Using Constant Sources in RC Circuits: Parallel & Series

[Medium176]

Why does a constant current source need to be used in a parallel RC circuit?

Why does a Constant Voltage source need to be used in a Series RC circuit?

 

[Mindscrape]

I(t) = C dv/dt

 

[Medium176]

could you explain?

 

[John Creighto]

I doesn't.

 

[Mindscrape]

I doesn't.

Does to.

$$I = dQ/dt$$

$$C \equiv Q/V$$

so

$$I = \frac{d}{dt}(CV)$$

capacitance is constant

$$I = C \frac{dV}{dt}$$

to the original poster, could you please show some thoughts towards this problem so I, or someone else, knows where to begin helping?

 

[cepheid]

Mindscrape, I think John's answer was directed to the OP, i.e. he wasn't refuting your claim that I = CdV/dt, he was answering the original question:

Q1. Why does a constant current source need to be used in a parallel RC circuit?

Q2. Why does a Constant Voltage source need to be used in a Series RC circuit?

A. It doesn't (in both cases).

I tend to agree. I don't see any need to make those restrictions. It seems more likely that circuits of those two forms were introduced to the OP in a specific context?

I think we do need to know what that specific context was to say anything further.

 

[Mindscrape]

Oh, my bad, John. I think that the question is getting at how the two configurations will produce similar differential equations, but we'll have to wait for the OP to elaborate.

 

[John Creighto]

Oh, my bad, John. I think that the question is getting at how the two configurations will produce similar differential equations, but we'll have to wait for the OP to elaborate.

Well, that is because each circuit only has one energy storage device. The number of states in a circuit is equal to the number of energy storage devices. Well at least that is usually true if the energy storage devices are resisters and capacitors.

 

[John Creighto]

On another note, the current source and voltage sources are forcing terms which aren't part of the natural response. A non ideal voltage sources is a voltage source in series with a resister. A non ideal current source is a current source in parallel with a resistor. It can be shown that the two are equivalent.

 

[John Creighto]

One other thing I should mention is the the current though an inductor cannot change instantaneously and the voltage across a capacitor cannot change instantaneously. This is the most obvious restriction on a circuit. Therefore ideal voltage sources cannot be in parralel with a capacitor and ideal current sources cannot be in series with an inductor.