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Questions Related to physics

An electric motor requires $220\ V$ of alternating current to run but the supplied alternating voltage is $100\ V$. Then the device used to run the motor is 

physics electro-magnetism applications of electromagnets transformer transformers

  1. Diode

  2. Transistor

  3. Tranformer

  4. Capacitor

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Correct Option: A

The efficiency of a transformer is

physics electro-magnetism applications of electromagnets transformer transformers

  1. $\eta$ < 1

  2. $\eta = $ 1

  3. $\eta$ > 1

  4. $\eta = $ 0

Show answer
Correct Option: A
Explanation:

efficiency of a transformer $=\frac { Power _{ output } }{ { Power } _{ input } } $ for having efficiency >1 is not allowed by law of conservation of energy and e$=$1 is only for cases.
so e<1.

A transformer is used to

physics electro-magnetism applications of electromagnets transformer transformers

  1. convert DC into AC

  2. convert AC into DC

  3. obtain the required DC voltage

  4. obtain the required AC voltage

Show answer
Correct Option: D
Explanation:

A transformer is used to change the voltage low to high or vice versa depending upon the efficiency for power transfer and usage and since it is an application of the changing emf due to change in magnetic flux which is created by changing current it can be only applied to a.c. .

Which of the remain constant in a transformer?

physics electro-magnetism applications of electromagnets transformer transformers

  1. current

  2. potential

  3. power

  4. frequency

Show answer
Correct Option: D
Explanation:

A transformer changes Voltages and current but the power remains constant but that is case only for ideal transformer, in general there is loss in power in form of heat , sound , vibrations, eddy currents , etc .Only the frequency remains constant.

How are the e.m.f. in the primary and secondary coils of a transformer related with the number of turns in these coils?

physics electro-magnetism applications of electromagnets transformer transformers

  1. $E _s/ E _p = (N _s / N _p)^2$

  2. $E _s E _p = N _s N _p$

  3. $E _p/ E _s = N _s / N _p$

  4. $E _s/ E _p = N _s / N _p$

Show answer
Correct Option: D
Explanation:

In a transformer, source of alternating current is applied to the primary coil. Due to this, the current in the primary coil (called as magnetizing current) produces alternating flux in the core of transformer. This alternating flux gets linked with the secondary coil, and because of the phenomenon of mutual induction an emf gets induced in the secondary coil. 
Mutual induction:
An alternating current flowing in a coil produces alternating magnetic field around it. When two or more coils are magnetically linked to each other, then an alternating current flowing through one coil causes an induced emf across the other linked coils. This phenomenon is called as mutual induction.
From these principles, it can be derived that emf divided by number of turns is same for both primary and secondary winding.

That is,$ \dfrac { E1 }{ N1 } =\dfrac { E2 }{ N2 } $

where,
E1 is the induced EMF in the primary coil
E2 is the induced EMF in the secondary coil
N1 is the number of turns in the primary coil
N2 is the number of turns in the secondary coil

For what purpose are the transformers used? Can they be used with a direct current source?

physics electro-magnetism applications of electromagnets transformer transformers

  1. To step up the a.c. voltage. No, a transformer cannot be used with a d.c. source.

  2. To step down the a.c. voltage. No, a transformer cannot be used with a d.c. source.

  3. both A & B

  4. None of the above

Show answer
Correct Option: C
Explanation:

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice-versa. That is, step up or step down the voltage in a circuit.
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it's connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won't drive any electrons around. A changing magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday's law of induction. We get a changing field from an AC current, since the current which makes the field is changing. Hence, DC current cannot be used as a source for a generator.

For what purpose are the transformers used ?

physics electro-magnetism applications of electromagnets transformer transformers

  1. To step down the a.c. voltage.

  2. To step up the a.c. voltage.

  3. both A & B

  4. None of the above

Show answer
Correct Option: C
Explanation:

This question is already present in ID: 177624.

Transformers are found everywhere alternating current is used. This includes both large power stations to the power cord for our laptop computer. A transformer is an electrical device that trades voltage for current in a circuit, while not affecting the total electrical power. This means it takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice-versa. That is, step up or step down the voltage in a circuit. 
A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity. 
For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it's connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won't drive any electrons around. A changing magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday's law of induction. We get a changing field from an AC current, since the current which makes the field is changing. Hence, DC current cannot be used as a source for a generator.

The input and output voltages of a transformer are $220$ V and $44$ V  respectively. Find the turns ratio.

physics electro-magnetism applications of electromagnets transformer transformers

  1. $2 : 5$

  2. $1 : 2$

  3. $1 : 5$

  4. $5 : 1$

Show answer
Correct Option: C
Explanation:

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows you to change (transform) the voltage from the primary to the secondary coil.

The Turns Rule is: $\dfrac { Ns }{ Np } =\dfrac { Vs }{ Vp } $

Where,
$Ns =$ number of turns on the secondary coil
$Np =$ number of turns on the primary coil
$Vs =$ voltage across the secondary coil
$Vp =$ voltage across the primary coil
Hence, the ratio of input voltage to output voltage is equal to the ratio of the number of turns of primary coil to the number of turns of the secondary coil.
In the question given, the ratio of the input-output voltage is $\dfrac { 220V }{ 44V } $. That is, $1:5$.

Therefore, the turns ratio will also be $1:5$.

A transformer is used to light a $140W,24V$ bulb from a $240V$ a.c mains. The current in the main cable is $0.7A$. The efficiency of the transformer is

physics electro-magnetism applications of electromagnets transformer transformers

  1. $63.8$%

  2. $83.3$%

  3. $16.7$%

  4. $36.2$%

Show answer
Correct Option: B
Explanation:

$\text{Power source}=EI=240\times 0.7=166$
$\Rightarrow \eta =\dfrac{\text{Output}}{\text{Input}}=\cfrac{140}{166}$ $\Rightarrow \eta=83.3\%$

Assertion (A) : A transformer can’t work on dc.
Reason (R) : dc changes neither in magnitude nor in direction.

physics electro-magnetism applications of electromagnets transformer transformers

  1. A,R are true and R is the correct reason for A

  2. A,R are true and R is not the correct reason for A

  3. A is true, R is false

  4. A is false, R is true

Show answer
Correct Option: A
Explanation:

A transformer can't  work on dc current because in dc current, neither magnitude nor direction change, so, there is no change in flux, so, no emf is induced in secondary coil hence no voltage or current is there in secondary coil.