1. Physics 30 Unit 2 Review Electrical Forces and Fields

2. Electrostatics: Like charges _______ and unlike charges __________. Law of Charges: Movement of Charges: Only _________ move within insulators and conductors. Grounding: The _______ is both an unlimited depository or source of ___________. Electrical Forces & Fields repel attract electrons earth electrons end

3. Charging by Friction: Rubbing two ___________ together will force the movement of ________ from one object into the other. Charging by Conduction: Touching two charged objects will cause a ___________ of charge. Charging by Induction: Causing a ____________ ____within an object without ___________ it. insulators electrons transfer charge separation touching end

4. There are three equal sized spheres. Sphere A has a charge of +2.0 mC, sphere B has a charge of -4.0 mC, and sphere C has a charge of +8.0 mC. If sphere C touches A, then touches B, what are the charges on all spheres? Examples: A 2.0 B -4.0 C 8.0 C 5.0 A C 0.5 B A 5.0 B 0.5 C end

5. Explain how to charge an electroscope negatively through induction. You have a rod that when rubbed with fur becomes positively charged. Examples: Ground the top of the electroscope. Rub the rod with fur so it becomes positively charged through friction. Since opposite charges attract (law of charges), negative electrons will flow from the ground into the electroscope. Disconnect the ground wire. Remove the positively charged object. The electroscope is now negatively charged. Bring a positively charged object near the electroscope but do not touch it. end

6. Electric Field: Is the direction a small, __________ test charge will travel when placed in the field. - Drawing Electric Fields: + positive end

7. Drawing Electric Fields around charged objects: end

10. - + ?

11. Drawing Electric Fields between charged plates: +- Electric Fields inside a spherical conductor is _______________ zero Electric field is constant between the plates so long as the distance between them doesn’t change. end

12. Charges flow along the outside of a conductor. end If there’s an imperfection along the surface of the sphere, that location will have ______________ charge. more

13. Electric Field Formulas: Electric Field has this notation: _______________ Energy has this notation: _______________ Do not use +/- notation in formulas. Use the direction of a small ____________ test charge to find direction. E E positive Electric field between electric plates. Units of N/C or V/m Electric field is used to find the force a charged object experiences when placed in the electric field. Units of N/C Electric field a distance from a charged object. Units of N/C end

14. Calculate the electric field 2.55 x 10 -3 m away from a 3.82  C object. Examples: If a 1.12  C charged sphere is brought to a distance of 2.55 x 10 -3 m away from the object in the last question, what electrical force does it experience? = (8.99x10 9 )(3.82x10 -6 C) (2.55x10 -3 ) 2 = 5.28x10 9 N/C = (5.28x10 9 )(1.12x10 -6 C)5.92x10 3 N = If the 1.12  C charged sphere has a mass of 3.00 g, what acceleration will it experience in the electric field? = 5.92x10 3 N 3.00x10 -3 kg = 1.97x10 6 m/s 2 end

15. Calculate the electric field between the plates below. Draw the direction of the electric field as well. Examples: If an electron is placed in between the plates, what electrical force does it experience? What is the direction of this force? + - 9 V separated by 2.50 cm The direction is up towards the positive plate. end

16. Calculate the electric field midpoint between two point charges of 4.00  C and 3.25  C that are 6.84 x 10 -1 m apart. Examples: 4.03.25 0.342 m The electric fields point in opposite directions so subtract. to the right end

17. Calculate the electric field at point “p” Examples: +1.66  C -2.82  C p 4.00 cm 3.00 cm end

18. +1.66  C -2.82  C p 4.00 cm 3.00 cm Combine the electric fields using pythagorean theorem. end

19. E of S end

20. Electrical Energy: Electric charges have electrical ____________ energy when placed near other charged _________ or between __________ plates. Electric potential is also referred to as __________. potential objectscharged voltage Unlike electric field, the ___________ experienced by a charge does change as it moves between electric plates. voltage end

21. Potential Difference: end There is 100 V between the plates. What is the potential difference experienced by the positive particle in locations 1, 2, 3 and 4? + plate - plate +1 4 2 3 0 V 50 V 100 V 75 V

22. Potential Difference: end There is 100 V between the plates. What is the potential difference experienced by the negative particle in locations 1, 2, and 3? + plate - plate -1 3 2 100 V 0 V 25 V

23. A proton is moved through two parallel plates that are 5.00 cm apart and have a potential difference of 90.0 V between them. How much work is done against the electric field when the proton moves 3.00 cm parallel to the plates? Examples: + - The proton is not moved through a potential difference. No work is done against the electric field. + end What other force could be present to ‘make’ the electron follow a straight path between the plates? A magnetic force. What direction would the field need to be oriented in?

24. A proton is moved through two parallel plates that are 5.00 cm apart and have a potential difference of 90.0 V between them. How much work is done against the field when the proton moves 3.00 cm towards the positive plate? Examples: + - + 3.00 cm Method 1 The proton is moved through 3/5 of the potential difference. V=(3/5) (90.0 V) = 54 V end

25. + - + 3.00 cm Method 2 The electric field is constant between the plates. end

26. 3.50x10 3 V 1.50x10 3 V Calculate the electric potential difference gained by the moving charge. + Though the electric field remains constant, the charged object does experience a change in voltage. end

27. - Calculate the speed of the 3.00x10 -12 C and 4.00x10 -15 kg charge as it reaches the opposite plate if it started from rest.. 3.50x10 3 V end

28. Calculate the vertical displacement of the electron as it travels between the plates if the plates are 9.00 cm long. Examples: + - 90 V separated by 6.50 cm e- 7.00 X 10 6 m/s end 9.00 cm

29. + - e- 7.00 X 10 6 m/s 9.00 cm HorizontallyVertically 0 end

30. How many electrons travel through a wire in 0.100 s if the current is 3.00 A ? Examples: end

31. Coulomb’s Law: Coulomb used a __________ balance to measure the electric forces acting on between charged spheres so he could derive the following formula: His apparatus looked like: torsion end

32. Cavendish: Cavendish used a __________ balance to measure the gravitational forces acting on between charged spheres so he could derive the following formula (this was done before Coulomb’s exp): His apparatus looked like: torsion end

33. What is the electric force between two charges of 5.00  C and 3.00  C if they are separated by 2.00 cm? Examples: end

34. Examples: end

35. Two point charges produce an electric force of 6.00 x 10 -2 N. What will be the force between them if their charges are tripled and the distance between them halved? Examples: end

36. Find the net electric force on the middle sphere. Examples: -2.00  C3.00  C 4.00 cm end

37. Two equally charged, identical spheres are 0.100 m apart and have an electric force of 4.00 x 10 -2 N between them. What is the magnitude of the charge on each sphere? Examples: end

38. Find the net electric force on the middle sphere. Examples: -2.00  C3.00  C 4.00 cm end

39. Use pythagorean theorem to combine the forces. 33.7 end