CHAPTER 16. ELECTROSTATICS 16.4
sphere will have half of the total charge:Q = Q^1 + 2 Q^2=^9 ,^6 ×^10− (^18) +(− 9 , 6 × 10 − 18 )
2
= 0C
So each sphere is now neutral.
No net charge means that there is no excess of electrons or protons.
Example 7: Conservation of charge
QUESTION
Two identical, metal spheres have different charges. Sphere 1 has a charge of
− 9 , 6 × 10 −^18 C. Sphere 2 has 30 excess electrons. If the two spheres are brought
into contact and then separated, what charge will each have? How many electrons
does this correspond to?
SOLUTION
Step 1:Analyse the problem
We need to determine what will happen to the charge when the spheres
touch. They are metal spheres so we know they will be conductors.
This means that the charge is able to move so when they touch it is
possible for the charge on each sphere to change. We know that charge
will redistribute evenly across the two spheres because of the forces
between the charges. We need to know the charge on each sphere, we
have been given one.
Step 2:Identify the principles
This problem is similar to the earlier worked example. This time we
have to determine the total charge given a certain number of electrons.
We know that charge is quantised and that electrons carry the base unit
of charge which is− 1 , 6 × 10 −^19 C. The total charge will therefore be:
Q 2 = 30×− 1 , 6 × 10 −^19 C
= 4, 8 × 10 −^18 C
Physics: Electricity and Magnetism 271