The wattage rating of an appliance W varies jointly as the square of the current I and the resistance R. If the wattage is 1.5 watts when the current is 0.1 ampere and the resistance is 150 ohms, find the wattage when the current is 0.3 ampere and the resistance is 50 ohms.

Answer 1

W = i^2 R (what else is new?)

1.5 = .01 R
R = 150 Ohms

W = .3^2 (50) = 4.5 Watts

or using just proportions

W = 1.5 (.3/.1)^2 * 50/150
= 1.5 * 9/3 = 1.5*3 = 4.5 Watts, remarkable :)