<h2>20. How much charge is on sphere B after A and B touch and are separated?</h2><h3>Answer:</h3>

<h3>Explanation:</h3>
We'll solve this problem by using the concept of electric potential or simply called potential  , which is <em>the energy per unit charge, </em>so the potential
, which is <em>the energy per unit charge, </em>so the potential  at any point in an electric field with a test charge
 at any point in an electric field with a test charge  at that point is:
 at that point is:

The potential  due to a single point charge q is:
 due to a single point charge q is:

Where  is an electric constant,
 is an electric constant,  is value of point charge and
 is value of point charge and  is  the distance from point charge to  where potential is measured. Since, the three spheres A, B and C are identical, they have the same radius
 is  the distance from point charge to  where potential is measured. Since, the three spheres A, B and C are identical, they have the same radius  . Before the sphere A and B touches we have:
. Before the sphere A and B touches we have:

When they touches each other the potential is the same, so:

From the principle of conservation of charge <em>the algebraic sum of all the electric charges in any closed system is constant. </em>So:

Therefore:

So after A and B touch and are separated the charge on sphere B is:

<h2>21. How much charge ends up on sphere C?</h2><h3>Answer:</h3>

<h3>Explanation:</h3>
First: A and B touches and are separated, so the charges are:

Second:  C is then touched to sphere A and separated from it. 
Third: C is to sphere B and separated from it
So we need to calculate the charge that ends up on sphere C at the third step, so we also need to calculate step second. Therefore, from the second step:
Here  and C carries no net charge or
 and C carries no net charge or  . Also,
. Also, 

Applying the same concept as the previous problem when sphere touches we have:

For the principle of conservation of charge:

Finally, from the third step:
Here  . Also,
. Also, 

When sphere touches we have:

For the principle of conservation of charge:

So the charge that ends up on sphere C is:

<h2>
22. What is the total charge on the three spheres before they are allowed to touch each other.</h2><h3>Answer:</h3>

<h3>Explanation:</h3>
Before they are allowed to touch each other we have that:

Therefore, for the principle of conservation of charge <em>the algebraic sum of all the electric charges in any closed system is constant, </em>then this can be expressed as:

Lastly, the total charge on the three spheres before they are allowed to touch each other is:
