Here a question which contains, as it is delivered, an apparent paradox:
Joule's law tells us that the power dissipation is directly proportional to the electrical resistance (R i = P ^ 2)
However, if is introducing a resistor in a circuit is too small, the power increases exponentially: exists a classic experiment in which a "mine of graphite with a thickness of 0.5 mm, connected to a generator, became the first hot and then burns out - which means and power consumption has
How to explain this phenomenon?
Well, sure, the point is this: the power dissipation is not always directly proportional to the resistance.
In general, the power is P = V xi (product of the DDP with the intensity of current)
The two cases are when you insert a resistance:
- generator in a "current", the current remains constant -> we can replace V with the first Ohm's law and obtain the known formula P = R ^ 2
- in a generator "voltage", which is the potential difference is kept constant. In this case, always the first Ohm's law, we
P = V ^ 2: R, and thus the power dissipation is inversely proportional to the resistance (resistance too small a power grow until the resistance melts and the circuit is opened). It 's the case of the graphite mine.
