Electric Forces and Electric Fields

Coulomb’s Law F = k C q 1 q 2 ______ r 2 A coulomb (C) is a lot of charge. Most objects can hold about a microCoulomb of charge. Suppose the circles above represent balloons. Find the force F 12, the force from charge 1 on charge 2. Use only the magnitude of the charge on q, not the sign. Determine the direction of the force based on the picture. q 1 = -4.0 μCq 2 = = -1.5 μC -- r = 0.03 m r

Coulomb’s Law: Ex. 1 F 12 = k C q 1 q 2 = ______ r 2 Use only the magnitude of the charge on q, not the sign. Determine the direction of the force based on the picture. q 1 = -4.0 μCq 2 = = -1.5 μC -- r = 0.03 m r F 12 is the force from q 1 on q 2

Coulomb’s Law: Ex 2 F 12 = k C q 1 q 2 = ______ r 2 Use only the magnitude of the charge on q, not the sign. Determine the direction of the force based on the picture. q 1 = -4.0 μCq 2 = = -1.5 μC -- r = 0.03 m r What is F 21 ?

Coulomb’s Law: Ex 3 F 12 = Use only the magnitude of the charge on q, not the sign. Determine the direction of the force based on the picture. q 1 = 3.2 x Cq 2 = = -1.6 x C - r = m r What is F 12 ? + +

Electric Field, E The electric field is the space around an electrical charge just like a gravitational field is the space around a mass.

Electric Field Arrows point in the direction a positive “test charge” (charge with small mass) would move.

Electric Field Lines Lines indicate the strength and direction of the electric field. The more dense the lines, the stronger the field.

Electric Field Lines of two Positive Charges

Electric Field Lines of three charges

Coulomb’s Law, revisited F = k q Q ______ r 2 q = test charge, Q = central charge What if we want to know how strong the force field is, unrelated to test charge q?

Electric Field Vector, E E = F/ q (Electric field = force per charge) E = k Q ______ r 2 Unit: N/C E is analogous to gravitational field, g. Remember g = GM/r 2 (units: N/kg)

Example 1 A charge of 3µC (q) is used to test the electric field of a central charge of +6C (Q) that causes a force of 800N. What is the magnitude of the electric field? the direction? Answer: 2.7 x10 8 N/C, away from Q

Example 2 A test charge of +3µC is located 5m to the east of a -4C charge. (draw picture) A) Find the electric force felt by the test charge. B) Find the electric field at that location. Answer: 4.32x10 3 N west, 1.44 x 10 9 N/C west.

Example 3 If a test charge is moved to a location three times as far as its original location, how does the magnitude of the electric field change? If a satellite is moved to a location three times as far as its original location, how does the magnitude of the gravitational field change?

Electric Field inside a conductor The electric field is zero inside a charged conductor. Excess charge on an isolated conductor resides on the surface. Excess charge accumulates on sharp points. Practical application: Electric field lines meet the conductor perpendicular to the surface of the conductor.

Thought questions 1. Where are you safer during a thunderstorm and why? A) In a car or B) Outdoors 2. Why can you not get radio reception in a tunnel or in a steel bridge?