# RC Circuit/Node voltage question

• Engineering
• RED119
In summary, the conversation revolved around finding the voltage across node D while a capacitor is charging. It was suggested to use the equation for Vc(t) and voltage division to find Vb, but there was a gap in the reasoning as the resulting expression did not match the expected outcome. It was later discovered that adding IRb to the voltage drop across Rb was necessary for the correct solution.
RED119

## The Attempt at a Solution

So you need to find the voltage across node D while the capacitor is charging, so it is going to be related to the voltage of the capacitor, so a function of time, so you need to come up with the equation for Vc(t), which I did. Then you do voltage division to find the voltage of Vb, which would be (Rb/(Ra+Rb))(Vc(t), then to find the voltage on node D you just need to use KVL and add Vb and Vc(t). But this doesn't simplify down to the expression it should be equal to, so there has to be some gap in my reasoning or something.

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I (think I) see you work it out in terms of ##V_C## which isn't very useful in this case.
The expression for ##V_C(t)## looks good to me. Now add ##IR_B##

BvU said:
I (think I) see you work it out in terms of ##V_C## which isn't very useful in this case.
The expression for ##V_C(t)## looks good to me. Now add ##IR_B##
I have Vb or voltage drop across Rb from (Rb/(Ra+Rb))(Vc(t), and I am adding that to Vc(t), but that isn't correct

Oh ?

## 1. How do I calculate the equivalent resistance in an RC circuit?

The equivalent resistance in an RC circuit can be calculated by adding the resistance of the resistor and the impedance of the capacitor in series. The formula for calculating impedance of a capacitor is Z = 1/(jωC), where j is the imaginary number, ω is the angular frequency, and C is the capacitance in Farads. Once you have calculated the impedance, you can add it to the resistance of the resistor to get the equivalent resistance.

## 2. How do I calculate the time constant in an RC circuit?

The time constant in an RC circuit can be calculated by multiplying the resistance (R) and the capacitance (C). The formula for time constant is τ = RC. This value represents the amount of time it takes for the capacitor to charge to approximately 63% of its maximum voltage.

## 3. What is the difference between a series RC circuit and a parallel RC circuit?

A series RC circuit has the resistor and capacitor connected in a series, meaning the current flows through one and then the other. In a parallel RC circuit, the resistor and capacitor are connected in parallel, meaning they share the same voltage source but have separate branches for the current to flow through.

## 4. How do I calculate the voltage across a capacitor in an RC circuit?

The voltage across a capacitor in an RC circuit can be calculated by using Ohm's law. Since the capacitor is connected in series with the resistor, the voltage across the resistor is equal to the voltage across the capacitor. The formula for voltage is V = IR, where I is the current and R is the resistance. Alternatively, you can use the equation V = V0(1-e^(-t/RC)), where V0 is the initial voltage and t is time.

## 5. How can I analyze the behavior of an RC circuit over time?

To analyze the behavior of an RC circuit over time, you can use the equations for charging and discharging a capacitor. When a capacitor is charging in an RC circuit, the voltage across it increases exponentially as it approaches the maximum voltage. When discharging, the voltage decreases exponentially. You can also use a graph to visualize the behavior of the voltage over time in an RC circuit.

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