1. Physics 2102 Lecture 13: WED 11 FEB
Jonathan Dowling
We Are Borg.
Resistance Is Futile!
Physics Lecture 13: WED 11 FEB
Capacitors III / Current & Resistance
Ch25.6–7
Ch26.1–3
Georg Simon Ohm
( )

2. Sec. 2 (Dowling) Average: 67/100
Exam 01:
Sec. 2 (Dowling) Average: 67/100
A: 90– B: 80– C: 60– D: 50–59
Graders:
Q1/P1: Dowling (NICH 453, MWF 10:30AM-11:30AM)
Q2/P2: Schafer (NICH 222B, MW 1:30-2:30 PM
Q3/P3: Buth (NICH 222A, MF 2:30-3:30 PM )
Q4/P4: Lee (NICH 451, WF 2:30-3:30 PM
Solutions:
Go over the solutions NOW.
Material will reappear on FINAL!

3. This problem from our slides first week of class.
F≠E! Units!
Most common mistake to Compute Magnitude and Direction of Electric Field instead of Electric Force at Central Point.
(ii) (4 pts) What is the direction of the net electrostatic force on the central particle due to the other particles?
F≠E!

4. Common mistake: Wrong x.
This is Sample Problem from Book We worked on Board x q3
Charges Alone Could Cancel to Left and To Right. Must discuss Big & Small Charge versus Small and Big Distance: Q vs. 1/r2
x–L
F32
F13
Common mistake: Wrong x.
Common mistake: To put
q3 at –L to Left “By Symmetry” This would only make sense if q3 and q2 were held and q1 was free. But we are told q1 and q2 are fixed and q3 is free to move. So no symmetry!
Not to right.

5. Dielectric Constant C =  A/d
If the space between capacitor plates is filled by a dielectric, the capacitance INCREASES by a factor 
This is a useful, working definition for dielectric constant.
Typical values of are 10–200 +Q –Q
DIELECTRIC
C =  A/d
The  and the constant
o are both called dielectric constants. The  has no units (dimensionless).

6. Atomic View
Emol
Molecules set up counter E field Emol that somewhat cancels out capacitor field Ecap.
This avoids sparking (dielectric breakdown) by keeping field inside dielectric small.
Hence the bigger the dielectric constant the more charge you can store on the capacitor.
Ecap

7. Example Capacitor has charge Q, voltage V
Battery remains connected while dielectric slab is inserted.
Do the following increase, decrease or stay the same:
Potential difference?
Capacitance?
Charge?
Electric field?
dielectric
slab

8. Example
Initial values: capacitance = C; charge = Q; potential difference = V; electric field = E;
Battery remains connected
V is FIXED; Vnew = V (same)
Cnew = kC (increases)
Qnew = (kC)V = kQ (increases).
Since Vnew = V, Enew = V/d=E (same)
dielectric
slab
Energy stored? u=e0E2/2 => u=ke0E2/2 = eE2/2

9. Summary Any two charged conductors form a capacitor.
Capacitance : C= Q/V
Simple Capacitors: Parallel plates: C = e0 A/d Spherical : C = 4 e0 ab/(b-a) Cylindrical: C = 2 e0 L/ln(b/a)
Capacitors in series: same charge, not necessarily equal potential; equivalent capacitance 1/Ceq=1/C1+1/C2+…
Capacitors in parallel: same potential; not necessarily same charge; equivalent capacitance Ceq=C1+C2+…
Energy in a capacitor: U=Q2/2C=CV2/2; energy density u=0E2/2
Capacitor with a dielectric: capacitance increases C’=kC

10. What are we going to learn? A road map
Electric charge  Electric force on other electric charges  Electric field, and electric potential
Moving electric charges : current
Electronic circuit components: batteries, resistors, capacitors
Electric currents  Magnetic field  Magnetic force on moving charges
Time-varying magnetic field  Electric Field
More circuit components: inductors.
Electromagnetic waves  light waves
Geometrical Optics (light rays).
Physical optics (light waves)

11. Resistance is NOT Futile!
Electrons are not “completely free to move” in a conductor. They move erratically, colliding with the nuclei all the time: this is what we call “resistance”.
The resistance is related to the potential we need to apply to a device to drive a given current through it. The larger the resistance, the larger the potential we need to drive the same current.
Ohm’s laws
Georg Simon Ohm
( )
"a professor who preaches such heresies
is unworthy to teach science.” Prussian
minister of education 1830
Devices specifically designed to have a constant value of R are called
resistors, and symbolized by

12. Current density and drift speed
The current is the flux of the current density! E i J dA
If surface is perpendicular to a constant electric field, then i=JA, or J=i/A
Units:
Drift speed: vd :Velocity at which electrons move in order to establish a current. E i L A
Charge q in the length L of conductor:
n =density of electrons, e =electric charge

13. Resistivity and resistance
Metal
“field lines”
These two devices could have the same resistance
R, when measured on the outgoing metal leads.
However, it is obvious that inside of them different things go on.
resistivity:
Resistivity is associated
with a material, resistance
with respect to a device
constructed with the material.
( resistance: R=V/I )
Example: A L V + -
Makes sense!
For a given material:

14. Resistivity and Temperature
Resistivity depends on temperature:
r = r0(1+a (T-T0) )
At what temperature would the resistance of a copper conductor be double its resistance at 20.0°C?
Does this same "doubling temperature" hold for all copper conductors, regardless of shape or size?

15. Power in electrical circuitsb a
A battery “pumps” charges through the resistor (or any device), by producing a potential difference V between points a and b. How much work does the battery do to move a small amount of charge dq from b to a?
dW = –dU = -dq•V = (dq/dt)•dt•V= iV•dt
The battery “power” is the work it does per unit time:
P = dW/dt = iV
P=iV is true for the battery pumping charges through any device. If the device follows Ohm’s law (i.e., it is a resistor), then V=iR and
P = iV = i2R = V2/R