Everything Science Grade 11

CHAPTER 17. ELECTROSTATICS 17.2

where m 1 and m 2 are the masses of the two particles, r is the distance betweenthem, and G is the
gravitational constant.

Both laws represent theforce exerted by particles (masses or charges) oneach other that interactby
means of a field.
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Example 1: Coulomb’s Law I

QUESTION

Two point-like charges carrying charges of +3× 10 −^9 C and− 5 × 10 −^9 C are 2m apart.

Determine the magnitude of the force between them and state whetherit is attractive or

repulsive.

SOLUTION

Step 1 : Determine what is required

We are required to findthe force between two point charges given the charges

and the distance between them.

Step 2 : Determine how to approach the problem

We can use Coulomb’sLaw to find the force.

F = k

Q 1 Q 2

r^2

Step 3 : Determine what is given

We are given:

• Q 1 = +3× 10 −^9 C


• Q 2 =− 5 × 10 −^9 C


• r = 2m
  We know that k = 8, 99 × 109 N· m^2 · C−^2.
  We can draw a diagramof the situation.

Q 1 = +3× 10 −^9 C Q 2 =− 5 × 10 −^9 C

� �

2 m

Step 4 : Check units

All quantities are in SI units.

Step 5 : Determine the magnitude of the force

Using Coulomb’s Law we have

F = k

Q 1 Q 2

r^2

= (8, 99 × 109 N· m^2 /C^2 )

(3× 10 −^9 C)(5× 10 −^9 C)

(2m)^2

= 3, 37 × 10 −^8 N