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Homework Help: RC Circuit: Charges faster than discharges?

  1. Jan 6, 2013 #1
    1. The problem statement, all variables and given/known data

    This isn't a problem from a textbook, but it is homework.
    For a lab in class, we worked with simple RC circuits on breadboards. The strange thing I noticed is that the capacitor always charges faster than it discharges. I know that the Time constant should be T=RC, so I don't understand why the time constants would differ.

    Could someone give a plausible explanation for why this is?
  2. jcsd
  3. Jan 6, 2013 #2

    Simon Bridge

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    I take it it's a small effect - but not accounted for by experimental error?
    Take a closer look at the differences between the circuits?

    The time constants are different because conditions are different.
    The charging circuit has an extra component in it - and, I'm guessing, one of the other components is in a different state.
    The details of the components can matter - what is the cap made of? The resistor? The voltage source? How do all these things affect the circuit?
  4. Jan 6, 2013 #3
    Simon, the difference in time constants is *very* obvious -it can't be accounted for by experimental error.
    Strangely, when charging, the time constant calculated from T=RC is accurately depicted in the graph ("nearly" completely charged at 5T). But when discharging, there's a dramatic difference.

    I actually can't find the answers to those questions because my class didn't spend much time on the lab. Is there anything that could potentially make a dramatic difference?
    I'm just curious.
  5. Jan 6, 2013 #4

    Simon Bridge

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    In the typical setup, the cap discharges through a different path to the charging.
    An extra resistance on that path would give a longer time constant for discharge than charge. An uncontrolled resistance like this is called a "stray" resistance. It could be that you discharged the cap through a different physical resistor than you charged it? Even if they are rated the same value, they may be physically different - especially if the equipment is old. Similarly, there may be longer wires on the discharging side of the circuit or some of the connections were bad on that side.

    Without knowing the details of the circuit I can only guess.
    Also see:

    ... an electrolytic capacitor requires a bit extra work to polarize the dielectric which can present as a different resistance charging to discharging but I don't know the details off-hand (it's been a while).
  6. Jan 6, 2013 #5


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    What was the circuit used?
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