Electric Force Unit 7.3

Coulomb’s Law We previously learned that a force is defined as the cause of a change in motion.

Coulomb’s Law Because two charged objects near one another may experience motion, either toward or away from each other, each object exerts a force on the other object called electric force.

Coulomb’s Law If you rub a balloon against your hair, you will find that the closer the balloon is to your hair, the stronger the attraction.

Coulomb’s Law Likewise, the repulsion between two charged balloons becomes stronger as the distance between the balloons decreases.

Coulomb’s Law The distance between the two objects affects the magnitude of the electric force between them.

Coulomb’s Law It also seems to follow that the amount of charge on the objects will also affect the magnitude of the electric force.

Coulomb’s Law In the 1780’s, Charles Coulomb conducted a variety of experiments in an attempt to determine the magnitude of the electric force between two charged objects.

Coulomb’s Law Coulomb found that the electric force between two charges is proportional to the product of the two charges.

Coulomb’s Law He also found that the electric forces are inversely proportional to the square of the distance between the charges.

Coulomb’s Law F electric = kC q1q2 r2 The following equation, known as Coulomb’s Law, expresses these conclusions. F electric = kC q1q2 r2 Electric force = Coulombs x charge 1 charge 2 (distance)2

kC is called Coulomb’s constant. It has a value of 8.99 * 109 Nm2 C2 Coulomb’s Law kC is called Coulomb’s constant. It has a value of 8.99 * 109 Nm2 C2

Coulomb’s Law When dealing with Coulomb’s law, remember that force is a vector quantity. The electric force between two objects always acts along the line between the objects.

Coulomb’s Law Also note that Coulomb’s law applies exactly only to point charges or particles and to spherical distributions of charge.

Coulomb’s Law When applying Coulomb’s law to spherical distributions of charge, use the distance between the center of the spheres as r.

Coulomb’s Law The Coulomb force is the second example we have studied of a force that is exerted by one object on another even though there is no physical contact between the two objects. This is a field force.

Coulomb’s Law Electric force can be compared to gravitational forces since both are field forces and both forces are inversely proportional to the square of the distance of separation.

Coulomb’s Law However, there are some differences between electric and gravitational forces.

Coulomb’s Law First of all, electric forces can be either attractive or repulsive. Gravitational forces are always attractive.

Coulomb’s Law This is because objects can have either positive or negative charge while mass is always positive.

Coulomb’s Law Another difference between gravitational and electric force is their relative strength. The electric force between charged atomic particles is much stronger than their gravitational attraction to Earth.

Coulomb’s Law Frequently, more than two charges are present and it is necessary to find the net electric force on one of them.

Coulomb’s Law The resultant force on any single charge equals the vector sum of all of the individual forces exerted on that charge, which is the principle of superposition.

Coulomb’s Law Once the magnitudes of the individual electric forces are found by Coulomb’s law, they are added together exactly like forces.

Coulomb’s Law Remember objects that are at rest are said to be in equilibrium. According to Newton’s first law, the net external force acting on a body in equilibrium must be zero.

Coulomb’s Law In electrostatic situations, the equilibrium position of a charge is the location at which the net electric force on a charge is zero.

Coulomb’s Law To find this location, you must find the position at which the electric force from one charge is equal and opposite the electric force from another charge.

Coulomb’s Law This can be done by setting the forces equal and then solving for the distance between either charge and the equilibrium position.

Coulomb’s Law Coulomb measured electric forces between charged objects with a torsion balance.

Coulomb’s Law A torsion balance consists of two small spheres fixed to the ends of a light horizontal rod. The rod is made up of an insulating material and is suspended by a silk thread.

Coulomb’s Law Coulomb was able to get the rod to rotate due to increasing the charges. By getting the angular displacement, a quantitative measure of the electric force was attained.

Coulomb’s Law He was able to establish the equation for electric force. More recent experiments have verified these results to within a small degree of uncertain numbers.