1. Electric Potential III
Physics 2113
Jonathan Dowling
Physics Lecture: 14
Electric Potential III

2. Conservative Forces, Work, and Potential Energy
Work Done (W) is Integral of Force (F)
Potential Energy (U) is Negative of Work Done
Hence Force is Negative Derivative of Potential Energy

3. Coulomb’s Law for Point Charge
Force [N]
= Newton
Electric
Field
[N/C]=[V/m]
Potential Energy [J]=Joule
Potential Voltage [J/C]=[V]
=Volt q2 q2 q1 P2 P1 P2 P1

4. Potential At Center of Ring of Charge
Divide the charge distribution into differential elements
Write down an expression for potential voltalge from a typical element — treat as point charge
Integrate!
Simple example: circular rod of radius r, total charge Q; find V at center. r dq
Same result holds along z-axis!

5. Potential Along Axis of Ring of Charge
Divide the charge distribution into differential elements
Write down an expression for potential voltalge from a typical element — treat as point charge
Integrate! R’ dq dq

6. Potential Voltage of Continuous Charge Distribution: Return of the Rod!
Uniformly charged rod
Total charge +Q
Length L
What is V at position P?
r = x P
dq=λdx L a dq
Units: [Nm2/C2][C/m]=[Nm/C]=[J/C]=[V]

7. Potential Voltage of Continuous Charge Distribution: Return of the Rod!
Uniformly charged rod
Total charge +Q
Length L
What is V at position P?
What about a>>L?
r = x P
dq=λdx L a
In this limit we recover V=kq/r for point charge!

8. Potential Voltage of Continuous Charge
Uniformly charged rod
Total charge +Q
Length L
What is V at position P?

9. Potential Voltage of Continuous Charge
What about d>>L?
We recover formula
V=kq/r for a point charge
Again!

10. Electric Field & Potential: A Simple Relationship!
Focus only on a simple case: electric field that points along +x axis but whose magnitude varies with x.
Notice the following:
Point charge:
E = kQ/r2
V = kQ/r
Dipole (far away):
E = kp/r3
V = kp/r2
E is given by a DERIVATIVE of V!
Of course!
Note:
MINUS sign!
Units for E: VOLTS/METER (V/m)

11. E from V: Example Uniformly charged rod Total charge +Q Length L
We Found V at P!
Find E from V? x P dq L a
Units:
√ Electric Field!

12. Electric Field & Potential: ICPP
Hollow metal spherical shell of radius R has a charge on the shell +q
Which of the following is magnitude of the electric potential voltage V as a function of distance r from center of sphere? R V r
r=R
(b) V r
r=R
(a)
Hint: Inside sphere there are no charges so E=0.
But E=dV/dr=0. What can V be?

13. (a) Since Δx is the same, only |ΔV| matters!
|ΔV1| =200, |ΔV2| =220, |ΔV3| =200
|E2| > |E3| = |E1|
The bigger the voltage drop the stronger the field. Δx
(b) = 3
(c) F = qE = ma
accelerate leftward

14. Equipotentials and Conductors
Conducting surfaces are EQUIPOTENTIALs
At surface of conductor, E is normal (perpendicular) to surface
Hence, no work needed to move a charge from one point on a conductor surface to another
Equipotentials are normal to E, so they follow the shape of the conductor near the surface. V E

15. Conductors Change the Field Around Them!
An Uncharged Conductor:
A Uniform Electric Field:
An Uncharged Conductor in the Initially Uniform Electric Field:

16. Sharp Conductors (NASA)
Charge density is higher at conductor surfaces that have small radius of curvature
E = σ/ε0 for a conductor, hence STRONGER electric fields at sharply curved surfaces!
Used for attracting or getting rid of charge:
lightning rods
Van de Graaf -- metal brush transfers charge from rubber belt
Mars pathfinder mission -- tungsten points used to get rid of accumulated charge on rover (electric breakdown on Mars occurs at ~100 V/m)
(NASA)

17. Sharp Conductors: Lightning Rods

18. Ben Franklin Invents the Lightning Rod!

19. LIGHTNING SAFE CROUCH
If caught out of doors during an approaching storm and your skin tingles or hair tries to stand on end, immediately do the "LIGHTNING SAFE CROUCH ”. Squat low to the ground on the balls of your feet, with your feet close together. Place your hands on your knees, with your head between them. Be the smallest target possible, and minimize your contact with the ground.

20. WE’LL BOTH BE DEAD IN SECONDS!
QUICK — TAKE A SELFIE!
WE’LL BOTH BE DEAD IN SECONDS!

21. Summary:
Electric potential: work needed to bring +1C from infinity; units = V = Volt
Electric potential uniquely defined for every point in space -- independent of path!
Electric potential is a scalar -- add contributions from individual point charges
We calculated the electric potential produced by a single charge: V = kq/r, and by continuous charge distributions :
dV = kdq/r
Electric field and electric potential: E= -dV/dx
Electric potential energy: work used to build the system, charge by charge. Use W = U = qV for each charge.
Conductors: the charges move to make their surface equipotentials.
Charge density and electric field are higher on sharp points of conductors.