tum ratio in a step up transformer is 1 : 2 if a Lechlanche cell of 1.5 V is connected across the input, then the voltage across the output will be 

physics electro-magnetism applications of electromagnets transformer transformers

  1. 0.1 V

  2. 1.5 V

  3. 0.75 V

  4. Zero

Show answer
Correct Option: D
Explanation:

A transformer can not step up a d.c. input so output potential here will be zero. No potential will be induced in the secondary coil.

Large transformers, when used for some time, become very hot and are cooled by circulating oil. The heating of the transformer is due to

physics electro-magnetism applications of electromagnets transformer transformers

  1. The heating effect of current alone

  2. Hysteresis loss alone

  3. Both the heating effect of current and hysteresis loss

  4. Intense sunlight at noon

Show answer
Correct Option: C
Explanation: 

Heat loss in a magnetic circuit is due to two reasons one is hysteresis and  other is due to ohmic loss due to eddy current .Laminations and circulatingoil are some of methods to avoid heating. Hysteresis loss is entirelymaterial property while eddy current loss depends on geometry of core and amount of current.

A certain transformer has a turns ratio of 1 and a 0.85 coefficient of coupling. When 2 V ac is applied to the primary, the secondary voltage is

physics electro-magnetism applications of electromagnets transformer transformers

  1. 1.7 V

  2. 0.85 V

  3. 1 V

  4. 0 V

Show answer
Correct Option: A
Explanation:

Coefficient coupling  $\eta = 0.85$
Input voltage in primary coil   $V _p = 2 \ V$
Turn ratio  $\dfrac{N _s}{N _p} = 1$
Thus output voltage in secondary coil   $V _s = \eta \dfrac{N _s}{N _p}V _p$
$\implies \ V _s = 0.85\times 1\times 2 = 1.7 \ V$

To step 120 V ac up to 900 V ac, the turns ratio must be

physics electro-magnetism applications of electromagnets transformer transformers

  1. 75

  2. 750

  3. 7.5

  4. 0.13

Show answer
Correct Option: C
Explanation:

Input voltage in primary coil  $V _p = 120 \ V$
Output voltage in secondary coil   $V _s = 900 \ V$
Turn ratio    $  \dfrac{N _s}{N _p} = \dfrac{V _s}{V _p} = \dfrac{900}{120} = 7.5$

The primary winding of a power transformer should always be

physics electro-magnetism applications of electromagnets transformer transformers

  1. open

  2. shorted

  3. switched

  4. fused

Show answer
Correct Option: D
Explanation:

When the secondary load is overloaded, it draws maximum current in the primary side due to which damaging the primary side as well as appliances. Therefore primary winding of power transformer is fused. Suppose peak voltage of power source is varying then current will also fluctuate so also to limit the current it is fused.

State whether True or False :

A transformer responds only to dc.

physics electro-magnetism applications of electromagnets transformer transformers

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

Transformer does not work on DC. Direct current have a variable magnetic field induced if fed through the primary winding of transformer. Thus, only a constant magnetic field will act o secondary and shall not induce any voltage across the terminals of the same. So, it works on AC circuits only.

State whether given statement is True or False
A transformer consists of two or more cores that are electrically coupled on a common core.

physics electro-magnetism applications of electromagnets transformer transformers

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

A transformer consists of only single core and it has windings wound on the core which provides necessary magnetic field. The transformer may contain two or more windings.