RLC circuit
Hi:
I have a question about the RLC circuit. A RLC circuit cotains a DC voltage source. The voltage source initially have 2 volts of voltage. It stays like this for a very long time. Then the DC voltage switches to 5 volts. Immediately after switching, will the voltage across the capacitor change instantly or stay same? Will the current around the RLC circuit change instantly or stay same? thanks.
[413 byte] By [
wingstech] at [2007-11-20 1:55:15]
# 1 Re: RLC circuit
This is a computer programming related site, so I don't know if you'll get many responses...
Is this a homework question?
Viggy
# 2 Re: RLC circuit
Yes, it was a homework question
# 3 Re: RLC circuit
The voltage across a capacitor can never change instantaneously. The nature of the circuit that the capacitor is in doesn't matter; in other words, voltage across the capacitor can't change instantaneously in an RLC circuit, or an RC circuit, or whatever.
Mike
# 4 Re: RLC circuit
Start with I = dq/dt :)
This is a very basic thing - take a look at fundamentals of physics by Resnick and Halliday. Irodov's book has some great problems to practice, electricity is easier (its mechanics is superb! I enjoyed a lot when I used to solve the problems)
Now thats all history :( 7 years.. needs a brush up sometime, if I get some time.
# 5 Re: RLC circuit
Start with I = dq/dt :)
Here's what I can remember: Since DC voltage is appied, there will be not current passing through the capacitor except leakage. :)
Kheun at 2007-11-9 13:04:31 >
# 6 Re: RLC circuit
Here's what I can remember: Since DC voltage is appied, there will be not current passing through the capacitor except leakage. :)
That's true only in steady state. As a function of time, however, things can be different. As exterminator points out, I=dq/dt, from which you can calculate that the voltage across a capacitor cannot change instantaneously.
Mike
# 7 Re: RLC circuit
Hi:
Yes, capacitor can't change instantaneously. That is what I got. But how about the current around the RLC circuit. Will it change instantaneously or stay same after DC voltage switch from 2 volts to 5 volts?
# 8 Re: RLC circuit
Yes, capacitor can't change instantaneously. That is what I got.
That's not exactly correct. A capacitor *can* change the current through it instantaneously; it cannot change voltage across it instantaneously.
But how about the current around the RLC circuit. Will it change instantaneously or stay same after DC voltage switch from 2 volts to 5 volts?
You need to refer to a circuit analysis reference. The answer changes depending on the exact nature of the circuit.
Mike
# 9 Re: RLC circuit
Hi:
Yes, capacitor can't change instantaneously. That is what I got. But how about the current around the RLC circuit. Will it change instantaneously or stay same after DC voltage switch from 2 volts to 5 volts?
Well, just as the voltage across a capacitor cannot change instantaneously, the current through a coil cannot change instantaneously.
So, if your RLC circuit is all in series, then neither voltage nor current changes instantaneously.
Viggy
# 10 Re: RLC circuit
That's not exactly correct. A capacitor *can* change the current through it instantaneously; it cannot change voltage across it instantaneously.
You need to refer to a circuit analysis reference. The answer changes depending on the exact nature of the circuit.
Mike
It was a typo, I should say "Yes, capacitor can't change voltage instantaneously" rather than "Yes, capacitor can't change instantaneously"
# 11 Re: RLC circuit
Well, just as the voltage across a capacitor cannot change instantaneously, the current through a coil cannot change instantaneously.
So, if your RLC circuit is all in series, then neither voltage nor current changes instantaneously.
Viggy
good, this was what I answered for the homework.
# 12 Re: RLC circuit
A bit late in the thread, but just an observation.
If the capacitor has had 2 volts applied to it for a long time, then it will probably have filled up by the time the 5 volts are applied. Therefore there will be no current going through the capacitor by the time the 5 volts are applied. Hence there is no change in current (there is no current, because the capacitor is still full) and therefore there is no induction accross the coil due to the change in current, and therefore the voltage accross the capacitor will change to 5 volts (virtually) instantaneously. I think.
# 13 Re: RLC circuit
A bit late in the thread, but just an observation.
If the capacitor has had 2 volts applied to it for a long time, then it will probably have filled up by the time the 5 volts are applied. Therefore there will be no current going through the capacitor by the time the 5 volts are applied. Hence there is no change in current (there is no current, because the capacitor is still full) and therefore there is no induction accross the coil due to the change in current, and therefore the voltage accross the capacitor will change to 5 volts (virtually) instantaneously. I think.
Nope. As the current starts to flow, a magnetic field builds in the coil. This changing magnetic field also induces a "back" current in the wire (i.e. this current wants to flow in the opposite direction of the first current), thus the current cannot change instantaneously.
With the cap, it takes time for the charge (voltage) to build on the plates.
Viggy
# 14 Re: RLC circuit
This site has a nice Java applet: http://www.ngsir.netfirms.com/englishhtm/RLC.htm
It shows AC response, and not the response to a DC step change, so I suppose the debate will rage on ;)
Mike
# 15 Re: RLC circuit
As the current starts to flow ...
There is no current.
The capacitor is already full because it has been attached to a DC source for a long time. The capacitor therefore has (nearly) infinite resistance. No current, no induction, no delay.
:)
EDIT
Except that, with the higher voltage, the capacitor will start filling up even more, and then you do get current.
Ok, I was wrong.
:)
# 16 Re: RLC circuit
Yep, that's the trick to the question! :D
Viggy
