# CBSE Board Exam 2020: Important MCQs (with Answers) for Class 12 Physics - Chapter 3 Current Electricity; Also useful for JEE Main, UPSEE, WBJEE

Mayank Uttam

Get important MCQs (with Answers) for Class 12 Physics - Chapter 3 Current Electricity. Thee questions are very important for the preparation of upcoming CBSE Class 12 Physics Board Exam 2020.  As per CBSE  Time Table 2020, CBSE Board Exam 2020 for Class 12 Physics subject is scheduled to be held on 02nd March. In this article, you will get important MCQs for Class 12 Physics, Chapter 3 - Electricity. These questions are also useful for other competitive exams like JEE Main 2020 preparation.

1. In how many different ways three 1 Ω resistances can be combined?

(a) three

(b) four

(c) five

(d) six

Sol: (b)

Three 1 Ω resistances can be combined in four different ways.

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2. Two wires A and B of equal masses and of the same metals are taken. The diameter of the wire A is half the diameter of the wire B. If the resistance of A is 24 Ω then find the resistance of wire B?

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(a) 0.5

(b) 1

(c) 1.5

(d) 3

Sol: (c)

3. If 400 Ω resistance is made by adding four 100 Ω resistance of tolerance 5%, then the tolerance of the combination is

(a) 5%

(b) 10%

(c) 15%

(d) 20%

Sol: (a)

If four resistances of 100Ω and tolerance 5% are connected in series then, total resistance will be 400 Ω but tolerance will remain 5%.

Sol: (a)

The resistivity of a given material is constant and does not depend on the shape and size of the material. So, graph given in option (a) is correct.

5. Resistance n, each of r Ω, when connected in parallel give an equivalent resistance of R Ω. If these resistances were connected in series, the combination would have a resistance in Ω, equal to

(a) n2R

(b) R / n2

(c) R / n

(d) nR

Sol: (a)

When n resistors of resistance r are combined in parallel then their equivalent resistance is given by (r/n) = R ⇒ r = n R.

When n resistors of resistance R are combined in series then their equivalent resistance is given by n r ⇒ n (n R) ⇒ n2R.

7. The bulbs A, B and C are connected as shown in the figure given below. The bulbs B and C are identical. If the bulb C is fused then:

(a) Both A and B will glow more brightly

(b) Both A and B will glow less brightly

(c) A will glow less brightly and B will glow more brightly

(d) A will glow more brightly and B will glow less brightly

Sol: (c)

When C gets fused then the overall resistance of the circuit increases and the current through A gets reduced due to which it gets dimmer.

Also, the current passing through bulb B will increase so it will glow brighter.

8. A cylindrical wire is stretched to increase its length b 10 %. What is the percentage increase in resistance?

(a) 10 %

(b) 41 %

(c) 1 %

(d) 21 %

Sol: (d)

9. A copper wire is stretched to make it 0.2%. What is the percentage change in its resistivity?

(a) 0.4 %

(b) 2.0 %

(c) 4.0 %

(d) None of these

Sol: (d)

There will be no change in its resistivity because resistivity of a material depends on nature of the material and temperature of its surroundings.

10. A steel wire is stretched to make it 0.2% longer. What is the percentage change in its resistance?

(a) 4 %

(b) 0.4 %

(c) 2 %

(d) 0.2 %

Sol: (b)

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