Transformer

Q. Draw the typical input and output characteristics of an n-p-n transistor in CE configuration. Show how these characteristics can be used to determine (a) the input resistance $\left(r_i\right)$, and (b) current amplification factor $\beta$.

Input charcteristics: It is the graph drawn between input current and input voltage when output voltage is kept constant.

Output characteristics : It is the graph drawn between output current and output voltage when input current is kept constant.

(a) Input resitance $(R_i$

$={\left[\frac{\text{Change in emitter base voltage}(△V_{BE})}{\text{Change in base current}(△I_B)}\right]}_{\text{at constant}{V}_{CE}}$


or $R_i={\left(\frac{△V_{BE}}{△I_B}\right)}_{V_{CE}}$

(b) Current amplification factor, $\left(\beta \right)={\left(\frac{△I_C}{△I_B}\right)}_{V_{CE}}$

= $\frac{△I_C}{△I_E-△I_C}$

= $\frac{\frac{△I_C}{△I_E}}{\frac{△I_C}{△I_E}-\frac{△I_C}{△I_E}}$

= $\frac{\alpha }{1-\alpha }$

$\Rightarrow \beta =\frac{\alpha }{1-\alpha }$

Q.

A bird perches on a bare high power line, and nothing happens to the bird. A man standing on the ground touches the same line and gets a fatal shock. Why?

Q.

The internal resistance of a cell of emf 2 V is 0.1 ohm. It is connected to a resistance of 3.9 ohm. The voltage across cell will be (in volts)

1. 0.5

2. 1.9

3. 2.0

4. 1.95

Q.

A circular coil of radius 8.0 cm and 20 turns is rotated about its vertical diameter with an angular speed of 50 rad s⁻¹ in a uniform horizontal magnetic field of magnitude 3.0×10⁻² T. Obtain the maximum and average emf induced in the coil. If the coil forms a closed loop of resistance 10Ω, calculate the maximum value of current in the coil. Calculate the average power loss due to Joule heating. Where does this power come from?

Q. the peak voltage in the output of a half-wave diode rectifier fed with a sinusoidal signal without filter is 10 v. the d.c. conponent of the output voltage is

ans. 10/π

Q.

An electrical power line, having a total resistance of $2\Omega$, delivers $1kW$ at $220V$. The efficiency of the transmission line is approximately:

1. $72\%$

2. $91\%$

3. $85\%$

4. $96\%$

Q. In an ideal transformer, number of turns in the primary coil are $140$ and those in the secondary coil are $280$. If current in the primary coil is $4\text{A}$, then that in the secondary coil is -

1. $4\text{A}$
2. $2\text{A}$
3. $6\text{A}$
4. $8\text{A}$

Q. A copper wire is wound on a wooden frame, whose shape is that of an equilateral triangle. If the linear dimension of each side of the frame is increased by a factor of $3$, keeping the number of turns of the coil per unit length of the frame the same, then the self inductance of the coil:

1. decreases by a factor of 9
2. decreases by a factor of $9\sqrt{3}$
3. inreases by a factor of 3
4. inreases by a factor of 27

Q.

Given below are two statements:

Statement I: PN junction diodes can be used to function as transistor, simply by connecting two diodes, back to back, which acts as the base terminal.

Statement II: In the study of transistor, the amplification factor $\beta$ indicates ratio of the collector current to the base current.

In the light of the above statements, choose the correct answer from the options given below.

1. Statement I is false but Statement II is true.

2. Both Statement I and Statement II are true.

3. Statement I is true but Statement II is false.

4. Both Statement I and Statement II are false.

Q. A small town with a demand of 800 kW of electric power at 220 V issituated 15 km away from an electric plant generating power at 440 V.The resistance of the two wire line carrying power is 0.5 W per km.The town gets power from the line through a 4000-220 V step-downtransformer at a sub-station in the town. (a) Estimate the line power loss in the form of heat. (b) How much power must the plant supply, assuming there isnegligible power loss due to leakage? (c) Characterise the step up transformer at the plant.

Q.

What are the parts of the transformer?

Q. 35. Figure shows the variation of electric field intensity E versus distance x. What is the potential difference betweenthe points at x = 2 m and at x = 6 m from O?

Q. The ratio of number of turns of primary coil to secondary coil in a transformer is $2:3$. If a source of $6\text{V}$ ac is connected across the primary coil, then e.m.f. across the secondary coil (in volts) will be

Q.

(a) Six lead-acid type of secondary cells each of emf 2.0 V and internal resistance 0.015 Ω are joined in series to provide a supply to a resistance of 8.5 Ω. What are the current drawn from the supply and its terminal voltage?

(b) A secondary cell after long use has an emf of 1.9 V and a large internal resistance of 380 Ω. What maximum current can be drawn from the cell? Could the cell drive the starting motor of a car?

Q. A step down transformer is used to reduce the main supply of $220\text{V}$ to $10\text{V}$. If the primary draws $5\text{A}$ and secondary draws $88\text{A}$ current, calculate the efficiency of the transformer.

1. $70$%
2. $60$%
3. $80$%
4. $90$%

Q. 131. 1 Calculate the electric current passing through 3 ohm and 6 ohm resistor 2 pd across each resistor

Q. Transformers are used

(a) in DC circuits only
(b) in AC circuits only
(c) in both DC and AC circuits
(d) neither in DC nor in AC circuits

Q. A transformer works on the principle of -

1. self induction.
2. electrical inertia.
3. magnetic effect of electric current.
4. mutual induction.

Q.

A closed path in which electricity flows is known as

1. Generator

2. Electric meter

3. Electric circuit

Q.

A step-down transformer reduces the voltage of a transmission line from $2200V$ to $220V$. The power delivered by it is $880W$ and its efficiency is $88\%$. The input current is

1. $4.56mA$

2. $0.045A$

3. $0.45A$

4. $4.65A$

Q.

A thermal power plant produces electric power of 600 kW at 4000 V, which is to be transported to a place 20 km away from the power plant for consumers usage. It can be transported either directly with a cable of large current carrying capacity or by using a combination of step-up and step-down transformers at the two ends. The drawback of the direct transmission is the large energy dissipation. In the method using transformers, the dissipation is much smaller. In this method, a step-up transformer is used at the plant side so that the current is reduced to a smaller value. At the consumers end, a step-down transformer is used to supply power to the consumers at the specified lower voltage. It is reasonable to assume that the power cable is purely resistive and the transformers are ideal with power factor unity. All the currents and voltages mentioned are rms values. If the direct transmission method with a cable of resistance 0.4 Ω km−1 is used, the power dissipation (in %) during transmission is

1. 20

2. 30

3. 40

4. 50

Q.

A transformer has 50 turns in the primary and 100 in the secondary. If the primary is connected to a 220 V DC supply, what will be the voltage across the secondary ?

Q.

How does electricity generated at the power stations reach our homes?

Q.

A coil is wound as a transformer of rectangular cross-section. If all the linear dimensions of the transformer are increased by a factor 2 and the number of turns per unit length of the coil remain the same, the self inductance increased by a factor of

Q.

An ideal inductor is connected in series with a resistor and an ideal battery. The battery supplies energy at a rate P(t), the resistor dissipates energy at a rate $P_R\left(t\right)$, and the inductor stores energy at a rate $P_L\left(t\right)$. What can be concluded about the relationship between $P_R\left(t\right)$ and $P_L\left(t\right)$?

1. $P_R\left(t\right)>P_L\left(t\right)$ for all times t while charging

2. $P_R\left(t\right)=P_L\left(t\right)$ for some value of t while charging

3. $P_R\left(t\right)>P_L\left(t\right)$ only near the beginning of the charging

4. $P_R\left(t\right)<P_L\left(t\right)$ only near the beginning of the charging

Q. The number of turns of the primary and the secondary coils of a transformer are $20$ and $200$ respectively. Primary voltage and the current are given as $5V$ and $1A.$ Assuming the efficiency to be $90\%,$ the secondary voltage and current are

1. $50V$ and $0.09A$
2. $50V$ and $10A$
3. $50V$ and $0.11A$
4. $0.5V$ and $9.0A$

Q. 1 Hertz is equal to how many electron volts(eV) ?

Q. A coil of Cu wire (radius-r, self inductance-L) is bent in two concentric turns each having radius $\frac{r}{2}$ . The new self inductance is

1. 2L
2. L
3. 4L
4. $\frac{L}{2}$

Q.

The average power dissipation in pure resistive circuit is:

1. 

2. 

3. 

4. 

Q. A power transmission line feeds power at 2200 v with a current of 5 A to a step down transformer with its primary winding having 4000 turns calculate the number of turns and the current in the secondary in order to get output power at 220v