Electricity

MCQs Part 3

ASSERTION & REASON TYPE

Directions: Read the following questions and choose:

  1. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
  2. If both Assertion and Reason are true, but Reason is not correct explanation of the Assertion.
  3. If Assertion is true, but the Reason is false.
  4. If Assertion is false, but the Reason is true.

36. Assertion: A series combination of cells is used when their internal resistance is much smaller than the external resistance

Reason: It follows from the relation where the symbols have their standard meaning.

Answer: Here, Rs =nr. If R << nr, then = current due to one cell.

If nr <<R, current due to one cell.

Which is useful. Hence the reason is a correct explanation of the assertion.

37. Assertion: Though the direction of electric current is well defined, yet it is treated as a scalar.

Reason: Current does not follow the laws of vector addition.

Answer: Both the assertion and reason are true and reason is correct explanation of the assertion.

38. Assertion: The relation V = IR is valid even in the case of non-ohmic devices.

Reason: =R=a constant, as per Ohm's law

Answer: For non-ohmic devices, but R is not a constant. Thus ohm's law is not obeyed by such devices. Reason is true, but not a true explanation of the assertion.

39. Assertion: When a wire is stretched so that its thickness is halved, its resistance would become 16 times.

Reason: The data is insufficient to predict.

Answer: Thickness i.e. diameter D is halved. As , therefore, a becomes As = constant, therefore l becomes 4 times. From R becomes 16 times.

Assertion is true, but the reason is false.

40. Assertion: V-I graphs for a conductor at two different temperatures T1 and T2 are shown in fig here T1 > T2.

Reason: Resistance of a conductor increases with rise in temperature.

Answer: For given V; (I) is greater for T1. Therefore is smaller for T1 i.e. R1 < R2.

T1 < T2. The assertion is wrong.

41. Assertion: Units of conductivity are ohm—1 m—1

Reason: It follows from

Answer: From

42. Assertion: On interchanging the positions of cell and galvanometer in a standard wheatstone bridge, in fig (a) the condition remains unchanged.

Reason: In fig (b)

i.e. condition remains unchanged.

Answer: Apply wheatsone bridge. Both the assertion and reason are true and the latter is correct explanation of the former.

43. Assertion: In a Metre bridge, length of wire can vary from 1 m to several metres.

Reason: Greater is the length, better is the accuracy.

Answer: In a metre bridge, length of wire is fixed at one metre. The assertion is not true.

44. Assertion: The direction of current inside a cell is from negative to positive terminal. However, it is reversed during charging of the cell.

Reason: Current flows always from positive to negative.

Answer: In any electrical circuit, current flows from positive to negative-outside the cell. Inside the cell, it must be from negative to positive (to maintain the continuity).

However, to charge a cell, its positive terminal is connected to positive of the charger and negative terminal is connected to negative of the charger. Therefore, inside the cell, corrent is forced to flow from positive to negativel. Reason is true but it is not a correct explanation of the assertion.

45. Assertion: The composition of manganin is Cu 84%, Mn 12% and Ni 4%. The composition of eureka is Cu 60% Ni 40%.

Reason: Both manganin and eureka are alloys prepared with these specifications.

Answer: Both the assertion and reason are true and reason is correct explanation of the assertion.

46. Assertion: X and Yare the diametrically opposite ends of a circular wire of radius 7 in; having resistance 0·1 ohm/m. Effective resistance between X and Y is 1·1 ohm.

Reason: Resistance of each half wire

= 2.2 ohm.

The two halves are connected in parallel. Therefore

Rp = 1.1 ohm

Answer: Total length of wire = r

Lenght of each half

Resistance of each half

Between X and Y, the two halvs are connected in parallel; as shown. Both the assertion and reason are true and reason is correct explanation of the assertion.

47. Assertion: The value of I in the adjoining.

Reason: I seems to be 3 ampere.

Answer: Applying Kirchhoff's first law, I + 2 = 3+ 4, I = 7 —2 = 5 A. The reason is false.

48. Assertion: The value of I in the Fig. shown here is 0 . 225 A.

Reason: Use Ohm's law.

Answer: If R is effective resistance between A and B, then

Acc. to ohm's law

The reason is true, but not a correct explanation of the assertion.

49. Assertion: The equivalent resistance between the points X and Y in the Fig. 13.240 is 10 ohm.

Reason: Use the principle of Wheatstone bridge.

Answer: the bridge is balanced.

RABC = 10 + 10 = 20 ohm.

RADC = 10 + 10 = 20 W.

Between X and Y these resistances are connected in parallel. Therefore,

RXY = 10W. Reason is true, but not correct explanation of assertion.

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