Tag: electro-magnetism

Questions Related to electro-magnetism

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

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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.

State whether given statement is True or False
An autotransformer is one in which the primary and secondary are automatically coupled.

physics electro-magnetism applications of electromagnets transformer transformers

  1. True

  2. False

Show answer
Correct Option: B
Explanation:

An auto transformer is an electrical transformer with only one winding. The portion of the same winding acts both primary and secondary side of the transformer. 

In a transformer the number of turns in the primary coil is 200 and secondary coil is 600. The ratio between the electric current in the primary and secondary respectively is

physics electro-magnetism applications of electromagnets transformer transformers

  1. 1 : 4

  2. 4 : 1

  3. 1 : 3

  4. 3 : 1

Show answer
Correct Option: D
Explanation:

In a transformer, we know that 
$\frac{V _2}{V _1}=\frac{N _2}{N _1}$
where $V _1$ and $V _2$ are voltage across primary and secondary coils respectively and $N _1$ and $N _2$ are respective number of turns. 

Also, the transformer doesn't add or take away energy, so the power in
the primary and secondary is equal:
$V _1I _1=V _2I _2$
Therefore, $\frac{I _1}{I _2}=\frac{N _2}{N _1}$ 
$\frac{I _1}{I _2}=\frac{600}{200}=\frac{3}{1}$ 
Hence, desired ratio is 3:1. (D)

In transformer, core is made of soft iron to reduce

physics electro-magnetism applications of electromagnets transformer transformers

  1. Hysteresis losses

  2. Eddy current losses

  3. Force opposing electric current

  4. None of the above

Show answer
Correct Option: A
Explanation:

Transformer cores are made up of soft iron because soft iron has less losses due to hysteresis.

The quantity that remains unchanged in the output with respect to the input in an ideal transformer is

physics electro-magnetism applications of electromagnets transformer transformers

  1. Voltage

  2. Current

  3. Frequency

  4. None of these

Show answer
Correct Option: C
Explanation:

Transformer does not change the frequency of input AC.