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State the principle on which transformer works.

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#### Solution

A transformer is a device with the help of which, a given alternating voltage can be increased or decreased to any desired value. The first type of transformer which delivers an output voltage smaller than the input voltage

is called a step down transformer. The second type of transformer which delivers an output voltage larger than the input voltage is called a step up transformer.

Principle of working of a transformer:

A transformer works on the principle that whenever the magnetic flux linked with a coil changes, an emf is induced in the neighbouring coil.

#### RELATED QUESTIONS

Explain transformer working with construction.

Derive an expression for ratio of e.m.f.s and currents in terms of number of turns in primary and secondary coil.

Draw a labeled diagram of a step-down transformer.

A group of students while coming from the school noticed a box marked "Danger H.T. 2200 V" at a substation in the main street. They did not understand the utility of a such a high voltage, while they argued, the supply was only 220 V. They asked their teacher this question the next day. The teacher thought it to be an important question and therefore explained to the whole class.

Answer the following questions:

(i) What device is used to bring the high voltage down to low voltage of a.c. current and what is the principle of its working ?

(ii) Is it possible to use this device for bringing down the high dc voltage to the low voltage? Explain

(iii) Write the values displayed by the students and the teacher.

Derive an expression for e.m.f. and current in terms of turns ratio

A transformer converts 240 V AC to 60 V AC. The secondary has 75 turns. The number of turns in primary are _______.

(A) 600

(B) 500

(C) 400

(D) 300

A power transmission line feeds input power at 2200 V to a step-down transformer with its primary windings having 300 turns. Find the number of turns in the secondary to get the power output at 220 V.

Find the ratio of primary and secondary currents in terms of turn ratio in an ideal transformer.

How much current is drawn by the primary of a transformer connected to 220 V supply when it delivers power to a 110 V − 550 W refrigerator?

Given the input current 15 A and the input voltage of 100 V for a step-up transformer having 90% efficiency, find the output power and the voltage in the secondary if the output current is 3 A.

Describe briefly, with the help of labelled diagram, working of a step-up transformer.

A step-up transformer converts a low voltage into high voltage. Does it not violate the principle of conservation of energy? Explain.

A transformer is used to step up an alternating emf of 200V to 440V. If the primary coil has 1000 turns, calculate the number of turns in the secondary coil.

An a.c generator generates an emf 'ε' where ε = 314 Sin (50πt) volt. Calculate the frequency of emf ε.

What is the turns ratio i.e., transformer ratio, n_{s}: n_{p}, in an ideal transformer which in-creases ac voltage from 220 V to 33000 V?

In an ideal transformer, an output of 66 kV is required when an input voltage of 220 V is available. If the primary has 300 turns, how many turns should the secondary have?

What is the ideal transformer?

The primary and secondary coils of a transformer each have an inductance of 200 x 10^{-6} H. The mutual inductance between the windings is 4 x 10^{-6} H. What percentage of the flux from one coil reaches the other?

**Choose the correct option:**

A circular coil of 100 turns with a cross-sectional area of 1 m^{2} is kept with its plane perpendicular to the magnetic field of 1 T. The magnetic flux linkage is ______.

An ideal transformer has 100 turns in the primary and 250 turns in the secondary. The peak value of the AC is 28 V. The rms secondary voltage is nearest to ______

State the mathematical relation between a number of turns in the primary coil to a secondary coil in the step-up transformer.

Distinguish between Step up and Step Down Transformer.

What is Transformer?

Explain step up and step down transformer?

The primary of a transformer has 40 turns and works on 100 V and 100 W. Find a number of turns in the secondary to step up the voltage to 400 V. Also calculate the current in the secondary and primary.

Describe the construction and working of a transformer with a neat labelled diagram.

In a transformer, the number of turns in the primary and the secondary are 410 and 1230 respectively. If the current in primary is 6A, then that in the secondary coil is

A step-down transformer reduces the supply voltage from 220 V to 11 V and increase the current from 6 A to 100 A. Then its efficiency is

What are step-up and step-down transformers?

Explain the working of the transformer.

Mention the various energy losses in a transformer.

Give the advantage of AC in long distance power transmission with an illustration.

Find out the phase relationship between voltage and current in a pure inductive circuit.

A step-down transformer connected to the main supply of 220 V is used to operate 11V,88W lamp. Calculate

- Voltage transformation ratio and
- Current in the primary

A 200V/120V step-down transformer of 90% efficiency is connected to an induction stove of resistance 40 Ω. Find the current drawn by the primary of the transformer.

The 300 turn primary of a transformer has resistance 0.82 Ω and the resistance of its secondary of 1200 turns is 6.2 Ω. Find the voltage across the primary if the power output from the secondary at 1600V is 32 kW. Calculate the power losses in both coils when the transformer efficiency is 80%

A step-up transformer has 300 turns of primary winding and 450 turns of secondary winding. A primary is connected to 150 V and the current flowing through it is 9A. The current and voltage in the secondary are

The output power in step-up transformer used in practice is ______.

A 220 V input is supplied to a transformer. The output circuit draws a current of 2.0 A at 440 V. If the ratio of output to input power is 0.8, then the current drawn by primary winding is ______.

**Assertion:** A transfonner cannot work on D.C. supply.

**Reason:** D.C. changes neither in magnitude nor in direction.

Which among the following, is not a cause for power loss in a transformer?

The primary winding of a transformer has 100 turns and its secondary winding has 200 turns. The primary is connected to an a.c supply of 120 V and the current flowing in its is 10 A. The voltage and the current in the secondary are ______.

A transformer is used ______

A step-down transformer connected to an ac mains supply of 220 V is made to operate at 11 V, 44 W lamp. Ignoring power losses in the transformer, what is the current in the primary circuit?

A step down transformer connected to an ac mains supply of 220 V is made to operate at 11 V, 44 W lamp. Ignoring power losses in the transformer, what is the current in the primary circuit?

For an LCR circuit, the power transferred from the driving source to the driven oscillator is P = I^{2}Z cos φ.

- Here, the power factor cos φ ≥ 0, P ≥ 0.
- The driving force can give no energy to the oscillator (P = 0) in some cases.
- The driving force cannot syphon out (P < 0) the energy out of oscillator.
- The driving force can take away energy out of the oscillator.

The line that draws power supply to your house from street has ______.

- zero average current.
- 220 V average voltage.
- voltage and current out of phase by 90°.
- voltage and current possibly differing in phase `phi` such that `|phi| < pi/2`.

1 MW power is to be delivered from a power station to a town 10 km away. One uses a pair of Cu wires of radius 0.5 cm for this purpose. Calculate the fraction of ohmic losses to power transmitted if

- power is transmitted at 220 V. Comment on the feasibility of doing this.
- a step-up transformer is used to boost the voltage to 11000 V, power transmitted, then a step-down transfomer is used to bring voltage to 220 V. (ρ
_{Cu}= 1.7 × 10^{–8}SI unit)

A transformer operating at primary voltage 8 kV and secondary voltage 160 V serves a load of 80 kW. Assuming the transformer to be ideal with purely resistive load and working on unity power factor, the loads in the primary and secondary circuit would be:

A transformer consisting of 300 turns in the primary and 150 turns in the secondary gives output power of 2.2 kW. If the current in the secondary coil is 10 A, then the input voltage and current in the primary coil are ______.

In a transformer, number of turns in the primary coil are 140 and that in the secondary coil are 280. If current in primary coil is 4 A, then that in the secondary coil is ______.

The magnetic flux through a coil perpendicular to its plane is varying according to the relation Φ = (5t^{3} + 4t^{2} + 2t - 5) Weber. If the resistant of the coil is 5 ohm, then the induced current through the coil at t = 2 sec will be ______.

Magnetic flux Φ in weber in a closed circuit of resistance 10Ω varies with time Φ (sec) as Φ = 6t^{2} - 5t + 1. The magnitude of induced current at t = 0.25s is ______.

An ideal transformer converts 220 V a.c. to 3.3 kV a.c. to transmit a power of 4.4 kW. If primary coil has 600 turns, then alternating current in secondary coil is ______.

Two coils P and Q are kept near each other. When no current flows through coil P and current increase in coil Q at the rate 10A/s, the emf in coil P is 15mV. When coil Q carries no current and current of 1. 8A flows through coil P, the magnetic flux linked with the coil Q is ______.

An iron rod is placed parallel to magnetic field of intensity 2000 Am^{-1}. The magnetic flux through the rod is 6 × 10^{−4} Wb and its cross-sectional area is 3 cm^{2}. The magnetic permeability of the rod in Wb A^{-1}m^{-1} is ______.

Magnetic flux passing through a coil is initially 4 × 10^{-4} Wb. It reduces to 10% of its original value in t second. If the emf induced is 0. 72 mV then t in second is ______.

Efficiency of transformer is the ratio of ______.

The self-inductance of a closely wound coil of 200 turns is 10 mH. Determine the value of magnetic flux through the cross-section of the coil when the current passing through the coil is 4 mA.

The primary coil of a transformer has 60 turns whereas its secondary coil has 3000 turns.

If a 220 V ac voltage is applied to the primary coil, how much emf is induced in the secondary coil?

The primary coil of a transformer has 60 turns whereas its secondary coil has 3000 turns.

If a current of 5A flows in the primary coil, how much current will flow in a load in the secondary coil? State the assumption you have made regarding the transformer, in this calculation.

What is a transformer?

Derive the equation for a transformer.