Calculating Charge on Capacitors in Parallel | 40 pF & 70 pF [SOLVED]

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In summary, the problem involved a 40 pF capacitor charged to 3 kV being connected in parallel to an uncharged 70 pF capacitor. The new charge on the second capacitor was found by taking the total charge on both capacitors (120 nC) and using it to find the equivalent capacitance (110 pF), then using Q = CV to find the new charge on the second capacitor, which is approximately 109.1 nC. The mistake in the initial attempt was using the voltage of 3 kV for the second capacitor instead of the voltage on both capacitors after being connected.
  • #1
soccerj17
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[SOLVED] capacitor problem

Homework Statement


A 40 pF capacitor is charged to 3 kV and then removed from the battery and connected in parallel to an uncharged 70 pF capacitor. What is the new charge on the second capacitor? Answer in units of nC.


Homework Equations


pF= 1 F x 10^-12
kV= 1 V x 10^3
nC= 1 C x 10^-9
Q = CV


The Attempt at a Solution


I thought that because they're connected in parallel that means the voltage is the same across them so I used Q=CV to find charge. For the second capacitor the capacitance is 70 x 10^-12 C and I used the voltage of 3 x 10^3 V. Multiplying them together I got
210 x 10^-9 or 210 nC. I submitted this online to our answer service and it was wrong, and I don't know what I did wrong.
 
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  • #2
The voltage on capacitor #2 after you connect them is equal to the voltage on capacitor #1 after you connect them, but this is NOT equal to the voltage on capacitor #1 before you connect them, so you can't use the voltage of 3 x 10^3 V

Use the fact that the total charge on both capacitors doesn't change after the connection.
 
  • #3
So total charge is the original capacitance (40 pF) times the 3 KV so 120 nC. Then I find voltage by V = Qtotal/Ceq which is C1 + C2 then i use that to find Q by using Q=CV?
 
  • #4
Alright i got it!
 

1. What is a capacitor?

A capacitor is an electronic component that stores electrical energy in the form of an electric field. It is made up of two conductive plates separated by an insulating material called a dielectric.

2. How do capacitors work?

Capacitors work by accumulating and storing electrical charge. When a voltage is applied across the plates of a capacitor, an electric field is created between the plates. This causes one plate to accumulate a positive charge and the other plate to accumulate a negative charge. The capacitor can then release this stored energy when it is connected to a circuit.

3. What are some common problems with capacitors?

Some common problems with capacitors include short circuits, open circuits, and leakage. Short circuits occur when the dielectric between the plates breaks down, causing the capacitor to lose its ability to store or release energy. Open circuits occur when the connection between the plates and the terminals is broken, rendering the capacitor useless. Leakage occurs when the dielectric material is not able to insulate the plates effectively, causing current to leak out of the capacitor.

4. How do I test a capacitor?

To test a capacitor, you can use a multimeter set to the capacitance function. Connect the probes to the capacitor terminals and the multimeter will display the capacitance value. You can also test for continuity with a multimeter set to the resistance function. If the capacitor is functioning properly, the multimeter should display a low resistance value. Additionally, you can use an oscilloscope to test for any fluctuations in voltage across the capacitor.

5. What should I do if my capacitor is not working?

If your capacitor is not working, it is best to replace it with a new one. It is not recommended to try and repair a faulty capacitor as they are relatively inexpensive and can be easily found at electronic stores. It is important to properly dispose of the old capacitor as they may contain hazardous materials. Be sure to follow safety precautions when handling capacitors as they can hold a charge even when disconnected from a power source.

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