Transformers

Q.

A lamp consumes $1000\mathrm{J}$ of electrical energy in $10\mathrm{s}$. What is its power?

Q.

Why is DC current not used in homes?

Q.

Explain the function of 1) armature 2) brushes 3) split rings in an electric motor?

Q.

Give two points of difference between a step-up and a step-down transformer.

Q.

Why inductor is not used in DC?

Q. Explain the use of commutator in D.C Generator.

Q.

The quantity that remains unchanged in a transformer is

1. Voltage

2. Current

3. Frequency

4. None of these

Q.

Calculate the amount of heat released when 5.0 g of water at 20 °C is changed into ice at 0 °C.

Specific heat capacity of water = 4.2J g^-1 °C^-1, Specific latent heat of fusion of ice = 336 J g^-1.

1. 2100 J

2. 4200 J

3. 1680 J

4. 420 J

Q.

Why is the iron core of a transformer made laminated instead of being in one solid piece?

Q.

In a step up transformer :
(a) $N_s=N_p$ (b) $N_s<N_p$
(c) $N_s>N_p$ (d) nothing can be said.

Q.

What is a rheostat symbol?

Q. In what ways can a motor be made more powerful

Q.

In an ideal step-up transformer, the ratio of turns of primary to secondary is $1:10$ and primary voltage is $230V$. If the load current is $2A$, then current in the primary is

1. $10A$

2. $2A$

3. $20A$

4. $1A$

Q.

Commercial electric motors do not use

1. An electromagnet to rotate the armature

2. Effectively large number of turns of conducting wire in the current-carrying coil

3. A permanent magnet to rotate the armature

4. A soft iron core on which the coil is wound

Q.

Does an induction cooker work in the principle of electromagnetic induction ? If so then how?

Q.

Why the armature as used as a stator?

Q.

How do the input and output powers in a transformer compare? State the assumption made.

Q.

Why commutator is used in DC motors ?

Q. Which of the following is/are true for an ideal step-down transformer?

1. Primary voltage < Secondary voltage
2. Number of turns in primary coil > Number of turns in secondary coil
3. Primary current < Secondary current
4. Power in primary coil < Power in secondary coil

Q. Why power is inversely proportional to the resistance when voltage is kept constant and Why power is directly proportional to resistance when current is kept constant?

Q.

A transformer has $1500$ turns in the primary coil and $1125$ turns in the secondary coil. If the voltage in the primary coil is $200V$, then the voltage in the secondary coil is

1. $100V$

2. $150V$

3. $200V$

4. $250V$

Q.

In the common emitter amplifier, what is the relationship between the phase of the input and output signals?

Q.

What is a commutator?

Q.

Which starter is used for slip ring induction motor of high ratings?

Q.

The energy conversion that takes place during the working of a dc motor is

1. Mechanical energy is converted into Electrical energy

2. Electrical energy is converted into Mechanical energy

3. Electrical energy is converted into Magnetic energy

4. Magnetic energy is converted into Electrical energy

Q.

If the coefficient of mutual induction of the primary and secondary coils of an induction coil is 5 H and a current of 10 A is cut-off in $5\times {10}^{-4}$ s, the emf inducted (in volt) in the secondary coil is

1. $5\times {10}^4$

2. $1\times {10}^5$

3. $25\times {10}^5$

4. $5\times {10}^6$

Q.

Name two kinds of energy losses in a transformer. How are they minimised?

Q. The principle on which transformer works is?

1. Mutual induction
2. Self induction
3. Electromagnetic induction
4. None of these

Q. Which of these components can be used for storing charge?

1. Resistor
2. Capacitor
3. Inductor
4. Transistor

Q.

Does a transistor amplify voltage?