# Initial Energy/Final energy on 2 capacitors

• MarcL
In summary, two capacitors of values 4μF and 9μF are connected in series with a 30 V battery. When the battery is removed and the plates of like sign are connected, the initial and final energies for each capacitor can be found by using the equations Q=C*V and U=q^2/2C, as well as Ceq = ∑Ci and 1/Ceq= ∑1/Ci. The initial energy is calculated to be 325.125 J for the first capacitor and 735 J for the second capacitor. The final energies are found to be 51 μC and 115.02 μC for the first and second capacitors, respectively, using the formula Energy=
MarcL

## Homework Statement

Two capacitors, C1 = 4μF and C2 = 9μF, are connected in series with a 30 V battery. The battery is removed and the plates of like sign are connected. Find the initial and final energies for each capacitor.

## Homework Equations

Q=C*V
U=q^2/2C
Ceq = ∑Ci ( don't know how to note it on the computer) --> in parallel
1/Ceq= ∑1/Ci

## The Attempt at a Solution

1) find ceq ( in series) =>2.77uF
2)Find totaly charge ( same on both cause in series) =>93.077uC => Get initial energy from this (?)
3)Voltage on C1 => q/c => 20.78 V
4)Voltage on C2 => 1/c => 9.23 V
5)After they are reconnected, it needs to reach equilibrium ( I think?) So my total charge is
2*83.077 = 166.154 uC ??
Sooooo then my voltage on capacitor 1 would be 166.154/13 => 12.78V ( approx.)
6)Q1 final => c*v => 4uF*12.78 =>51uC
7)Q2 final => c*v => 9uF*12.78 =>115.02uC
9) Energy Final on 1 => q^2/2C => 325.125 J
10) Energy Final on 1=> q^2/2C=> 735 J

MarcL said:

## Homework Statement

Two capacitors, C1 = 4μF and C2 = 9μF, are connected in series with a 30 V battery. The battery is removed and the plates of like sign are connected. Find the initial and final energies for each capacitor.

## Homework Equations

Q=C*V
U=q^2/2C
Ceq = ∑Ci ( don't know how to note it on the computer) --> in parallel
1/Ceq= ∑1/Ci

## The Attempt at a Solution

1) find ceq ( in series) =>2.77uF
2)Find totaly charge ( same on both cause in series) =>93.077uC => Get initial energy from this (?)
3)Voltage on C1 => q/c => 20.78 V
4)Voltage on C2 => 1/c => 9.23 V
5)After they are reconnected, it needs to reach equilibrium ( I think?) So my total charge is
2*83.077 = 166.154 uC ??
Sooooo then my voltage on capacitor 1 would be 166.154/13 => 12.78V ( approx.)
6)Q1 final => c*v => 4uF*12.78 =>51uC
7)Q2 final => c*v => 9uF*12.78 =>115.02uC
9) Energy Final on 1 => q^2/2C => 325.125 J
10) Energy Final on 1=> q^2/2C=> 735 J

It is correct, but what are the initial energies?
You could have used also the formula Energy=U^2*C/2 for the final energies.

ehild