Chapter 17 Notes
College Physics by Giambattista et al.
Energy Methods
Electric field carries energy which may be transferred into or out of a charged particle:
+q
FE
HIGH
V
LOW
V
FE
–q
Energy is stored whenever you do work against the field, i.e. electrical potential energy
increases:
ΔUE = qΔV
Electric Potential
 analogous to pressure
 changes in V cause energy transfers to occur
 SI unit: volt = joule/coulomb
Accelerator Problems, Part II:
Total mechanical energy is conserved, so
ΔE = ΔK + ΔUE = 0  ΔK = –qΔV
Equipotential Surfaces
 surfaces of constant potential
 always perpendicular to field lines
 E always points toward surfaces at lower V
Note:
W = ΔK 
FEd = –qΔV
or
E
V
d
gradient formula
SI unit for field: V/m = N/C.
Capacitors
 popular electronic device that stores charge and energy
 “conductor sandwich”
 capacitance = measure of storage capacity, SI unit: farad (μF, nF, pF)
Q  CV
U E  12 C (V ) 2
John B. Ross, Ph.D.
storage formulas
IUPUI Physics Dept.
Chapter 17 Notes
College Physics by Giambattista et al.
Parallel-Plate Design – simplest
C0 
area A
0 A
d
ε0 = 8.85×10–12 F/m
Q
“permittivity of free space”
gap d
(air gap formula)
–Q
Dielectrics
 insulation, filling
 used to increase C without increasing A or d.
How do they work?
Q
–
+
–Q
Dielectric polarizes,
sets up an opposing field
thereby reducing the net
field and potential difference:
E = E0/κ
ΔV = ΔV0/κ
–
C = κC0
+
κ is the dielectric constant = magnification factor, Table 17.1, p.619.
Capacitor Combinations [Chapter 18-6]
Manufacturers make caps with only certain values of C. What if you need a cap with a
different value and don’t have the time, resources, or $$ to build one from scratch??
Make an equivalent capacitance out of combos of standard caps.
I. Parallel Combination – like a ladder, same voltage difference across each.
+
Q1
ΔV
–
C1
Q2
C2
Qtotal = Q1 + Q2 + Q3 
John B. Ross, Ph.D.
Q3
C3
Ceq = C1 + C2 + C3 = Σ Ci
IUPUI Physics Dept.
Qtotal
Ceq
Chapter 17 Notes
College Physics by Giambattista et al.
II. Series Combination – like a train, same charge stored on each.
Q
Q
Q
C1
C2
C3
+
ΔV
ΔV = ΔV1 + ΔV2 + ΔV3 
John B. Ross, Ph.D.
Ceq
–
1
1
1
1
1




C eq C1 C 2 C 3
Ci
IUPUI Physics Dept.
+
ΔV
–