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
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
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 ?
(c) 1 Ω
V = E + Ir
7.2 = 6 + 2 r
2 r = 1.2
r = 0.6 ohm
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
Resistance R = ρL/A => L= RA/ρ
Since V = IR and V = EL => E = V/L
Q. Find the equivalent resistance of the circuits shown in figure between the points a and b. Each resistor has a resistance r
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 .
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?