Find Charges on Capacitor w/ Floating Plates

In summary: So the net charge on that circuit node cannot change, right?In summary, the conversation is about finding the charges on capacitors in a circuit with two batteries and three capacitors. The person is struggling to solve the problem due to the presence of floating plates, which cannot receive or release charge. They are advised to use superposition theorem and to consider the floating plates as isolated from the circuit. The conversation ends with the realization that the net charge on the circuit node with the floating plates cannot change.
  • #1
Hydrous Caperilla
62
7

Homework Statement


Ignore those darkened black lines as they are just consequences of my editing and there's a SWITCH" between A and C2 which is opened initially

So here's the question I was working with.

The switch is closed at t=O,find the charges on the capacitor.The batteries are both 2OV and the C1=2uF,C2=4uF and C3=2uF.
K0lO2.jpg

Homework Equations



3. The Attempt at a Solution
After the switch is closed at t=O ,the charges on the capacitors will change so I used Khirchoff rule on the 2 loops but it didn't help me because there are 3 unknown charges and my loops are 2 so I got stuck.When I asked my teacher ,He said that the circled plates are floating plates because these plates are isolated from the circuit so charges can't come on these plates from outside and so sum of charges on these plates would be zero but I couldn't catch that...Isn't C2 connected with the battery so charges can come from the battery?Anyway why are the circled plates floating plates how can I identify them
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  • #2
Would the charges change?
 
  • #3
Use superposition theorem.
Consider one battery at a time and short the other one. You can find the voltages across the capacitors in both the cases and then add them to get the resultant.
 
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  • #4
I believe so because the capacitors would cease to remain in series as seen when the path through capacitor 3 was open
 
  • #5
My doubt is not about solving the question but in identifying the floating plates...Why should I consider those circled plates to be floating plates
 
  • #6
Hydrous Caperilla said:
My doubt is not about solving the question but in identifying the floating plates...Why should I consider those circled plates to be floating plates
They have no wired connection to anything but other capacitor plates. Can you trace a path (via wire) from any of those plates to one of the battery terminals?
 
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  • #7
Nope...they are only connected to other capacitors
 
  • #8
Hydrous Caperilla said:
Nope...they are only connected to other capacitors
So the net charge on that circuit node cannot change, right? There's no path for new charge to enter or to leave.
 
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  • #9
Okay I get this
 

FAQ: Find Charges on Capacitor w/ Floating Plates

1. How do you find the charges on a capacitor with floating plates?

The charges on a capacitor with floating plates can be found by using the formula Q=CV, where Q is the charge, C is the capacitance, and V is the voltage.

2. What is the capacitance of a capacitor with floating plates?

The capacitance of a capacitor with floating plates can be calculated using the formula C=εA/d, where ε is the permittivity of the medium between the plates, A is the area of the plates, and d is the distance between the plates.

3. How does the distance between the plates affect the charges on a capacitor with floating plates?

The distance between the plates has an inverse relationship with the charges on a capacitor with floating plates. As the distance increases, the charges decrease, and vice versa.

4. Can the charges on a capacitor with floating plates be negative?

Yes, the charges on a capacitor with floating plates can be negative. This occurs when the capacitor is connected to a power source with a negative voltage, which causes the charges on the plates to become negative.

5. How does the permittivity of the medium between the plates affect the charges on a capacitor with floating plates?

The permittivity of the medium between the plates has a direct relationship with the charges on a capacitor with floating plates. As the permittivity increases, the charges on the plates also increase, and vice versa.

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