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Current Electricity

Multiple Choice Questions

411.

A thin wire of resistance 4Ω is bent to form a circle. The resistance across any diameter is

4 Ω
2 Ω
1 Ω
8 Ω

412.

A battery of e.m.f. E and internal resistance r is connected to an external resistance R the condition for maximum power transfer is

r < R
r > R
r = 1/R
r = R

Short Answer Type

413.

Two wires each of same material and same length have 2 : 1 ratio of their radii. If the first wire has a resistance of 2 Ω then what will be the equivalent resistance of the two wires when they are connected in series ?

414.

What should be the resistance of an ideal ammeter and an ideal voltmeter ?

415.

A 60 W- 220 V bulb is connected in series with another bulb 40 W-220 V. Which bulb will give more illuminance ?

416.

A wire of length 5 m is stretched by 1 mm by a force of 1 N. What is the value of energy stored in the wire ?

Multiple Choice Questions

417.

One kg of copper is drawn into a wire of 1 mm diameter and a wire of 2 mm diameter. The resistance of the two wires will be in the ratio

2 : 1
1 : 2
16 : 1
4 : 1

418.

An electrical cable having a resistance of 0.2 Ω delivers 10 kW at 200 V DC to a factory. What is the efficiency of transmission ?

65 %
75 %
85 %
95 %

419.
A wire of resistance 5 Ω is drawn out so that its new length is 3 times its original length. What is the resistance of the new wire ?

45 Ω

15 Ω

5/3 Ω

5 Ω

45 Ω

$Since, volume is constant, i . e ., {πr}_{1}^{2} l_{1} = {πr}_{2}^{2} l_{2} \Rightarrow {(\frac{r_{1}}{r_{2}})}^{2} = (\frac{l_{2}}{l_{1}}) = (\frac{3 l}{l}) = 3 Now, \frac{R_{1}}{R_{2}} = \frac{ρ \frac{l_{1}}{A_{1}}}{ρ \frac{l_{2}}{A_{2}}} = \frac{l_{1} / {r_{1}}^{2}}{l_{2} / {r_{2}}^{2}} = \frac{l_{1}}{l_{2}} \times \frac{{r_{2}}^{2}}{{r_{1}}^{2}} \Rightarrow \frac{R_{1}}{R_{2}} = \frac{1}{3} \times \frac{1}{3} \Rightarrow R_{2} = 9 R_{1} = 9 \times 5 = 45 Ω$

420.

Two identical cells each of emf E and internal resistance r are connected in parallel with an external resistance R. To get maximum power developed across R, the value of R is