Two charges, one +Q and the other −Q, are held a distance d apart. Consider only points on the line passing through both charges and clearly explain your answers to the following: [You can answer this problem without any calculations]. Do not consider any points at infinite distance from the charges. [5 points](a) Find the location of all points, if any, where the electric potential is zero.(b) Find the location of all points, if any, where the electric field is zero.

a. d/2 mid-way between the charges.

b. d/2 mid-way between the charges.

Explanation:

(a) Find the location of all points, if any, where the electric potential is zero.

Since the charges are of equal magnitude and opposite charge and separated by a distance, d, the electric potential due to the +Q charge is V = kQ/x and that due to the -Q charge is V' = -kQ/(d - x) where x is the point of zero electric potential.

The potential is zero when  V + V' = 0, and this can only be midway between the charges. This is shown below

So, kQ/x + [-kQ/(d - x)] = 0

kQ/x - kQ/(d - x) = 0

kQ/x = kQ/(d - x)

1/x = 1/(d - x)

(d - x) = x

d = x + x

d = 2x

x = d/2 which is mid-way between the charges.

(b) Find the location of all points, if any, where the electric field is zero.

Since the charges are of equal magnitude and opposite charge and separated by a distance, d, the electric field due to the +Q charge is E = kQ/x² and that due to the -Q charge is E' = -kQ/(d - x)² where x is the point of zero electric field.

The electric field is zero when  E + E' = 0 and this can only be midway between the charges. This is shown below.

So, kQ/x² + [-kQ/(d - x)²] = 0

kQ/x² - kQ/(d - x)² = 0

kQ/x² = kQ/(d - x)²

1/x² = 1/(d - x)²

(d - x)² = x²

d - x = ± x

d = x ± x

d = x - x or x + x

d = 0 or 2x

d = 0 or d = 2x

Since d ≠ 0, d = 2x ⇒ x = d/2 which is midway between the charges.

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