Current Electricity Class 12 Physics MCQs Pdf
51. A car battery is of 12 V. Eight dry cells of 1.5 V connected in series also give 12 V, but such a combination is not used to start a car. Why?
Answer/Explanation
Answer:
Explaination:
Dry cells used in series will have high resistance (=10 Ω) and hence provide low
current, while a car battery has low internal resistance (0.1 Ω) and hence gives
high current for the same emf, needed to start the car.
52. Graph showing the variation of current versus voltage for a material GaAs is
shown in the figure. Identify the region of
(i) negative resistance, [Dehradun 2019]
(ii) where Ohm’s law is obeyed. [Delhi 2015].
Answer/Explanation
Answer:
Explaination:
(i) DE is the negative resistance region.
(ii) Ohm’s law is obeyed in the region AB.
53. Define the term ‘electrical conductivity’ of a metallic wire. Write its SI unit. [Delhi 2014]
Answer/Explanation
Answer:
Explaination:
Electric conductivity () is defined as the reciprocal of electric
resistivity
i.e. \(\sigma=\frac{1}{\rho}\)
Its SI unit is Ω-1 m-1, mho m-1 or siemen
m-1.
54. Give an example of a material each for which temperature coefficient of
resistivity is
(i) positive and
(ii) negative. [CBSE Sample Paper 2016]
Answer/Explanation
Answer:
Explaination:
The temperature coefficient of resistivity is
(i) positive for Cu (copper) and nichrome, and
(ii) negative for Si (semiconductor).
55. Show variation of resistivity of copper as a function of temperature in a graph. [Chennai 2019, Delhi 2014]
Answer/Explanation
Answer:
Explaination:
56. Two identical cells, each of emf E, having negligible internal resistance r, are connected in parallel with each other across an external resistance R. What is the current through this resistance? [AI2013]
Answer/Explanation
Answer:
Explaination:
For parallel arrangement of two identical cells, the total resistance of the
circuit = R (cells have negligible internal resistance) Potential difference
across the resistor = E.
So, current through the resistance, \(I=\frac{E}{R}\)
57. The plot of the variation of potential difference across a combination of three
identical cells in series, versus current is as shown here. What is the emf of each
cell?
Answer/Explanation
Answer:
Explaination:
When7=0, Vmax = E = 6V
Thus, an emf of each cell = \(\frac{6}{3}\) = 2 volts.
58. Why resistance becomes more in series combination?
Answer/Explanation
Answer:
Explaination:
It is due to the increase in effective length of the conductor, because R ∝ l.
59. Why resistance becomes less in parallel combination?
Answer/Explanation
Answer:
Explaination:
It is due to the increase in effective area of cross-section ofthe conductor,
because R ∝ l/A.
60. Two similar wires of same length and same area of cross-section but of different material,, having resistivity p, and p2 are connected end to end (in series). Calculate the effective resistivity of their combination.
Answer/Explanation
Answer:
Explaination:
61. Two similar wires of same length and same area of cross-section but of different material having resistivity pj and p2 are connected side by side i.e. in parallel. Calculate the effective resistivity of their combination.
Answer/Explanation
Answer:
Explaination:
62. In an experiment on meter bridge, if the balancing length AC is x, what would be its value, when the radius of the meter bridge wire AB is doubled? Justify your answer. [AI 2011C]
Answer/Explanation
Answer:
Explaination:
At the balance point, we have
As R1 and R2 remain the same, then x will also remain the
same. It will not depend upon the diameter of the wire.
63. A cell of negligible internal resistance is connected in series to the wire of a potentiometer. If potentiometer wire is changed and in its place another wire of different material having diameter twice of the first wire is used keeping the length constant, then how the balance point will change?
Answer/Explanation
Answer:
Explaination: Balance point will not change.
64. How can we increase the sensitivity of a potentiometer? [Foreign 2017]
Answer/Explanation
Answer:
Explaination:
By reducing potential gradient.
Potential gradient can be reduced by
(i) increasing length of the wire, and
(ii) reducing current in the main circuit.
65. Why is the potentiometer preferred .to a voltmeter for measuring emf of a cell? [Dehradun 2019]
Answer/Explanation
Answer:
Explaination:
Emf measured by the potentiometer is more accurate because the cell is in open
circuit giving no current.