1. Electric Potential Mr. Finn Honors Physics March 20012

2. Spring Return to “Energy” • Recall: two types of energy • Kinetic energy = energy due to motion • Potential energy = energy due to forces/interactions • Energy stored in: • gravitational fields = GPE • stretched/compressed springs = EPE • E = “elastic” • electric fields = QPE • Q = charge

3. F F “electric potential” = potential energy = volt charge Spring Electric Potential = “voltage” • Put 2 positive charges near each other Force must be applied to keep charges near each other Charges act like compressed spring - store energy Also works if opposite charges - then charges act like stretched spring, but still store energy

4. Spring Dangerous Balloons? Voltage = average QPE/chg (like temperature = average KE) Also need total charge (like heat = total energy) High voltage = It takes a lot of work to move more electrons onto the balloon. Total work = Total electrical energy stored in balloon (work/electron)(# electrons) Balloon has few electrons, but each electron has lots of energy! (like 4th of July sparkler) tiny large Figure 9.14 (Hewitt)

5. Hill Roller Coaster slowly convert to KE rapidly convert to KE GPE

6. Just to confuse you! • Not the same as Potential Energy!! • Potential = QPE/unit charge Hill Potential • represent fields as a type of “energy” or POTENTIAL and focus on energy transformations • Idea useful when forces and fields are too complex to be used directly and no need for details on motion • Still provides a picture showing how charges affect surroundings  resembles contour map

7. Steep rise from 8600 ft to 9000 ft Slow rise from 9000 ft to 9200 ft Hill Contour Map Hill at ~9000 ft (positive charge) Valley below 8800 ft (negative charge) Lines of constant elevation (8800 ft) Flat region between 8860 ft and 8880 ft Lake = perfectly flat region = conductor

8. distance between 2 points As potential goes up, field points down. Only differences in potential matter, not actual value. Work done BY field ON charge q so ∆V<0 or loss in QPE = gain in KE How are these related? • Field difference in electric potential or • Work W = -q ∆V = ∆KE • Force F = q E = -q (∆V/∆x) If we know Electric Potential, we can find anything else.

9. Key Terms • Electric Force: Fe: force between 2 charges • vector • Electric Field, E: force/charge or rate V changes • vector • Electric PE, QPE: electrical analog to GPE • scalar • Electric Potential, V: QPE/charge • scalar • Potential Difference = ∆V (Voltage) • scalar

10. UNITS • Force: N = force to change velocity of 1 kg at rate of 1 m/s every sec • Work or Energy: J = work done when force of 1 N applied over distance 1 m • electron volt (eV) = energy 1 electron gets from 1 V battery • Potential: V (volt): 1 J/C = energy per charge • Charge: C = charge on 6.24  1018 electrons • Electric Field: N/C or V/m = force per charge or slope of potential Used for energy at atomic scales