Current Electricity

MCQs Part 1

1.

Þ

  1. 106
  2. 107
  3. 1013
  4. 1019

Answer: A beam of electrons moving at a speed of 106 m/s along a line produces a current of 1.6 ´ 106 A. The number of electrons in the 1 metre of the beam is

2. In the Bohr's model of hydrogen atom, the electrons moves around the nucleus in a circular orbit of a radius 5 ´ 1011 metre. It's time period is 1.5 ´ 1016 sec. The current associated is

  1. Zero
  2. 1.6 ´ 1019 A
  3. 0.17 A
  4. 1.07 ´ 103 A

Answer:

3. A potential difference of V is applied at the ends of a copper wire of length l and diameter d. On doubling only d, drift velocity

  1. Becomes two times
  2. Becomes half
  3. Does not change
  4. Becomes one fourth

Answer: Drift velocity doesn't depends upon diameter.

4. A wire has a non-uniform cross-sectional area as shown in figure. A steady current i flows through it. Which one of the following statement is correct

  1. The drift speed of electron is constant
  2. The drift speed increases on moving from A to B
  3. The drift speed decreases on moving from A to B
  4. The drift speed varies randomly

Answer: For a conductor of non-uniform cross-section

5. Masses of three wires are in the ratio 1 : 3 : 5 and their lengths are in the ratio 5 : 3 : 1. The ratio of their electrical resistance is

  1. 1 : 3 : 5
  2. 5 : 3 : 1
  3. 1 : 15 : 125
  4. 125 : 15 : 1

Answer:

6. Equal potentials are applied on an iron and copper wire of same length. In order to have same current flow in the wire, the ratio of their radii must be [Given that specific resistance of iron = 1.0 107 Wm and that of copper = 1.7 ´ 108 Wm]—

  1. About 1.2
  2. About 2.4
  3. About 3.6
  4. About 4.8

Answer: V = constant., i = constant.

So R = constant

Þ

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7. The V-i graph for a conductor at temperature T1 and T2 are as shown in the figure. (T2 T1) is proportional to—

  1. cos 2q
  2. sin q
  3. cot 2q
  4. tan q

Answer: As we know, for conductors resistance
µ Temperature.

From figure R1µ T1Þ tanq µ T1 Þtanq=kT1…. (i)

(k = constant)

and R2µT2Þ tan (90o q)µT2Þcotq = kT2 .....(ii)

From equation (i) and (ii)

Þ (T2 T1) µ cot 2q

8. The resistance of a wire at 20oC is 20 W and at 500oC is 60W. At which temperature resistance will be 25W

  1. 50oC
  2. 60oC
  3. 70oC
  4. 80oC

Answer: By using

Again by using the same formula for 20W and 25W

Þ t = 80oC

9. Two resistance R1 and R2 provides series to parallel equivalents as then the correct relationship is

Answer: Series resistance and parallel resistance

10. What will be the equivalent resistance of circuit shown in figure between points A and D

  1. 10 W
  2. 20 W
  3. 30 W
  4. 40 W

Answer: The equivalent circuit of above fig between A and D can be drawn as

So

11. The effective resistance between point P and Q of the electrical circuit shown in the figure is

  3. 2r + 4R

Answer: The points A, O, B are at same potential. So the figure can be redrawn as follows

12. In the following circuit if key K is pressed then the galvanometer reading becomes half. The resistance of galvanometer is—

  1. 20 W
  2. 30 W
  3. 40 W
  4. 50 W

Answer: Galvanometer reading becomes half means current distributes equally between galvanometer and resistance of 40 W. Hence galvanometer resistance must be 40 W.

13. A torch bulb rated as 4.5 W, 1.5 V is connected as shown in the figure. The emf of the cell needed to make the bulb glow at full intensity is —

  1. 4.5 V
  2. 1.5 V
  3. 2.67 V
  4. 13.5 V

Answer: When bulb glows with full intensity, potential difference across it is 1.5 V. So current through the bulb and resistance of 1W are 3 A and 1.5 A respectively. So main current from the cell i = 3 + 1.5 = 4.5 A. By using E = 1.5 + 4.5 2.67 = 13.5 V.

14. The wire used in the arrangement shown in fig. has a resistance r ohm per metre. The equivalent resistance between points A and B if radius of circle is 1 m is—

Answer: Here R1, R2 and R3 are in parallel for points A and B, where

15. The two ends of a uniform conductor are joined to a cell of emf E and some internal resistance. Starting from the mid point P of the conductor, we move in the direction of current and return to P. The potential V at every point on the path is plotted against the distance covered (x). Which of the following best represent the resulting curve?

Answer: In a uniform conductor, when we move in the direction of current, the potential V decreases linearly. When we pass through the cell, from its negative pole towards positive pole, the potential V increases by an amount equal to the terminal potential difference of cell, which is less than the emf of a cell, as some potential drop occurs across the internal resistance of a cell.

16. In the circuit shown in fig. all wires have equal resistance R (= 5 W). The equivalent resistance between A and B is—

  1. 10/7
  2. 15/4
  3. 7
  4. 3

Answer: The points C and F are symmetrically located w.r.t. points A and B. Hence these points C and F are at the same potentianl. Hence the resistance of arm CF is ineffective. Similarly the resistance of arm DE is also ineffective. The equivalent circuit will be as shown.

The resistance of arm ADCB = 3R

The resistance of arm AEFB = 3R

The resistance of arm AB = R

These three resistances are in parallel. Their effective resistance

17. Eight resistance each of 4 ohm are connected in the circuit as shown. The equivalent resistance between A and B is

Answer: The equivalent circuit will be as shown in fig (a) and (b).

The effective resistance between A and B is

18. Current passing through 1W resistance is zero. Then the emf E is

  1. 8 V
  2. 10 V
  3. 6 V
  4. 16 V

Answer: Potential difference across 2 W resistance from left side is 10 V. Therefore, potential difference across 2 W rsistance from right side cell should also be 10 V. This is possible only if E= 10 V as the current passing through 1 W resistance is zero.

19. The resistance of a carbon filament at 0ºC is 104 ohm. It is connected in series to an iron wire. The temperature coefficient of resistivity of carbon and iron are —0.0003 and +0.00052 per ºC respectively. What must be the resistance of the iron wire so that the combined resistance does not change with temperature?

  1. 15 ohm
  2. 60 ohm
  3. 75 ohm
  4. 156/104 ohm

Answer: Combined resistance will not change if the total resistance of two rods in series with change in temperature is zero i.e.

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