## Suppose you have three resistors of 20ohm, 50 ohm and 100 ohm. What minimum and maximum resistances can you obtain from these resistors?

Q. Suppose you have three resistors of 20 ohm , 50 ohm and 100 ohm . What minimum and maximum resistances can you obtain from these resistors?
(a) 15Ω , 170Ω
(b) 12.5Ω , 170Ω
(c) 12.5Ω , 150Ω
(d) 15Ω, 150Ω
Ans: B

For minimum resistance all the resistors are connected in parallel.

1/Rmin = 1/20 + 1/50 + 1/100

For maximum Resistance , all the resistors are connected in series

Rmax = 20 + 50 + 100 = 170 ohm

## The potential difference between the terminals of a 6 V battery is 7.2V when it is being charged by a current of 2 A. What is the internal resistance of the battery?

Q. The potential difference between the terminals of a 6 V battery is 7.2V when it is being charged by a current of 2 A. What is the internal resistance of the battery ?

(a) 0.4Ω
(b) 0.6Ω
(c) 1 Ω
(d) 0.5Ω
Ans: B

Sol.

V = E + Ir

7.2 = 6 + 2 r

2 r = 1.2

r = 0.6 ohm

## Calculate the electric field in a copper wire of cross-sectional area 2 mm² carrying a current of 1A. The resistivity of copper = 1.7 x 10^-8 ohm-m

Q. Calculate the electric field in a copper wire of cross-sectional area 2 mm2 carrying a current of 1A . The resistivity of copper ρ = 1.7 x 10-8 ohm-m

(a) 8.5 mvm-1
(b) 8 mvm-1
(c) 9 mvm-1
(d) 7 mv/m
Ans: A

Resistance R = ρL/A => L= RA/ρ

Since V = IR and V = EL => E = V/L

## Find the equivalent resistance of the circuits shown in figure between the points a and b. Each resistor has a resistance r

Q. Find the equivalent resistance of the circuits shown in figure between the points a and b. Each resistor has a resistance r (a) r/2
(b) r/3
(c) r/4
(d) r/5
Ans: A
The upper circuit forms Wheatone bridge . At first find the equivalent resistance of upper circuit , then this equivalent resistance of the upper circuit is in parallel with resistor between a and b .

## A voltmeter consists of a 25 ohm coil connected in series with a 575 ohm resistor. The coil takes 10mA for full scale deflection. What maximum potential difference can be measured on this voltmeter?

Q. A voltmeter consists of a 25 ohm coil connected in series with a 575 ohm resistor. The coil takes 10mA for full scale deflection. What maximum potential difference can be measured on this voltmeter?
(a)3V
(b) 6V
(c) 4V
(d) 5V
Ans: B