Ohm's Law is not really a "Law"

Doug H. · 2007-05-23T13:21:53+00:00

Relax - you already knew this. Conductor resistance is a function of conductor temperature and conductor temperature is a function of amperage which is a function of restance.... Interesting circular influences there. Fortunately, as temperature increases so does resistance. Otherwise we'd always be battling a current/thermal runaway scenario. So, Ohm's Law is "close enough" for constant temperatures within limits. But, if you think that I = E/R is really an immutable "Law", hook up a 12vdc battery to an incandescent 12v lamp. With a dual trace o-scope, capture the applied voltage on one channel and the lamp current on the other during the short period that the fillament is heating up. Ohm's Law implies a linear curve - but what you capture is quite different. Of course, to get a current input you can measure the voltage across a very low valued resistor in series with the lamp which would also heat up and affect the outcome. But, choosing a very low resistance value helps minimize the resistor's influence.Yeah - OK - I'm just killing time and fighting off boredom.

May 23, 2007 20 Replies

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JackShephard 19 years ago

Damn. You got one right! I might be impressed if I wasn't yawning.

It is called "Voltage induced stress", and it will most certainly shift the "resistance value" due to all the leakage through the resistance medium that was use to make the resistor.

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Ohm's Law is not really a "Law"

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