# Disconnected and Reconnected Capacitors

• nns91
In summary, the conversation discusses the redistribution of charge and resulting potential difference when a 100pF and 400pF capacitor charged to 2kV are connected together in parallel. The resulting charge and voltage are calculated, but it is found that the new voltage is larger than the initial voltage, leading to confusion and the question of whether the initial charges should be averaged. The recommended solution is to calculate the initial charges and use the difference to find the new voltage.
nns91

## Homework Statement

A 100pF and a 400pF capacitor are both charged to 2kV. They are then disconnected from the voltage source and are connected together, positive plate to negative plate.

(a). Find the resulting potential difference across each capacitor
(b). Find the energy lost when the connections are made

Q=C.V

## The Attempt at a Solution

So when they are reconnected they are in parallel. However the charge is redistributed because positive plate is hooked to negative plate.

I have calculated the total charge using Q=C.V= (100pF+400pF)*2kV=1$$\mu$$C

The I use Q1/V1=Q2/V2 and Q1+Q2=1$$\mu$$C to calculate the new charge and get Q1= 0.2$$\mu$$C and Q2=0.8$$\mu$$C.

However, the new voltage calculated using V=Q/C is 2*10^9 V which is larger than initial voltage. It does not make sense at all. Where did I screw up ?

Do I have to calculate the initial charge of each capacitor and average them to 0.5microC and then calculate the voltage from there ?

nns91 said:
Do I have to calculate the initial charge of each capacitor and average them to 0.5microC and then calculate the voltage from there ?

Calculate the initial charges. As the capacitors are connected positive plate to negative, the total charge will be the difference of the initial charges. The new capacitor has capacitance C1+C2, the charge is Q2-Q1, from that you get the voltage.

ehild

yucheng

## 1. What is a disconnected capacitor?

A disconnected capacitor is a capacitor that is not connected to any circuit or power source. It is essentially isolated and not functioning in any way.

## 2. How does a disconnected capacitor affect a circuit?

A disconnected capacitor does not have any effect on a circuit because it is not connected and therefore not able to store or release any electric charge.

## 3. Can a disconnected capacitor still hold a charge?

Yes, a disconnected capacitor can still hold a charge if it was previously charged while connected to a circuit. However, the charge will dissipate over time due to leakage and other factors.

## 4. What is the purpose of reconnecting a capacitor?

The purpose of reconnecting a capacitor is to restore its function in a circuit. Capacitors are used to store and release electric charge, so reconnecting them allows them to perform this function again.

## 5. Are there any safety concerns when handling disconnected capacitors?

Yes, there are safety concerns when handling disconnected capacitors as they may still hold a charge. It is important to discharge them properly before handling to avoid electric shock.

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