EXERCISES

1.A piece of wire of resistance is cut into five equal parts, These parts are tien
connected in parallel. If the equivalent resistance of this combination is R”, then the
ratio R/R’is –
(a) 1/25

(b) 1/5
(c) 5
(d) 25

Ans:- (d)25

2. Which of the following terms does not represent electrical power in a circuit?
(a) I2R
(b) IR2
(c) VI
(d) V/R

Ans:-(b) IR2

3. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V. the
power consumed will be –
(a) 100 W
(b) 75 W
(c) 50 W
(d) 25 W

Ans:-(d) 25 W

4. Two çonducting wires of the same material and of equal lengths and equal diameters
are first connected in series and then parallel in a circuit across the same potential
difference. The ratio of heat produced in series and parallel combinations would be
(a) 1:2

(b) 2:1
(c) 1:4
(d) 4:1

Ans:-(c) 1:4

5. How is a voltmeter connected in the circuit to measure the potential difference
between two points?

6. A copper wire has diameter 0.5 mm and resistivity of 1.6 x 108 2m. What will be
the length of this wire to make its resistance 10 Q? How much does the resistance
change if the diameter is doubled?
7. The values of current I flowing in a given resistor for the corresponding values of
potential difference V across the resistor are given below-
I (amperes)
0.5
1.0
2.0
3.0
4.0
V(volts)
1.6
3.4
6.7
10.2
13.2
Plot a graph between Vand I and calculate the resistance of that resistor.
8. When a 12 V battery is connected across an unknown resistor, there is a current of
2.5 mA in the circuit. Find the value of the resistance of the resistor.
9. A battery of 9 V is connected in series with resistors of 0.2 20.3 Q, 0.4 20.52
and 12 2, respectively. How much current would flow through the 12 Q resistor?
10. How many 176 2 resistors (in parallel) are required to carry 5 A on a 220 V line?
11. Show how you would connect three resistors, each of resistance 6 2, so that the
the combination has a resistance of (i) 92, (ii) 4 Q.
12. Several electric bulbs designed to be used on a 220 V electric supply line, are rated
10 W. How many lamps can be connected in parallel with each other across the two
wires of 220 V line if the maximum allowable current is 5 A?
13. A hot plate of an electric oven connected to a 220 V line has two resistance coils A
and B, each of 24 Q resistance, which may be used separately, in series, or in
parallel. What are the currents in the three cases?
14. Compare the power used in the 2 2 resistors in each of the following circuits:
(i) a 6 V battery in series with 12 and 22 resistors, and (ii) a 4 V battery in parallel
with 12 Q and 2 Q resistors.

15. Two lamps, one rated 100 W at 220 V. and the other 60 W at 220 V, are connected
in parallel to an electric mains supply. What current is drawn from the line if the supply
voltage is 220 V?
16. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
17. An electric heater of resistance 8 2 draws 15 A from the service mains 2 hours.
Calculate the rate at which heat is developed in the heater.
18. Explain the following.
(a)Why is the tungsten used almost exclusively for filament of electric lamps?
(b)Why are the conductors of electric heating devices, such as bread-toasters
and electric irons, made of an alloy rather than a pure metal?
(c)Why is the series arrangement not used for domestic circuits?
(d)How does the resistance of a wire vary with its area of cross-section?
(e)Why are copper and aluminum wires usually employed for electricity
transmission?

Ans:-  a. The resistivity and melting point of tungsten is very high. Due to this property, it doesn’t burn readily when heated. Electric lamps operate at high temperature. Hence, tungsten is a choice of metal for the filament of electric lamps.

b. The conductors of electric heating devices are alloys because of their high resistivity. Due to its high resistivity, it produces large amount of heat.

c. The voltage is divided in a series circuit as a result each component in the circuit receives a small voltage because of which the number of current decreases and the device gets hot and does not work properly. This is the reason why series circuits are not used in domestic circuits.

d. Resistance is inversely proportional to the area of cross-section. When the area of cross-section increases the resistance decreases and vice versa.

e. Copper and aluminum are good conductors of electricity and have low resistivity because of which they are usually employed for electricity transmission.