Coulomb's Law Date _______________
When two charged objects are near each other they may experience an acceleration either towards or away from each other because each object exerts a force on the other object.
This force is called the electric force. The unit of force is Newton (N).
In the 1780's Charles Coulomb performed experiments to determine the magnitude of the electric force between two objects.
Coulomb found that the electric force between two charged objects is proportional to the product of the two charges (q1 and q2). He also found that the electric force is inversely proportional to the square of the distance (r) between the two objects. Coulomb's constant (kc) is 8.99 x 109 Nm2/C2.
1
As the charges on the objects increase, the electric force between the objects ________________________.
As the distance between the objects increases, the electric force between objects, decreases by a factor of 1/r2.
If the distance doubles, the electric force decreases by ______________.
If the distance triples, the electric force decreases by ________________.
2
Sample Problem
Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of +12 x 10­9 C and the other is given a charge of ­18 x 10­9 C. What is the electric force exerted on one sphere by the other? (Coulomb's constant, kc = 8.99 x 109 Nm2/C2)
3
How does the electric force compare to the gravitational force?
Electric Force
Gravitational Force
Force is exerted by ___________ particles.
Force is exerted by anything with _______.
The electric force between two
particles is relatively large.
The gravitational force between two
particles is extremely small.
Force is only _______________.
Force may be ______________ or
_______________.
4
Calculating Gravitational and Electric Forces
The electrons and protons of a hydrogen atom are separated, on average, by a distance of about 5.3 x 10­11 m. Find the magnitudes of the electric force and the gravitational force that each
particle exerts on the other.
Mass of electron, me = 9.109 x 10­31 kg
Charge of electron, qe = ­1.60 x 10­19 C
Mass of proton, mp = 1.673 x 10­27 kg
Charge of proton,qp = 1.6 x 10­19 C
Universal gravitation constant, G = 6.673 x 10­11 Nm2/kg2
Coulomb's constant, kc = 8.99 x 109 Nm2/C2
Gravitational force, Fg = G memp
Electric force, Felectric = kc q1q2
2
r
r2
r = 5.3 x 10­11 m
5
Solving the problem
Substitute the values into the equations and solve.
Gravitational force
6
Electric force
7
The electric force is significantly stronger than the gravitational force. Although we feel our attraction to Earth by gravity, we do not usually feel the effects of the electric force. Explain why.
An ordinary nickel contains about 1024 electrons, all repelling each other. Why don't these
electrons fly off the nickel?
8
Practice Problems
1) Two charges of + 60 x 10 ­9 C and + 50 x 10 ­9 C are separated by 0.39 m. Calculate the repulsive electric force.
9
2) What is the electric force between a glass ball that has + 2.5 x 10­9 C of charge and a rubber ball that has ­ 5.0 x 10­9 C of charge when they are separated by a distance of 0.05 m? Is this force attractive or repulsive?
10