calculate the resistances of (a) 100-watt 120-volt incandescent lamp, (b) a 750-watt 115-volt toaster, (c) a 1300 watt 110-volt portable iron.

Answer 1

A 72-ohm coil of wire is connected in series with an adjustable resistor (rheostat) whose resistance can be varied from 0 to 88 ohms. If the line potential is 115 volts, calculate (a) the range through which the coil current can be varied, (b) the rheostat resistance when the power taken by the coil is 90 watts

Answer 2

a. 120I = 100 W.
I = 0.833A

R = V/I = 120/0.833 = 144 Ohms.

b. Same procedure as a.

c. Same procedure as a.

a. Imax = V/(72+0) = 115/(72+0) = 1.60A
Imin = 115/(72+88) = 0.719A

b. P = V^2/(Rc+R) = 90 W.
115^2/(72+R) = 90
90*(72+R) = 115^2
72+R = 146.94
R = 146.94-72 = 74.94 Ohms=The rheostat
resistance.