Move your finger in the direction of current I in a circuit (+ to -), and
your finger will experience a drop in voltage = IR when going from one side of
resistor R to the other side. This defines R, and the unit of R, the ohm
(abbreviated W ),
is volt/amp. This should be called Ohm's definition. The law is the
proportionality between the voltage drop and the current. They are almost never
exactly proportional, but most electrical applications do not demand high
precision, so V = IR is useful. It really should be
D V, = - IR, but as long as you know how to use it, V = IR is fine.
Incidentally, we could just as well write the constant on the V side of the equation, I = kV, where k is called the conductance. The unit of conductance is the mho, and mho = 1/ohm: inverses and inverse spellings. Conductance seldom used, but you might run into it sometime. The same relationship exists between resistivity and conductivity. These are constants for a given material at a given temperature, and knowing one of these enables you to find the resistance of a slab of known dimensions.
Why isn't Ohm's law exact? Because resistance varies with temperature, and it is difficult if not impossible to prevent temperature from increasing with increasing current. Metals increase their resistance when temperature increases due to increased scattering of electrons, and semiconductors decrease R with T increase due to an increase in the number of charge carriers.
Power is energy per time, and it is easy to show that P = VI = V2/R = I2R.
My other main pages:
Comments, questions: fredrick.gram @ tri-c.edu (remove spaces)