When exactly does a capacitator discharge?

1. May 12, 2007

Mr_Bojingles

I have a few questions concerning capacitators.

1.) When exactly does a capacitator discharge. Is it only when its voltage equals that of the power source and cancel each other out or is the capacitator always discharging at a slow rate.

2.) I read that when you use a capacitator in conjunction with a resistor you can make a timer device. How does that work? Does the resistor block off all the low current and only lets current flow whenever the capacitator builds charge greater than the resistance?

3.) When the capacitators emf equals the power sources emf and cancel each other out is the section of wire between the battery and the capacitator completely dead?

2. May 12, 2007

waht

1.) When exactly does a capacitator discharge. Is it only when its voltage equals that of the power source and cancel each other out or is the capacitator always discharging at a slow rate.

When charged with DC voltage, a capacitor will hold the charge indefinitely. With AC, current will flow through the capacitor, while continuously charing it and discharging it.

2.) I read that when you use a capacitator in conjunction with a resistor you can make a timer device. How does that work? Does the resistor block off all the low current and only lets current flow whenever the capacitator builds charge greater than the resistance?

With resistor you limit the current applied to the capacitor, hence you limit the amount of charge per unit time being deposited in the capacitor, as more charges accumulate, the voltage across the cap will increase until it reaches the maximum voltage that is avialable fro your power supply.

3.) When the capacitators emf equals the power sources emf and cancel each other out is the section of wire between the battery and the capacitator completely dead?

See question 1.

3. May 13, 2007

No! That's when it stops charging and current in the circuit dies if I remember correctly. It's voltage now opposes the source's and current stops. This isn't true obviously if the circuit is more complex than a simple battery and loop. When it's fully charged and you kill the power source it will discharge current the opposite way to which current flowed to charge it. Note all this is for DC only. AC works differently where it's behaviour becomes dependant on frequency.

Also capacitor's only discharge on DC when the source voltage is turned off or drops.

You can.

No not really. The resistor I suppose simply caps the maximum rate at which current can discharge and also causes the response to be delayed over time instead of all the charge being dumped instantly. I'm stretching my memory here to a few weeks ago but, I think that the time constant occurs at 37% of the capacitor's charge.

To get the time constant multiply R by C. After one time constant 37% of the charge is gone, after 5 time constants have passed you're down to less than 1% initial charge. Very reliable behaviour usable as a timer due to its predictability.

With DC yes but not with AC. Capacitors allow some AC through at certain frequencies but with DC yes it stops dead.

**Edit**

Waht beat me to it