Electric charge Forces between charged objects

1. Chapter 20 Electric Forces and Fields • Electric charge • Forces between charged objects Topics: Sample question: In electrophoresis, what force causes DNA fragments to migrate through the gel? How can an investigator adjust the migration rate? Slide 20-1

2. Checking Understanding Two spheres are touching each other. A charged rod is brought near. The spheres are then separated, and the rod is taken away. In the first case, the spheres are aligned with the rod, in the second case, they are perpendicular. After the charged rod is removed, which of the spheres is: i) Positive ii) Negative iii) Neutral Pick only one per poll Slide 20-13

3. Charge Model • Electric forces can be attractive or repulsive • There are two kinds of charges, positive (protons) and negative (electrons). In solids, electrons are charge carriers (protons are 2000 time more massive). • A charged object has a deficit of electrons (+) or a surplus of electrons (-). Neutral objects have equal numbers of + and - charges • Objects with the same sign of charge repel each other • Objects with the opposite sign of charge attract each other • Neutral objects are polarized by charged objects which creates attractive forces between them Slide 20-3

4. Electric Charges and Forces, Part III Slide 20-10

5. Charging an object • Rubbinghttp://phet.colorado.edu/simulations/index.php?cat=Electricity_Magnets_and_Circuits • Conduction (see previous slide) • Induction • Electroscope example • Charging two spheres exampleYou have two conducting spheres. How can you charge them with opposite charges without touching either one with a charged object? (Anything else is fair game) http://panda.unm.edu/Courses/Saul/Physics2/00-Charge/3-InducedChargeSpheres/Induction-2Spheres.html Slide 20-3

6. Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0e-8 C is placed on sphere A on the left and a charge of 6.0e-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. • Draw Free-Body Diagrams for the pucks and spheres • How do the Coulomb forces acting on spheres A & B compare? (Use a ratio) • Which sphere has the greater acceleration? How would your answer change if the mass of the puck under sphere A was reduced by 50%? Slide 20-3

7. Charged Spheres & Forces Two identical metal spheres are firmly fastened to and electrically insulated from frictionless plastic air pucks that ride on an air table as shown below. The pucks are held in place as a charge of 2.0e-8 C is placed on sphere A on the left and a charge of 6.0e-6 C is placed on sphere B on the right. The pucks are then released so that the pucks with the spheres attached are now free to move without across the table. • As the two spheres get farther away from one another, how would (if at all) the following quantities change?
 1) Force
 2) Speed 3) Acceleration Choices: • Increase • Decrease • Stay the same • Can’t tell Slide 20-3

8. The Charge Model Slide 20-11

9. Visualizing Charge • Charges on an insulator do not move. • Charges on a conductor adjust until there is no net force on any charge. We call this electrostatic equilibrium. Slide 20-12

10. Examples If a charged plastic rod is brought near an uncharged metal rod on an insulating stand, an uncharged metal ball near the other end of the metal rod is attracted to this end of the rod. Explain the motions of charges that give rise to this force. Follow-up: Describe a procedure by which you could give two identical metal spheres exactly equal charges. Slide 20-14

11. How does an electroscope work? Three cases • When you put charge on a neutral electroscope • When you bring a charged object near a neutral electroscope • When you bring a charged object near a charged electroscope Slide 20-3

12. Coulomb’s Law Slide 20-15

13. Checking Understanding A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the largest? Slide 20-16

14. Answer A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the largest? Slide 20-17

15. Two 0.10 g honeybees each acquire a charge of +23 pC as they fly back to their hive. As they approach the hive entrance, they are 1.0 cm apart. What is the magnitude of the repulsive force between the two bees? How does this force compare with their weight? Slide 20-29

16. Checking Understanding A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the smallest? Slide 20-18

17. Answer A small, positive charge is placed at the black dot. In which case is the force on the small, positive charge the smallest? Slide 20-19

18. Checking Understanding All charges in the diagrams below are of equal magnitude. In each case, a small, positive charge is placed at the black dot.In which case is the force on the small, positive charge the smallest? Slide 20-27

19. Answer All charges in the diagrams below are of equal magnitude. In each case, a small, positive charge is placed at the black dot.In which case is the force on the small, positive charge the smallest? Slide 20-28