Using Constant Sources in RC Circuits: Parallel & Series

In summary, the discussion revolves around the use of constant current and voltage sources in parallel and series RC circuits. The main contention is whether these sources are necessary in each respective circuit, with some arguing that they are not needed due to similar differential equations being produced. Additionally, the role of energy storage devices and the restrictions on ideal voltage and current sources in circuits are also discussed.
  • #1
Medium176
2
0
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?
 
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  • #2
I(t) = C dv/dt
 
  • #3
could you explain?
 
  • #4
I doesn't.
 
  • #5
John Creighto said:
I doesn't.

Does to.

[tex]I = dQ/dt[/tex]

[tex]C \equiv Q/V[/tex]

so

[tex]I = \frac{d}{dt}(CV)[/tex]

capacitance is constant

[tex]I = C \frac{dV}{dt}[/tex]

to the original poster, could you please show some thoughts towards this problem so I, or someone else, knows where to begin helping?
 
  • #6
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
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
Mindscrape said:
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.
 
  • #9
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
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.
 

FAQ: Using Constant Sources in RC Circuits: Parallel & Series

1. How do I calculate the total resistance in a series RC circuit?

In a series RC circuit, the total resistance is equal to the sum of the resistance of the resistor and the reactance of the capacitor. To calculate the reactance of the capacitor, use the formula XC = 1/(2πfC), where f is the frequency of the circuit and C is the capacitance of the capacitor. Once you have calculated the reactance, add it to the resistance of the resistor to get the total resistance.

2. How do I calculate the total capacitance in a parallel RC circuit?

In a parallel RC circuit, the total capacitance is equal to the sum of the individual capacitances of the capacitors. Simply add up the values of all the capacitors in the circuit to get the total capacitance.

3. What is the time constant in an RC circuit?

The time constant in an RC circuit is a measure of how quickly the capacitor charges or discharges. It is equal to the product of the resistance and capacitance in the circuit, represented by the symbol τ. The time constant is used to calculate the time it takes for the capacitor to reach 63.2% of its maximum charge or discharge.

4. How does the placement of the resistor and capacitor affect the behavior of an RC circuit?

In a series RC circuit, the resistor and capacitor are placed in series, meaning that the current flows through both components. This results in a voltage drop across both components, and the capacitor charges and discharges as the current passes through. In a parallel RC circuit, the resistor and capacitor are placed in parallel, meaning that the voltage across both components is the same. This results in the capacitor charging and discharging more quickly due to the lower total resistance in the circuit.

5. How does the frequency of the circuit affect the behavior of an RC circuit?

The frequency of the circuit affects the reactance of the capacitor, which in turn affects the total impedance of the circuit. As the frequency increases, the reactance of the capacitor decreases, resulting in a lower total impedance and a faster charging and discharging of the capacitor. At very high frequencies, the reactance of the capacitor approaches 0, resulting in a purely resistive circuit.

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