1. Wed. Jan. 21 – Physics Lecture #21 Electric Interactions 1. Charge & Matter 2. Coulomb’s Law 3. Unit Vectors and Vector Math

2. ConceptCheck: You rub a rubber balloon with wool, transferring electron from the wool onto the balloon. You bring your charged balloon near a second balloon. You notice that the second balloon is attracted to the first balloon. Which of the following is correct about the sign of the net charge on the second balloon? 1. Must be positive 2. Must be negative 3. Must have no net charge 4. Could be positive or have no net charge 5. Could be negative or have no net charge 6. Could be positive, negative or have no net charge

3. i ii iii ConceptCheck: Two spheres can be held and moved around by insulated handles; the spheres start in contact with no net charge. Then, (i) a negative charge is brought near the spheres; (ii) the spheres are separated; (iii) the charge is removed. Consider the following choices: +– 0 (no net charge) 1. Not enough information a)After the three steps, what would be the net charge on the right-most sphere if the two spheres were insulators? b)After the three steps, what would be the net charge on the right-most sphere if the two spheres were conductors?

4. Questions from lab…

5. ConceptCheck: Two small spheres, each with uniformly distributed net charge +Q, have their centers a distance r apart, as shown, and exert a force of magnitude F on each other. The charge on the left is increased so it now has net charge +3Q and the distance between the charges is doubled to 2r. a) What is the magnitude of the force on the +Q charge now? 1. F/24. 4F/3 2. 3F/45. 3F/2 3. F6. 2F b) What is the magnitude of the force on the +3Q charge now? ? +Q+Q+Q+Q r

6. ConceptCheck: An electron is a distance a 0 away from a proton, as shown. What direction is the electron moving? 1. Not moving 2. Up (towards top of screen) 3. Down (towards bottom of screen) 4. Left 5. Right 6. Not enough information

7. ConceptCheck. Consider two point particles lying fixed along the x axis separated by a distance d. These particles have charges that are equal in magnitude but opposite in sign, with the negative particle on the left. Where can a third positive point charge be placed so the net electrical force on this third charge is zero? 1.Halfway between the first two charges. 2.Some point on the x axis between the first two charges, but not exactly halfway. 3.Some point on the x axis to the left of the negative charge. 4.Some place on the x axis to the right of the positive charge. 5.A distance d from the first two charges (so the three charges form an equilateral triangle). 6.There is no point where the net force would be zero.

8. In-Class Exercise: a)Determine the magnitude of the electrical force of charge 1 on charge 3. b)Determine the unit vector pointing from charge 1 to charge 3. c)Determine the electrical force of charge 1 on charge 3. d)Determine the net electrical force acting on charge 3.