Grounding a circuit through a capacitor/parallel RC circuit

In summary, the conversation discusses two circuits - a clear one and an unclear one. The clear circuit is a parallel RC circuit where the capacitor is initially uncharged and eventually reaches 12V across the resistor. The unclear circuit raises questions about the charging of the capacitor and the voltage measured across the resistor at t=0s. The expert explains that the voltage across the resistor in the unclear circuit depends on the past history of the circuit and gives examples of how it can vary. It is also mentioned that the unclear circuit lacks enough connecting wires, making it difficult to understand. Overall, it is clarified that the voltage across the capacitor remains constant in the unclear circuit.
  • #1
jozefmak
3
1
Hi,
I have two similar circuits. One of them is clear for me but I am not sure how the second one works.

Let's start with the clear one:
1598446695058.png

a parallel RC circuit, which is explained here:

At time t = 0s there are zero volts across the resistor because the capacitor is fully uncharged. After some time, the capacitor will get charged to 12V so we will measure 12V across the resistor.

This is the unclear circuit for me:
1598447266491.png

Will the capacitor get charged? (What voltage?)
What voltage are we going to measure at t = 0s across the resistor?


I know that when I remove the capacitor:
1598447822903.png

there will be 12V across the resistor at time t = 0s because a positive terminal of battery will have 0V and negative terminal of battery will have -12V (with respect to those 0V). The problem, which makes this circuit unclear for me is the capacitor.
 
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  • #2
In your second circuit, there will be 12V across the resistor. But the voltage at the top of the capacitor is unknown, and could be anything (within limits set by the breakdown voltage of the capacitor). So the voltages at the resistor terminals could be +10V and -2V, or +100V and +88V, or -10V and -22V, or anything you like. The voltage that actually appears depends on the past history of the circuit.
 
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  • #3
I see, thanks for reply!

I am just wondering for better understanding what exactly this sentence means:
"The voltage that actually appears depends on the past history of the circuit."
Can you give some example, please?
 
Last edited:
  • #4
Where are you connecting the 9V battery?
 
  • #5
Suppose you take the capacitor and connect a wire to both sides and discharge it. Then there is no voltage across it. So when you connect the bottom side to 0V, the top is at 0V as well. So when you connect the battery and resistor to it, the top of the resistor will be at 0V and the bottom will be at -12V. On the other hand, suppose you connect the capacitor to a power supply and charge it up to 100V. Then when you connect the bottom of the capacitor to 0V, the top will be at +100V. So when you connect the battery and resistor to it, the top of the resistor will be at +100V and the bottom will be at +88V. All of this assumes an ideal capacitor, and that the capacitance of the capacitor is large compared to the capacitance of the battery and resistor. In a real circuit, there will be charge sharing and it gets more complicated.
 
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  • #6
It's a sort of hackneyed phrase but you really need a CIRCUIT with a path between the connections of any component. The 'Unclear' circuit is unclear because there are not enough connecting wires.
 
  • #7
sophiecentaur said:
The 'Unclear' circuit is unclear because there are not enough connecting wires.
Good point. The deficiency is not in the OP’s knowledge. It is a deficiency in the circuit.
 
  • #8
phyzguy said:
Suppose you take the capacitor and connect a wire to both sides and discharge it. Then there is no voltage across it. So when you connect the bottom side to 0V, the top is at 0V as well. So when you connect the battery and resistor to it, the top of the resistor will be at 0V and the bottom will be at -12V. On the other hand, suppose you connect the capacitor to a power supply and charge it up to 100V. Then when you connect the bottom of the capacitor to 0V, the top will be at +100V. So when you connect the battery and resistor to it, the top of the resistor will be at +100V and the bottom will be at +88V.
OK, now I understand. Voltage across the capacitor will be invariable (no charging, no discharging) when connected in the "unclear" scheme. Thank you!
 
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Related to Grounding a circuit through a capacitor/parallel RC circuit

1. What is grounding a circuit through a capacitor/parallel RC circuit?

Grounding a circuit through a capacitor/parallel RC circuit involves connecting one end of the capacitor or RC circuit to ground, which serves as a reference point for the voltage in the circuit. This allows for the removal of unwanted noise or interference from the circuit.

2. How does a capacitor/parallel RC circuit help in grounding a circuit?

A capacitor/parallel RC circuit helps in grounding a circuit by acting as a low impedance path to ground for high-frequency signals, while allowing low-frequency signals to pass through. This effectively filters out any unwanted noise or interference in the circuit.

3. What is the purpose of using a capacitor in a parallel RC circuit for grounding?

The purpose of using a capacitor in a parallel RC circuit for grounding is to provide a low impedance path to ground for high-frequency signals. This helps in reducing noise and interference in the circuit, resulting in a cleaner and more stable signal.

4. Can a capacitor/parallel RC circuit be used for grounding in all types of circuits?

Yes, a capacitor/parallel RC circuit can be used for grounding in all types of circuits. However, it is most commonly used in circuits that require high-frequency filtering, such as in audio and radio frequency circuits.

5. How do I calculate the value of the capacitor and resistor for a parallel RC circuit used for grounding?

The value of the capacitor and resistor for a parallel RC circuit used for grounding can be calculated using the formula: RC = 1/2πf, where R is the resistance in ohms and C is the capacitance in farads. The value of the capacitor and resistor should be chosen based on the frequency of the signal and the desired filtering effect.

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