Life is Less Than Ideal: When
resistors and capacitors mix
Charges in Conductors
 Electric fields are created when positive charges
and negative charges are separated
 A uniform electric field existing over a region sets
up a potential difference between points in that
region: DV=EDx, where Dx is the distance along
a field line.
 If I apply a potential difference across a
conducting object (including semiconductors),
charges experience a force, and charge carriers
will flow until the potential difference is removed.
What if charge can’t flow?
 Consider charge separated by two metal
plates
– A potential difference exists between the plates
– An electric field exists between the plates,
pointing from positive plate to negative plate
– No current can flow
Introducing, . . . Capacitance
The battery provides
the work needed to
move the charges
and increase their
potential energy
ELECTRIC POTENTIAL
(POTENTIAL)
•When a charge, q0, moves from one point to
another in an electric field, its potential energy
changes.
DU  U f Ui  Wif  i F  ds
f
F  q0E, so DU  q0 i E ds
f
ELECTRIC POTENTIAL
•Electric potential (or potential) is defined as the potential
energy per unit charge.
DU
f
DV 
  i E  ds
q0
ELECTRIC POTENTIAL
•For point charges, or for spherical charge
distributions, it is customary to choose the
zero of potential to be at infinity.
V   E  ds
r

What determines capacitance?
C = e0 A/d
More about capacitors
More about capacitors
U = Vq
1 eV = 1.602 x 10-19 J
= energy to move
electron through 1 volt
Before the next class, . . .
• Homework 12 (Due Monday, Feb. 25)
• Do Activity 11 Evaluation by Midnight
tonight
• Do the Readings for Class 12 on Magnetism
• Do Reading Quiz