1. Electric Force Chapter 17.2

2. Coulomb’s Law

3. 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

4. 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.

5. 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)

6. 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. • What is the magnitude of the electrical force between the two charged spheres? • Is this a repulsive or attractive force? • How many excess electrons are on the negative sphere? • How many electrons has the positive sphere lost?

7. Example Answer A What is the magnitude of the electrical force between the two charged spheres?

8. 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

9. Example Answer C How many excess electrons are on the negative sphere? -1C = 6.2x1018 Electrons

10. Example Answer D How many electrons has the positive sphere lost? -1C = 6.2x1018 Electrons

11. 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.

12. 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.