# Homework Help: RC Circuits

1. Feb 9, 2008

### 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?

2. Feb 9, 2008

### Mindscrape

I(t) = C dv/dt

3. Feb 9, 2008

### Medium176

could you explain?

4. Feb 9, 2008

### John Creighto

I doesn't.

5. Feb 10, 2008

### Mindscrape

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?

6. Feb 10, 2008

### cepheid

Staff Emeritus
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.

7. Feb 10, 2008

### 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.

8. Feb 10, 2008

### John Creighto

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.

9. Feb 10, 2008

### 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.

10. Feb 10, 2008

### 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.