1. Electric Forces and Fields Chapter 20

2. Charges and Forces Experiment 1 Nothing happens Nothing happens The objects are neutral The objects are neutral

3. Charges and Forces Experiment 2 The two rods repel each other The two rods repel each other A rubbed rod is charged A rubbed rod is charged Long range repulsive force Long range repulsive force

4. Charges and Forces Experiment 3 These rods attract each other These rods attract each other Positive and negative charges Positive and negative charges

5. Charges and Forces Experiment 4 When two rods are rubbed more vigorously the strength of the forces is greater When two rods are rubbed more vigorously the strength of the forces is greater The strength of the The strength of the charges decreases charges decreases as the distance as the distance between the rods between the rods increases increases

6. Charges and Forces Experiment 5 Rub a plastic rod with wool Rub a plastic rod with wool The rod is weakly attracted to the wool The rod is weakly attracted to the wool The rod is repelled by a piece of silk that has been used to The rod is repelled by a piece of silk that has been used to rub glass rub glass

7. Charges and Forces Experiment 6 Charged plastic rod held over paper Charged plastic rod held over paper The neutral paper is attracted to the rod The neutral paper is attracted to the rod A charged glass rod attracts the paper A charged glass rod attracts the paper A neutral rod has no effect on the paper A neutral rod has no effect on the paper

8. Charge Separation A charged object can induce a charge separation on an uncharged object that can lead to an attractive force. http://phet.colorado.edu/simulations/sims.p hp?sim=Balloons_and_Static_Electricity A charged object can induce a charge separation on an uncharged object that can lead to an attractive force. http://phet.colorado.edu/simulations/sims.p hp?sim=Balloons_and_Static_Electricity http://phet.colorado.edu/simulations/sims.p hp?sim=Balloons_and_Static_Electricity http://phet.colorado.edu/simulations/sims.p hp?sim=Balloons_and_Static_Electricity

9. Insulators and Conductors How does charge move on different materials? Charge a plastic rod by rubbing it with wool Charge a plastic rod by rubbing it with wool Touch a neutral metal sphere Touch a neutral metal sphere The sphere acquires the charge of the rod The sphere acquires the charge of the rod

10. Insulators and Conductors How does charge move on different materials? The metal sphere that is touched by the charged plastic rod will pick up small pieces of paper (it is charged) The metal sphere that is touched by the charged plastic rod will pick up small pieces of paper (it is charged) The other sphere will not (it remains neutral) The other sphere will not (it remains neutral)

11. Insulators and Conductors How does charge move on different materials? Touch one sphere with a charged plastic rod Touch one sphere with a charged plastic rod Both spheres will attract small bits of paper (they are charged) Both spheres will attract small bits of paper (they are charged)

12. Transfer of Charge Charge can be transferred from one object to another when the objects touch. Charge can be transferred from one object to another when the objects touch. Removing charge from an object is called discharging Removing charge from an object is called discharging Charge is conserved (it can’t be created or destroyed Charge is conserved (it can’t be created or destroyed

13. Transfer of charge Conductors – material through or along which charge easily moves Conductors – material through or along which charge easily moves Metal Metal Insulators – materials on or in which charges remain immobile Insulators – materials on or in which charges remain immobile Glass and plastic Glass and plastic Both insulators and conductors can be charged. They differ in the mobility of the charge Both insulators and conductors can be charged. They differ in the mobility of the charge

14. Charge diagram An insulating rod is charged by rubbing An insulating rod is charged by rubbing Charges on the insulator rod don’t move Charges on the insulator rod don’t move

15. Charge diagram Charges in a conductor are free to move Charges in a conductor are free to move

16. Electrostatic Equilibrium The charges on an isolated conductor are in static equilibrium other than the brief interval when the charges are adjusting The charges on an isolated conductor are in static equilibrium other than the brief interval when the charges are adjusting The charges are at rest The charges are at rest No net force on any charge No net force on any charge

17. Electroscope Charge polarization The charges move around but The charges move around but Charge is conserved Charge is conserved Slight separation of Slight separation of the positive and the positive and negative charge in a negative charge in a neutral object neutral object

18. Polarization Force The polarization force arises because the charges in the metal are separated NOT because the rod and metal are oppositely charges The polarization force arises because the charges in the metal are separated NOT because the rod and metal are oppositely charges

19. Fundamental Charge Charge is represented by the symbol q Charge is represented by the symbol q The SI unit of charge is the coulomb (C) The SI unit of charge is the coulomb (C) Protons and electrons have the same amount of charge but opposite signs Protons and electrons have the same amount of charge but opposite signs The fundamental or elementary charge (e) is the magnitude of the charge of a proton or electron The fundamental or elementary charge (e) is the magnitude of the charge of a proton or electron e = 1.60 X 10 -19 C e = 1.60 X 10 -19 C

20. Fundamental Charge Charge is conserved Charge is conserved The total amount of charge remains constant The total amount of charge remains constant

21. Coulomb’s Law Electric force increases for object’s with more charge and decreases as charged objects are moved farther apart. Electric force increases for object’s with more charge and decreases as charged objects are moved farther apart.

22. Coulomb’s Law Equation K is the electrostatic constant K is the electrostatic constant K = 8.99 x10 9 N m 2 /C 2 K = 8.99 x10 9 N m 2 /C 2 C are units of electric charge called coulombs C are units of electric charge called coulombs r is the distance the charges are apart r is the distance the charges are apart

23. Direction of forces The forces are directed along the line joining the two particles The forces are directed along the line joining the two particles The forces are repulsive for two like charges The forces are repulsive for two like charges The forces are attractive for two opposite charges The forces are attractive for two opposite charges These forces are an action/reaction pair equal in magnitude but opposite in direction. These forces are an action/reaction pair equal in magnitude but opposite in direction.

24. Forces are Vectors Electric forces are represented by vectors Electric forces are represented by vectors Like other forces they can be superimposed Like other forces they can be superimposed If multiple charges are acting on charge j, the net electric force on charge j is the sum of all the individual forces due to each charge. If multiple charges are acting on charge j, the net electric force on charge j is the sum of all the individual forces due to each charge. F net = F 1onj + F 2on j + F 3onj +… F net = F 1onj + F 2on j + F 3onj +…

25. Adding Electric Forces in 1 dimension Example 20.1 Example 20.1 A. Two +10nCcharged particles are 2.0 cm apart on the x-axis. What is the net force on a A. Two +10nCcharged particles are 2.0 cm apart on the x-axis. What is the net force on a +1.0 nC charge midway between them? +1.0 nC charge midway between them? F net =F 1on3 + F 2on3 F net =F 1on3 + F 2on3 F net = 0 F net = 0 q 1 and q 2 exert repulsive forces on q 3. Equal in magnitude, opposite in direction q 1 and q 2 exert repulsive forces on q 3. Equal in magnitude, opposite in direction

26. Adding Electric Forces in 1 dimension 20.1 b. What is the net force if the charged particle on the right is replaced by a -10 nC charge? 20.1 b. What is the net force if the charged particle on the right is replaced by a -10 nC charge? Forces are equal in magnitude and direction Forces are equal in magnitude and direction F net = 2F 1on3 F net = 2F 1on3

27. Adding Electric Forces in 1 dimension 20.1 B. The magnitude is given by Coulomb’s Law 20.1 B. The magnitude is given by Coulomb’s Law F 1on3 = 9.0 x 10 -4 N F 1on3 = 9.0 x 10 -4 N F net = 2(F 1on3 ) = F net = 2(F 1on3 ) = 1.8 X 10 -3 N to the right 1.8 X 10 -3 N to the right

28. Adding Electric forces in 2 dimension Example 20.2 Example 20.2 Three charges particle with q 1 = -50nC, q 2 = +50nC, and q 3 = +30nC are placed as shown. What is the net force on charge q 3 due to the other 2 charges Three charges particle with q 1 = -50nC, q 2 = +50nC, and q 3 = +30nC are placed as shown. What is the net force on charge q 3 due to the other 2 charges

29. Adding Electric forces in 2 dimension Define a coordinate system with q 3 at the origin. Define a coordinate system with q 3 at the origin. Draw the forces on the charge q 3 with direction determined by the signs of the charges. Draw the forces on the charge q 3 with direction determined by the signs of the charges.

30. Adding Electric forces in 2 dimension

31. Determine the net force on charge q3 using Coulomb’s law: Determine the net force on charge q3 using Coulomb’s law: =2.7 x 10 -3 N =2.7 x 10 -3 N

32. Adding Electric forces in 2 dimension Determine the net force on charge q 3 using Coulomb’s law: Determine the net force on charge q 3 using Coulomb’s law: Magnitude and distance are the same for F 2on3 Magnitude and distance are the same for F 2on3 =2.7 x 10 -3 N =2.7 x 10 -3 N

33. Adding Electric forces in 2 dimension Compute values for the components using trig. Compute values for the components using trig. (F 1on3 ) x =-1.9x10 -3 N (F 1on3 ) x =-1.9x10 -3 N (F 1on3 ) y =-1.9x10 -3 N (F 1on3 ) y =-1.9x10 -3 N (F 2on3 ) x =-1.9x10 -3 N (F 2on3 ) x =-1.9x10 -3 N (F 2on3 ) y =-1.9x10 -3 N (F 2on3 ) y =-1.9x10 -3 N

34. Adding Electric forces in 2 dimension Next add the components of the net force Next add the components of the net force F 3x =F (1on3)x + F (2on 3)x = F 3x =F (1on3)x + F (2on 3)x = -1.9 X 10 -3 N -1.9 x 10 -3 N = 3.8 x10 -3 N -1.9 X 10 -3 N -1.9 x 10 -3 N = 3.8 x10 -3 N F 3y = F (1on3)y + F (2on3)y = F 3y = F (1on3)y + F (2on3)y = +1.9 X 10 -3 N -1.9 x 10 -3 N = 0 +1.9 X 10 -3 N -1.9 x 10 -3 N = 0 Net force is F3 = Net force is F3 = 3.8 x10 -3 N, -x direction 3.8 x10 -3 N, -x direction

35. The Electric Field Grass seed in a pan of oil Grass seed in a pan of oil When charged spheres When charged spheres (+ and -) touch the surface, (+ and -) touch the surface, the grass seeds line up the grass seeds line up in a regular pattern. in a regular pattern. The pattern suggests that some kind of electric influence fills the space around the charges. This alteration of space could be the mechanism by which the long range Coulomb’s Law force is exerted.

36. The Field Model The alteration of space around charge is the agent that exerts a force on charge B The alteration of space around charge is the agent that exerts a force on charge B This alteration of space is called a field This alteration of space is called a field The charges make an alteration everywhere in space The charges make an alteration everywhere in space Other charges then respond to those alteration at their position Other charges then respond to those alteration at their position

37. The Field Model

38. The field model applies to many branches of science The field model applies to many branches of science Electric field-alteration of space around a charge Electric field-alteration of space around a charge Gravitational Field-alteration of space around a mass Gravitational Field-alteration of space around a mass Magnetic Field-alteration of space around a magnet Magnetic Field-alteration of space around a magnet

39. Field Model Source Charges alter the space around them by creating an electric field E. Source Charges alter the space around them by creating an electric field E. A separate charge in the electric field then experiences a force F exerted by the field A separate charge in the electric field then experiences a force F exerted by the field

40. Electric Field Diagram