Physics 102 Lecture 3 Electric Potential Todays lecture

Physics 102: Lecture 3 Electric Potential • Today’s lecture will cover Textbook Sections 17. 1 - 3 Honors Credit info on web under course description. Physics 102: Lecture 3, Slide 1

Overview for Today’s Lecture • Electric Potential Energy/ Work – Uniform fields – Point charges • Electric Potential (like height) – Uniform fields – Point charges Physics 102: Lecture 3, Slide 2

Recall Work from P 101 • Work done by the force given by: – – W = F d cos(q) Positive: Force is in direction moved Negative: Force is opposite direction moved Zero: Force is perpendicular to direction moved • Careful ask WHAT is doing work! – Opposite sign for work done by you! • Conservative Forces – D Potential Energy = -Wconservative Physics 102: Lecture 3, Slide 3

Preflight 3. 1 C A Uniform E In what direction does the force on a negative charge at point A point? 1) left 2) right 3) up Physics 102: Lecture 3, Slide 4 B

Preflight 3. 1 C F - A B Uniform E In what direction does the force on a negative charge at point A point? 1) left 2) right 3) up Physics 102: Lecture 3, Slide 5 Electric field points in the direction a POSITIVE charge would feel force.

Preflight 3. 2 C A B Uniform E When a negative charge is moved from A to C the ELECTRIC force does 1) positive work. 2) zero work. 3) negative work. Physics 102: Lecture 3, Slide 6

Preflight 3. 2 The work is zero because the path is perpendicular to the field F F F motion - C A B Uniform E When a negative charge is moved from A to C the ELECTRIC force does 1) positive work. 2) zero work. 3) negative work. Physics 102: Lecture 3, Slide 7

Preflight 3. 3 C A B Uniform E When a negative charge is moved from A to B the ELECTRIC force does 1) positive work. 2) zero work. 3) negative work. Physics 102: Lecture 3, Slide 8

Preflight 3. 3 The work is negative the electric force opposes the direction of motion C A B F F -F -F - motion Uniform E When a negative charge is moved from A to B the ELECTRIC force does 1) positive work. 2) zero work. 3) negative work. Physics 102: Lecture 3, Slide 9

Preflight 3. 5 C A B When a negative charge is moved from A to B, the electric potential energy of the charge 1) Increases 2) is constant 3) decreases Physics 102: Lecture 3, Slide 10

Preflight 3. 5 C - A - - - B - Uniform E When a negative charge is moved from A to B, the electric potential energy of the charge 1) Increases 2) is constant 3) decreases Physics 102: Lecture 3, Slide 11 UE = -WE field Like “climbing up hill” – increases potential energy

ACT: Electric Potential Energy + C A B When a negative charge is moved from A to B, the electric potential energy of the charge (1) increases (2) is constant (3) decreases Physics 102: Lecture 3, Slide 12

ACT: Electric Potential Energy + E A- AC: W=0 C B - - - CB: W<0 - When a negative charge is moved from A to B, the electric potential energy of the charge (1) increases (2) is constant (3) decreases 1) The electric force is directed to bring the electron closer to be proton. 2) Since the electron ends up further from the proton the electric field did negative work. 3) So the electric potential energy increased Physics 102: Lecture 3, Slide 13

Work and D Potential Energy W = F d cos(q) Gravity • Brick raised yi yf • FG = mg (down) • WG = -mgh • DUG= +mgh yf h yi Physics 102: Lecture 3, Slide 14 Electric • Charge moved ∞ rf • FE = kq 1 q 2/r 2 (left) • WE = -kq 1 q 2/rf • DUE= +kq 1 q 2/rf rf

Work done by YOU to assemble 3 charges • W 1 = • W 2 = • W 3 = • Wtotal = • WE = D UE = 5 m 1 Watch signs! Physics 102: Lecture 3, Slide 15 3 5 m 5 m 2

ACT: Work done by YOU to assemble 3 negative charges How much work would it take YOU to assemble 3 negative charges? 1. W = +19. 8 m. J 2. W = 0 m. J 3. W = -19. 8 m. J Physics 102: Lecture 3, Slide 16 5 m 1 3 5 m 5 m 2

ACT: Electric Potential Energy of 3 negative charges Likes repel, so potential energy is still positive! 1. UE = +19. 8 m. J 2. UE = 0 m. J 3. UE = -19. 8 m. J Physics 102: Lecture 3, Slide 17 5 m 1 3 5 m 5 m 2

Preflight 3. 11 1 5 m 2 + + 5 m 5 m - 3 The total work required by you to assemble this set of charges is: (1) positive (2) zero (3) negative Physics 102: Lecture 3, Slide 18

Preflight 3. 11 1 5 m 2 + + 5 m 5 m - 3 The electric potential energy of this set of charges is: (1) positive Bring in (1): zero (2) zero Bring in (2): positive (3) negative Physics 102: Lecture 3, Slide 19 Bring in (3): negative x 2

Electric Potential (like height)* • Units Joules/Coulomb Volts v. Batteries v. Outlets v. EKG • • Really Potential differences Equipotential lines at same height Field lines point down hill V = k q/r (distance r from charge q) v. V(∞) = 0 Physics 102: Lecture 3, Slide 20

Preflight 3. 7 The electric potential at point A is _______ at point B 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 21

Preflight 3. 7 To go from B to A, a positive charge must climb “up hill” – increases potential energy. Hence A is at higher potential than B The electric potential at point A is _______ at point B 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 22

Preflight 3. 9 conductor The electric potential at point A is _______ at point B 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 23

Preflight 3. 9 conductor The electric potential at point A is _______ at point B 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 24 The electric field is zero at any point within a conducting material

ACT: Electric Potential + E A C B The electric potential at A is ______ the electric potential at B. 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 25

ACT: Electric Potential + E C A B The electric potential at A is ______ the electric potential at B. 1) greater than 2) equal to 3) less than Physics 102: Lecture 3, Slide 26 1) Electric field lines point “down hill” 2) AC is equipotential path (perpendicular to E) 3) CB is down hill, so B is at a lower potential than (“down hill from”) A

Electric Potential due to Proton What is the electric potential a distance r = 0. 53 10 -10 m from a proton? (Let V( ) = 0) V = U/q = k q/ r = What is the electric potential energy of an electron a distance r = 0. 53 10 -10 m from a proton? U = Vq = rf = 0. 5 10 -10 m + Physics 102: Lecture 3, Slide 27 -

Comparison: Electric Potential Energy vs. Electric Potential • Electric Potential Energy (U) - the energy of a charge at some location. • Electric Potential (V) - found for a location only – tells what the EPE would be if a charge were located there (usually talk about potential differences between two locations): U = q. V • Neither has direction, just location. Sign matters! Physics 102: Lecture 3, Slide 28

Two Charges • Calculate electric potential at point A due to charges – Calculate V from +7 m. C charge – Calculate V from – 3. 5 m. C charge – Add (EASY!) 4 m • V = kq/r V 7 = V 3 = Vtotal = A Q=+7. 0 m. C 6 m Q=-3. 5 m. C How much work do you have to do to bring W=DU=DVq = a 2 m. C charge from far away to point A? Physics 102: Lecture 3, Slide 29

ACT: Two Charges • In the region II (between the two charges) the electric potential is 1) always positive 2) positive at some points, negative at others. 3) always negative I II Q=+7. 0 m. C Physics 102: Lecture 3, Slide 30 III Q=-3. 5 m. C

ACT: Two Charges • In the region II (between the two charges) the electric potential is 1) always positive 2) positive at some points, negative at others. 3) always negative I II Q=+7. 0 m. C III Q=-3. 5 m. C Very close to positive charge potential is positive Very close to negative charge potential is negative Physics 102: Lecture 3, Slide 31

To Do • Read 17. 5 -6, 18. 1 -2, 6 • Bring “Problem Solver” to discussion section • Complete preflight before 8: 00 AM on day of next lecture. Physics 102: Lecture 3, Slide 32