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Homework Help: Time constant 1/RC

  1. Dec 11, 2007 #1
    where is this equation from?
    what does it equal?
  2. jcsd
  3. Dec 11, 2007 #2


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    Well, presumably you're talking r as an electrical resistance, and c as an electrical capacitance. (Note that people ordinarily use the capital letters R and C for these quantities.) Also, you're not describing an equation, you're describing an expression.

    The expression RC has units of seconds. It is known as the "time constant" of a circuit formed by a resistor and capacitor. Imagine giving this circuit a step input, perhaps from 0V to 1V. The time constant is the length of time needed for the circuit's step response to be within a factor of 1/e of its final value. The factor 1/e is around 37%. Thus, after one time constant, the output voltage is about 63% of its final value.

    - Warren
    Last edited: Dec 11, 2007
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