Force Fields

1. Force Fields • Gravitationalfields-act on masses-Strength ~ Mass / d2-attractive-cannot be shielded-can store energy-overall very weak force • Electricalfields-acts on charges-Strength ~ Charge & 1 / d2-attractive or repulsive-can be shielded-can store energy-generally stronger than gravity

2. Electric Field

3. Definition • Electric field is the strength & direction of the forces in space surrounding a positive test charge

4. Top tips on electric fieldField lines go from + to – chargesCloser field lines are strongerfield lines can’t cross another!

5. Question 1 • Suppose an electric field points south • Which way would a proton move:a. south • B. north • C. west • D. east.

6. Question 1 answer • Suppose an electric field points south • Which way would a proton move:A. south Since electric field shows direction of a positive charge from high to low energy

7. Question 2 • Suppose an electric field points south • Which way would an electron move:a. south • B. north • C. west • D. east.

8. Question 2 answer • Suppose an electric field points south • Which way would an electron move:B. north Since electron is a negative charge it moves opposite direction of field

9. Electric field as coulomb’s law • F = q1 * (k q2 /d2) rewrite the force equation F = q1 * E q1 is the positive test charge E is the field created by charge(s) q2

10. Question 3 • The electric field in a certain region of space is 40 N/C. A. What is the force on a -10 C charge placed in the region?B. Is it attractive or repulsive?

11. Question 3 Answer • F=q*E • F= (40 N/*C)(-10 C) • F = - 400 N(attractive force since negative answer, +*- = - )

12. Question 4 • Imagine a metal ball placed in an electric field • IF an electric field moves charges and conductors are full of electrons which can move, where will all the electrons go?a. stay on left side • B. spread out on outer surface • C. stay in middle • D. don’t move

13. Question 4 answer • Imagine a metal ball placed in an electric field • IF an electric field moves charges and conductors are full of electrons which can move, where will all the electrons go?B. spread out on outer surface since the charges move away from another to lower their energy • the charges on opposite sides cancel out forces so E=0 inside

14. Electric Shielding- Faraday Cage • Inside a Conductor, the Electric Field MUST be ZERO! • Any net charge on the inside of a conductor will be pushed to the surface & distributed to equilibrium! • Electric field E is perpendicular to the outer surface,and zero inside any conductor (metal)

15. Faraday cage • Don’t try this at home!

16. Electric field demos Pith balls inside the metal screen don’t move since there is no electric field inside a faraday cage Metal cage blocks out the electric field from the radio station or cell phone tower-so no music!

17. Common Faraday Cages Purpose: • Keep in or out electro-magnetic fields used in electronics/telecommunications Examples: • Metal foil around wires inside phones(so less pick-up of unwanted noise/calls) • Rebar in concrete or bridges- bad reception • Metal boxes around electronic trigger switches

18. coulomb Charles Coulomb~1750 • How many charges are in 1C? coulomb? • 1c is about 1 million charges! • The man, the law

19. Joules of energy • Charges have a type of energy called electron potential energy High energy: + is close to +: ++ or + is far from –: + -

20. Question 5 • Imagine moving a positive charge in an electric field • If you move the + charge to the left against the electric field, what happens to the energy of the + test charge?a. no change • B. increases • C. decreases • D. not enough info + E

21. Question 5 • Imagine moving a positive charge in an electric field • If you move the + charge to the left against the electric field, what happens to the energy of the + test charge?B. increases since field points from high to low energy + E

22. Question 6 • Imagine moving a positive charge against an electric field • If it takes 2000 Joules of work to move the charge..?a. how much energy does the charge gain? • B. how much work does the field do to move the charge back to the right? + E

23. Question 6 • Imagine moving a positive charge against an electric field • If it takes 2000 Joules of work to move the charge..?a. 2000 J----positive work needed to add energy? • B. -2000 J--- negative work done by the field, (field loses energy while charge gains) + E

24. Electric Potential (Voltage) • V= PE/Q • voltage= energy/Charge • Volts= joules/coulombs • Analogies:voltage is like water pressureenergy= heightwater= charge

25. Question 7:coulombs, joules, voltmatch these! • Coulomb unit of energy/charge • Joule unit of charge • Volt unit of energy

26. Question 7:coulombs, joules, voltmatch these! • Coulomb unit of charge • Joule unit of energy • Volt unit of energy per charge

27. Volts is an energy density • Voltage is also called potential • 1 volt = 1 joule / 1 coulomb • Example: 12 V battery: every coulomb of charge has 12 joules of energy

28. Example of high voltage, low charge • What’s the voltage on a balloon if it has… 1 joule of energy (medium) .001 coulombs of charge (low) voltage= energy/charge v = 1 J / .001C = 1000 Volts !

29. Question 8 • What’s the voltage on a Van de Graaf static generator 1 joule of energy (medium) .00001 coulombs of charge (v.low) v = ?

30. Question 8 • What’s the voltage on a Van de Graaf static generator 1 joule of energy (medium) .00001 coulombs of charge (v.low) v = 1 j/ .00001c • = 100,000 Volts ouch!