1. My nephew (a high school student) asked me last week how time constants work. Time =resistance X capacitance. T=RC
He was please with this simple explaination:

T=RC T= VOLTS/AMPS x COULOMBS/VOLT

T= JOULES PER COULOMBE/COULOMBS PER SECOND x COULOMBS/JOULES PER COULOMBS

Reducing this shows that T=Seconds

2.

3. Since the definitions of the units used (Joules, Coulombs) is based on seconds ((kg.m2)/sec2 and Amp.sec) it seems a long way round to "prove" a tautology.

4. If you charge or discharge a capacitor through a resistor it will follow an exponential decay curve. The time constant will be the product of the resistance and the capacitance. Time constant for exponential decay is the time to reach 1-1/e of the final value.

RC time constant - Wikipedia, the free encyclopedia
Time constant - Wikipedia, the free encyclopedia

5. Originally Posted by Dywyddyr
Since the definitions of the units used (Joules, Coulombs) is based on seconds ((kg.m2)/sec2 and Amp.sec) it seems a long way round to "prove" a tautology.
Not a long way around to a beginner who wants to understand the equation. "T=RC. Now he knows and doesn't have to memorize.

6. Right...

7. Originally Posted by thinkatron
Originally Posted by Dywyddyr
Since the definitions of the units used (Joules, Coulombs) is based on seconds ((kg.m2)/sec2 and Amp.sec) it seems a long way round to "prove" a tautology.
Not a long way around to a beginner who wants to understand the equation. "T=RC. Now he knows and doesn't have to memorize.
All he's really learned is that RC has the units of time. You could use the same process to "prove" that kinetic energy is mv^2 or 5 mv^2 or whatever.

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