Question

(a) The electric power consumed by a device may be calculated by using either of the two expressions P = 12R or P =V2/R . The first expression indicates that it is directly proportional to R whereas the second expression indicates inverse proportionality. How can the seemingly different dependence of P on R in these expressions be explained?(b) (i) A 100 W electric bulb is connected to 220 V mains power supply. Calculate the strength of the electric current passing through the bulb. (ii) If the same bulb is taken to U.S.A where the main power supply is 110 V, how much electric current will pass through the bulb when connected to mains?

Answer 1

a) Both the expressions are correct. In the first case, I remain constant whereas the second expression is true when V remains constant

So, the expression P = I2R is used when resistances are connected in series because in series connection the current in each resistance is constant.

Similarly, when resistances are connected in parallel the voltage is constant and so power becomes inversely proportional to R.

Answer 2

Answer:

Both the expressions are correct.

Explanation:

It depends on wether Current is given or Potential Difference