1. AP Physics ST Potential Difference & Electric Potential Potential Differences in a Uniform Electric Field 4.bp.blogspot.com

2. Potential Difference Electric Potential Electrical potential difference is very similar to gravitational potential difference. Review features of Mechanics: Gravitational Potential Energy (GPE) Work-Energy Conservative Forces Non-conservative Forces Conservation of Energy ΔKE = -ΔU

3. Potential Difference Electric Potential Ball-Spring Activity Purpose: – Students will be able to FEEL +W E and –W E in an effort to better conceptualize “negative work.” shop-handmade.craftcompany.com

4. Potential Difference Electric Potential Gravitational Field (g) – F g - Conservative force – Path independent – Recall: stairs vs. ladder Electric Field (E) – F E - Conservative force – Path independent

5. Potential Difference Electric Potential Things to remember: – F E = conservative force – F ext ≠ conservative force – Work done by a conservative force is equal to the negative change of potential energy.

6. Potential Difference Electric Potential Natural tendency of +q o is to accelerate to the right. External force MUST do +W to move particle to the left without accelerating*. * acceleration means energy in the system is NOT conserved E F ext motion of charge +q o

7. Potential Difference Electric Potential Natural tendency of +q o is to accelerate to the right. External force must act only enough to keep the test charge from accelerating. – External force is “backing off”; letting the electric field do the work. E F ext motion of charge +q o

8. Potential Difference Electric Potential Natural tendency of -q o is to accelerate to the left. External force must act only enough to keep the test charge from accelerating. – External force is “backing off”; letting the electric field do the work. E F ext motion of charge -q o

9. Potential Difference Electric Potential Natural tendency of -q o is to accelerate to the left. External force must do work to make the charge move to the right without accelerating. E F ext motion of charge -q o

10. Potential Difference Electric Potential Natural tendency of -q o is to accelerate to the left. External force must do work to make the charge move to the right without accelerating. E F ext motion of charge -q o

11. Potential Difference Electric Potential Electric Potential Energy – Energy by virtue of location in an electric field. Work done by the electric field affects the magnitude of transfer of energy stored by the field. web.ncf.ca

12. Potential Difference Electric Potential If a test charge were released in an electric field … – it would accelerate away due to the electric field doing work on the charge

13. Potential Difference Electric Potential Every bit (differential element) of work done by the electric field is a result of the Coulomb force acting across a bit of distance. The resulting work done (energy transferred)…

14. Potential Difference Electric Potential To consider the contributions of all work done (potential energy transferred) across every differential distance element… integrate from point A to point B! Electric Potential Energy

15. Potential Difference Electric Potential v 1000 F ext F ext push 1 q o  1 W Fext  + 1ΔU F ext push 3 q o  3 W Fext  + 3ΔU F ext push 10 q o  10 W Fext  + 10ΔU F ext push 1000 q o  1000 W Fext  + 1000ΔU F ext push 2 q o  2 W Fext  + 2ΔU

16. Potential Difference Electric Potential The amount of electric potential energy transferred depends on the number of charges being moved! – This gets unrealistic to consider b/c the number of charges able to be moved is ridiculously large. Simply more convenient to consider the amount of work done PER charge… known as ELECTRIC POTENTIAL ELECTRIC POTENTIAL – the amount of potential energy PER charge.

17. Potential Difference Electric Potential 1 J of work to move 1 C of charge through a potential difference of 1 V

18. Potential Difference Electric Potential Potential Difference (ΔV) – Work that an external force must perform to move a body of charge from point A to B without changing its kinetic energy. ELECTRIC POTENETIAL DIFFERENCE

19. Potential Difference Electric Potential Electric Potential aka - Potential difference, Voltage Voltage (V) DOES NOT depend on the amount of charge present. Potential Energy (U) DOES depend on the amount of charge present. Larger amount of charge in an electrical field has more U elec just like a larger mass in a gravitational field has more U grav. Electric potential of any amount of charge at the same location is the same!

20. Potential Difference Electric Potential Electron volt – – Energy that an electron (or proton) gains (or loses) when moving through a potential difference of 1 V.

21. Potential Differences in a Uniform Electric Field Determine the potential difference (ΔV) between points A and B in a uniform electric field (E). v E AB ds

22. Potential Differences in a Uniform Electric Field v E AB ds

23. Potential Differences in a Uniform Electric Field Potential Difference in a Uniform Electric Field – “-” indicates location B is at a lower potential than A. – Logical since B is further from the source of E thus E is weaker at B NOTE – electric field lines always point in the direction of DECREASING electric potential (V).

24. Potential Differences in a Uniform Electric Field Determine the change in electric potential energy (ΔU) between points A and B in a uniform electric field (E). v AB ds

25. Potential Differences in a Uniform Electric Field v AB ds E

26. Potential Differences in a Uniform Electric Field If q is POSITIVE (+q):  ΔU  will be “ – “  A +q will experience a decrease in potential energy and an increase in KE Release q in E… q will accelerate in direction of E Analogy: drop a baseball AB ds E

27. Potential Differences in a Uniform Electric Field If q is NEGATIVE (-q):  ΔU  will be “ + “  A -q will experience an increase in potential energy and an decrease in KE Initiate motion of q in the direction of E… q will de- accelerate Analogy: throw a baseball up AB ds E

28. Potential Differences in a Uniform Electric Field General case for charge in a uniform electric field. ΔU = ? A B E C d

29. Potential Differences in a Uniform Electric Field A B E C d General Form

30. Potential Differences in a Uniform Electric Field Note that all point locations on a line (or surface) has the SAME electric potential! This surface is known as an “equipotential surface”

31. Potential Differences in a Uniform Electric Field Equipotential Surface – a surface consisting of a continuous distribution of points with the same electric potential Electric field lines: – ALWAYS perpendicular to the equipotential surface – NEVER cross – ALWAYS point in the direction of decreasing electric potential. www.pstcc.edu

32. Potential Differences in a Uniform Electric Field Gravitational equipotential surface = topo map Electrical equipotential surface = electric charge www.k4lrg.orgwww.physics.sjsu.edu

33. Lesson Summary (THINK! What are the BIG, MAIN, GLOBAL lesson ideas?)

35. Lesson Summary Electric Potential Energy – Energy by virtue of location in an electric field. Electric Potential – The amount of potential energy PER charge. In a uniform electric field: – Electric Potential Energy – Electric Potential

36. Example #1: 05-1ab

37. Example #2 An electron is released from rest in a uniform electric of magnitude 4x10 3 V/m directed down. a.What is the change in potential energy when the electron moves 2.1 m from A to B? b.What is the potential difference between A and B?

38. Example #3: 25-10 A uniform electric field of magnitude 325 V/m is directed in the negative y direction as shown. The coordinates of point A are (-0.200, -0.300) m and those of point are (0.400, 0.500) m. Calculate the potential difference using the dotted-line path. x y A B E