
Principles of Analog Electronics
108
4.4.8 Joule’s Law
Let’s apply the electric power denition, P = VI, to the
case of a resistance, and since we know Ohm’s law,
V= RI, we get
IRI
V
2
== =
so from the value of R and the voltage V across it, or
from the current I owing through it, it is possible to
identify the value of the dissipated power.
Example
We can apply the equation of electric power to the case of a common lamp, 100 W,
fed with the house voltage of 110 V. Its lament resistance is R =V
2
/P=121 Ω,
and the current owing is I =P/V≅0.91 A.
The power we are talking about is completely dissipated in the resistor.
Infact,when the cur ...