Current Electricity

Water boils in an electric kettle in 20 minutes after being switched on. Using the same main supply, the length of the heating element should be to times of its initial length if the water is to be boiled in 15 minutes.

1. increased, 34
2. increased, 43
3. decreased, 34
4. decreased, 43

A heater is designed to operate with a power of 1000 W in a 100 V line. It is connected in combination with a resistance of 10 and a resistance R, to a 100 V mains as shown in figure. For the heater to operate at 62.5 W, the value of R should be . [figure]

The value of unknown resistance (x) for which the potential difference between B and D will be zero in the arrangement shown, is : [figure]

1. 3
2. 42
3. 6
4. 9

A wire of resistance R and radius r is stretched till its radius became r/2. If new resistance of the stretched wire is x R, then value of x is .

The number of electrons flowing per second in the filament of a 110 W bulb operating at 220 V is : (Given e = 1.6 10-19 C)

1. 1.25 1019
2. 31.25 1017
3. 6.25 1018
4. 6.25 1017

In the given figure R1 = 10 , R2 = 8 , R3 = 4 and R4 = 8 . Battery is ideal with emf 12 V. Equivalent resistance of the circuit and current supplied by battery are respectively : [figure]

1. 12 and 11.4 A
2. 10.5 and 1.14 A
3. 10.5 and 1 A
4. 12 and 1 A

The electric field between the two parallel plates of a capacitor of 1.5 capacitance drops to one third of its initial value in 6.6 when the plates are connected by a thin wire. The resistance of this wire is . (Given, e 3 = 1.1)

The ratio of heat dissipated per second through the resistance 5 and 10 in the circuit given below is: [figure]

1. 4 : 1
2. 2 : 1
3. 1 : 2
4. 1 : 1

A wire of resistance 20 is divided into 10 equal parts. A combination of two parts are connected in parallel and so on. Now resulting pairs of parallel combination are connected in series. The equivalent resistance of final combination is .

The resistances of the platinum wire of a platinum resistance thermometer at the ice point and steam point are 8 and 10 respectively. After inserting in a hot bath of temperature 400 , the resistance of platinum wire is :

1. 10
2. 16
3. 8
4. 2