✔ 最佳答案
1. Both equations are right.
P = V.I when R is kept constant
P = V^2/R when I is kept constant
They apply in different situations.
2. The "volt" is defined as the work done in Joule by a coulomb of charge through a potnetial difference of 1 volt. Hence, 1 volt = 1 J/C
Hence, work done by 1 coulomb of charge through a 12 v lamp
= 12 J
3. Since the masses of the two conductors are the same, then
L.A = L'A'
where L and L' are the lengths of X and Y respectively
and A and A' are the cross-sectional areas of X and Y respectively
Hence, L.A = (L/2)A'
A'/A = 2
Let resistance of conductor X = R
R = pL/A where p is the resistivity of the material
Power dissipation on X, P = 1^2.R = R = pL/A
Resistance of conductor Y, R' = pL'/A'
Power dissipation on Y, P' = (3^2).R' = 9R' = 9pL'/A'
Hence P/P' = (pL/A).(A'/9pL') = (1/9).(L/L').(A'/A) = (1/9).(2).(2) = 4/9
i.e. P:P' = 4:9