# Tag: physics

### Questions Related to physics

Read the following statements and answer whether the given statement is true or false.

A transformer is an electrical device that works on the principle of self-induction

1. True

2. False

Correct Option: B
Explanation:

False

A transformer consists of two electrically isolated coils and operates on Faraday’s principle of mutual induction, in which an EMF is induced in the transformer secondary coil due to the magnetic flux generated by the voltages and currents flowing in the primary coil working. In self induction there is only one coil and emf is induced in that coil due to the change in flux linked with that coil.

A transformer works on both A.C. and D.C.

1. True

2. False

Correct Option: B
Explanation:

False

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

Why are transformers used in the transmission of electric power?

1. Because they can reduce transmission losses

2. Because they can conserve electrical energy

3. Because they speed up transmission

4. Because they help to distribute electrical energy efficiently

Correct Option: A,D
Explanation:

Transformer used in the transmission of electrical power because they help to distribute electrical energy efficiently and can reduce transmission loss.

Which of the following statement about the action of a transformer is correct?

1. Alternating current always flows in the secondary coil when an alternating voltage is applied to the primary coil.

2. The current in the secondary coil is always larger than the current in the primary coil.

3. An e.m.f. is induced in the secondary coil when an alternating voltage is applied to the primary coil.

4. An e.m.f. is induced in the secondary coil when a steady direct current flows in the primary coil.

Correct Option: C
Explanation:

Alternating current does not flow in the secondary coil unless electrical load is connected to its terminals.
In step-up transformer current in secondary coil is less than the current in primary coil.
No e.m.f. is induced in the secondary coil if the primary coil is connected to a d.c. supply.

The current drawn by the primary of a transformer, which steps down 200 V to 20 V to operate a device of resistance 20 $\Omega$ is: (Assume the efficiency of the transformer to be 80 %)

1. 0.125 A

2. 0.225 A

3. 0.325 A

4. 0.425 A

Correct Option: A
Explanation:
$V _{primary} =200\ V$
$V _{secondary}=20\ V$
$R=20\ \Omega$
Power secondary $=\dfrac {(V _{secondary}^2)}{R}$

$=20\ W$

Power primary $=\dfrac {20}{\dfrac {80}{100}}=25\ w\quad (80\%\ efficiency)$

Power primary $=(I _{primary}) (V _{primary})$
$25=I _P \times 200$
$I _P=0.125\ A$
Option $(A)$

In a step up transformer the turn ratio is 1:2. A Leclanche cell (emf = 1.5 V) is connected across the primary. The voltage across the secondary is

1. 3.0 V

2. 1.5 V

3. 0.75 V

4. 0

Correct Option: A
Explanation:

For the step up transformer -

$\dfrac { { V } _{ 1 } }{ { V } _{ 2 } } =\dfrac { { n } _{ 1 } }{ { n } _{ 2 } }$

$\dfrac { 1.5 }{ { V } _{ 2 } } =\dfrac { 1 }{ 2 }$

$\Rightarrow { V } _{ 2 }=3V$.

A transformer works on the principle of

1. self induction

2. electrical inertia

3. mutual induction

4. magnetic effect of the electrical current

Correct Option: C
Explanation:
Transformer is based on the principle of electromagnetic induction.
There are two coils named as primary and secondary wound on a soft iron core.
When AC is applied to the primary coi, the resulting current will the production of
continously varying magnetic flux.
Due to mutual induction, induced EMF is produced in secondary coil which in return results in current.

Transformer is used to

1. convert ac to dc voltage

2. convert dc to ac voltage

3. obtain desired dc power

4. obtain desired ac voltage and current

Correct Option: D
Explanation:
Transformer converts some value of voltage/current into some other desired value based upon the principles of mutual induction.
Transformer is based on the principle of electromagnetic induction. There are two coils named as primary and secondary wound on a soft iron core. When AC is applied to the primary coil, the resulting current will the production of continuously varying magnetic flux. Due to mutual induction, induced EMF is produced in secondary coil which in return results in current.

Transformer can be used on;

1. a.c. only

2. d.c. only

3. a.c. and d.c. both

4. none of these

Correct Option: A

The magnetic resonance imaging is used as a tool of diagnosis to detect the injuries in human body. It is based on the principle that:

1. Protons in different types of human tissues have specific magnetic field environment

2. Magnetic field pattern of human body resonates with the externally applied magnetic field

3. Either $(1)$ or $(2)$

4. Neither $(1)$ nor $(2)$

Correct Option: A
Explanation:

The basic principle of MRI is protons in different types of human tissues have specific magnetic field environment.