Simple problem:Voltage of capacitors AFTER closing a switch

In summary: To find the voltages after closing the switches, you need to apply Kirchhoff's laws. First, use Kirchhoff's voltage law to write an equation for the loop on the left side of the circuit (V1, V1', V2', V3), and another equation for the loop on the right side (V4, V4', V3', V2). Then, use Kirchhoff's current law to write an equation for the junctions at V1' and V4'. You should end up with 3 equations and 4 variables, which means you will need to make an assumption about one of the variables. Once you solve for the unknowns, you can then go back and plug in the determined
  • #1
easyyy
1
0
Hello guys. Please look at this, it's a VERY SIMPLE problem and I've almost solved it, but sth is not right...

It's a simple problem but i am confused regarding the method that I'm supposed to use to solve this kind of problems. So the problem is:
V1 , V2 , V3 and V4, E1, E2 are given. They are the voltages before closing the switches. We have to find the same voltages V1', V2', V3' and V4' after closing the switches. The values with - ' - are the values AFTER closing the switches.

so, MY WORK UP TO HERE:

I have the following equations:

(1) Q1+Q2+Q3+Q4=Q1'+Q2'+Q3'+Q4 , because the total charge won't change after closing the switches
(2) -E1+V1'+V2'+E2-V3'+V4'=0 , because of the 2nd rule of Kirchoff
(3) Q1 + Q2=Q1'+Q2' (because these two capacitors are in linear connection)

Equations (1) and (3) can be rewritten using that Q=C*V and Q'=C*V' , because the capacity will not change after closing the circuit.
So we have 3 equations and 4 variables (V1' , V2', V3', V4' ). So how am I supposed to find these variables?? Maybe the equation (3) or is not correct, or even the (2) but that's alla I could do...


I would be gratefull if anyone could help because it's not just for this problem, I want to see the way I should work to solve this. Thnx in advance :)
// The circuit is here: as an image:
circuit.jpg


URL is: http://img.photobucket.com/albums/v446/easyyy/circuit.jpg"
 
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  • #2
When I see capacitors in series or parallel, the first thing I do is rewrite them as a single simplified capacitor. Using your knowledge about capacitors in series or parallel, try simplifying the circuit. It's not necessary, but you'll have fewer variables to think about. Then, after solving for the unknowns in the simplified equations, you can go back and plug in the determined values.

For example, look at the 2uF and 4uF capacitors. You can rewrite them as one capacitor with the equation

[tex]\frac{1}{C_1} + \frac{1}{C_2} = \frac{1}{C_{combined}}[/tex].

This comes out to 4/3 uF. As you can see, the charge across the combined capacitors is the same as that of the simplified capacitor, so, after solving for the charge, you can go back and plug it back into the un-simplified equations. Start solving for variables after you've simplified the circuit.

It's similar to what you do for resistors, if you've done that already, except that what you do for series and parallel is flipped.
 
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  • #3
easyyy said:
...
I have the following equations:

(1) Q1+Q2+Q3+Q4=Q1'+Q2'+Q3'+Q4 , because the total charge won't change after closing the switches
...
Since there are voltage sources in the circuit, when you close the switches there will be currents flowing through the circuit, so there is no conservation of charges.
 

1. What happens to the voltage of a capacitor after closing a switch?

When a switch is closed, the voltage across a capacitor remains unchanged. This is because capacitors act like open circuits to DC voltage, allowing no current to flow through them. Therefore, the voltage across a capacitor before and after closing a switch will be the same.

2. Why does the voltage across a capacitor not change after closing a switch?

Capacitors store charge on their plates, creating an electric field between the plates. When a switch is closed, the capacitor continues to hold the same amount of charge, and therefore the same voltage, as before. This is due to the nature of the electric field and the insulating material between the plates of the capacitor.

3. Can the voltage of a capacitor change after closing a switch?

While the voltage across a capacitor typically remains the same after closing a switch, there are certain circumstances where it can change. If the capacitor is connected in a circuit with other components, such as resistors, the voltage may change due to the flow of current through the circuit. Additionally, if the capacitor is not fully charged before the switch is closed, the voltage may change as it charges to its full potential.

4. How does the voltage of a capacitor affect the rest of the circuit?

The voltage of a capacitor can have a significant impact on the rest of the circuit it is connected to. For example, if the capacitor is fully charged and the switch is closed, it can act as a temporary power source, supplying energy to the rest of the circuit. Additionally, the voltage of a capacitor can affect the overall stability and performance of the circuit it is connected to.

5. Are there any safety concerns when working with capacitors and voltage?

Yes, there are safety concerns when working with capacitors and voltage. Capacitors can store a significant amount of energy, and if not discharged properly, they can release this energy in a dangerous manner. It is important to follow proper safety precautions and use appropriate equipment when working with capacitors and high voltages to avoid any potential hazards.

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