Electric ForceChapter 17.2

Coulomb’s Law

Coulomb’s Constant

• The proportionality constant k in Coulomb’s law is similar to G in Newton’s law of gravitation.

• k = 9,000,000,000 N·m2/C2 or 9.0 × 109 N·m2/C2

• If a pair of charges of 1 C each were 1 m apart, the force of repulsion between the two charges would be 9 billion newton. • That would be more than 10 times the weight of a

battleship

Gravity vs. Electricity• Newton’s law of gravitation for masses is similar to Coulomb’s

law for electric charges.• Whereas the gravitational force of attraction between a pair of

one-kilogram masses is extremely small, the electrical force between a pair of one-coulomb charges is extremely large.

• The greatest difference between gravitation and electrical forces is that gravity only attracts but electrical forces may attract or repel.

Rules for Coulomb’s Law

• If the electrical force is negative, then it’s a attractive force.• If the electrical force is positive, then it’s an repulsive

force.• Like charges repel and opposite charges attract.

• Make sure that you enter all variables for Coulomb's Law in SI Units:• Charge – Coulomb’s (C)• Distance – Meters (m)• Force – Newton (N)

Example Problem• A charged sphere with an excess charge of +5.5μC is

placed 0.05m from another charged sphere, which carries a charge of -4.9μC.a) What is the magnitude of the electrical force

between the two charged spheres?b) Is this a repulsive or attractive force?c) How many excess electrons are on the negative

sphere?d) How many electrons has the positive sphere lost?

Example Answer A

What is the magnitude of the electrical force between the two charged spheres?

Example Answer B

Is this a repulsive or attractive force?

• Because the elective force came out to be a negative answer, it means that the force will be attractive.

• Opposites attractive, likes repel.

Attractive

Example Answer C

How many excess electrons are on the negative sphere?

-1C = 6.2x1018 Electrons

Example Answer D

How many electrons has the positive sphere lost?

-1C = 6.2x1018 Electrons

Electric Force is a Field Force• The electric force, just like the gravitational force, is a field

force.• The force created by any charge can be measured at any

radius around the sphere where the charge is at the center.• You can sense the force field that surrounds a charged Van de

Graaff generator.

Principle of Superposition• Superposition – The resultant force on any single charge

equals the vector sum of the individual forces exerted on that charge by all the other individual charges that are present.

• The method of finding the total force acting on a single charged particle is vector addition.

• Solve for the total sum of the electrical forces in the x- and y-direction.

• Then use the Pythagorean theorem and the tan function to find the magnitude and direction of the resulting force.

• The positive and negative sign for each force just means if it is a repulsive or an attractive force. It doesn't not mean left or right.

• It is best to draw a picture of the problem when solving.