1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Struggling with RC circuit concepts

  1. Mar 4, 2013 #1
    So we're covering RC circuits in my physics class and I'm having some trouble with the concepts.
    When you have a circuit with a resistor and a battery, current passing through the resistor produces a voltage drop. If the battery is zero for our potential, the battery imbues the circuit with some potential difference which creates a current. My book says that the resistor will suck all of the potential energy out of the charges passing through it, since the voltage drop through the resistor equals the voltage of the battery. I don't understand how this could be, because if the charges in the wire have no potential energy after they've passed through the resistor how could they continue to be drawn toward the other end of the battery?
    Also, what happens as charge flows through a wire attached to both terminals of a battery, with no other circuit components, if we assume the resistance to be "negligible?" According to V=IR, the voltage would go to zero! I remember reading that this is mostly an experimental law, and is only valid at certain temperatures in certain substances, but this is still a confusing point. Could someone help me clarify?
  2. jcsd
  3. Mar 4, 2013 #2
    I assume this is an ideal case where the resistor is connected to the battery terminals with ideal wires. This effectively means you should view the wires as an extension of the battery terminals (they're the same node) and as charge passes through the resistor, it simply passes between battery terminals.

    Ideal wire again? In that case, charge won't flow as the terminals of the battery are the same node and there can be no voltage between them.
  4. Mar 4, 2013 #3
    that was my exact problem, I was struggling with the ideal wire concept! thanks so much :)
  5. Mar 5, 2013 #4

    If a battery puts out zero volts, then it is dead and no current exists.

    Of course. If you make a battery supply a current continuously, eventually it will deenergize.

    Voltage is the electrical density of the charge (joules/coulomb). If the energy density at one point is more (higher voltage) than the energy density (lower voltage) at another point, then the charge will flow to the lower energy density provided there is a conduction path.

    You would have an infinity of current across a infinitesimal amount of resistance. That cannot happen in the "real" world. It is like asking the question of what happens when a unstoppable force encounters an immovable wall. It is more a philosophical question than a scientific one.

  6. Mar 6, 2013 #5
    If the battery dead, zero voltage across it's terminals, no current flows.

    The potential [voltage] energy is converted to heat energy in the resistor. electrons moving through the resistor continue to have potential energy and this moves them along the circuit and well as those ahead of them.

    In practice what happens, is that for a short time [no pun intended] a large current will flow until the potential energy of the ions [charge carriers] in the battery are depleted....the chemical energy in the battery is dissipated. The initial flow of current is determined by the resistance of the wire between the battery terminals....and as it gets hot, it's resistance typically increases....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook