Chapter 17 Electric Potential and Electric Energy;
Capacitance
Electric Potential and Electric
Energy; Capacitance












Review of Chapter 16
17.1 Electric Potential and Potential Difference
17.2 Relationship between Electric Potential and Electric Field
17.3 Equipotential Lines
17.4 The Electron Volt, a Unit of Energy
17-5 Electric Potential Due to Point Charges
17-6 Electric Dipoles
17-7 Capacitance
17-8 Dielectrics
17-9 Storage of Electric Charge
17-10 Cathode Ray Tube
17-11 The Electrocardiogram
Important stuff from Chapter 16:
Coulomb’s Law: F = kQ1Q2/r2
 where:
 k = 9.0 x 109 Nm2/C2
 Q1 & Q2 are two charges (coulomb)
 r = distance between two charges
Important stuff from Chapter 16:
 Electric Field (E)—force (F) exerted on a positive
test charge divided by the magnitude of the charge (q,
coulombs)
 E = F/q (units N/C)
  electric field goes from positive to negative (the
path of a positive test charge)
Important stuff from Chapter 16:
 Electric Field due to a Point Charge:
E = kQ/r2
Important stuff from Chapter 16:
 Electric potential energy—the energy stored in a
charged objects when its in an electric field
 positive when the two charges are the same (repulsive)
and negative when the two charges are opposite
(attractive)
Electric Potential and Potential
Difference
 To move an charge in an electric field work must be
done.
Electric Potential and Potential
Difference
  change in electric potential
energy (PEa – PEb) when a charge,
q, moves from point b to point a is
the negative work done by the
electric force to move the charge
from b to a
  PE of a charge is the largest
when it is closest to the plate with
the same charge
+
+
+b
+
+
+
+
+
+
+
a -
Electric Potential and Potential
Difference
Electric Potential (potential)—
the potential energy per unit
charge (V)
 Va = PEa/q --for a test charge, q,
at point a in an electric field
 Where is the test charge’s electric
potential the most, at point a or
b?
 Positive plate has higher potential
than negative (by definition, why?)
+
+
+b
+
+
+
+
+
+
+
a -
Electric Potential and Potential
Difference
 Can only measure differences in
PE; so can measure the potential
difference (difference in potential)
between two points
 Since potential difference (PEa –
PEb) =  W then

Vab = Va  Vb = Wba/q
+
+
+b
+
+
+
+
+
+
+
a -
Electric Potential and Potential
Difference
 Vab = Va  Vb = Wba/q
 Unit; volt (1 V = 1 J/C)
 Voltage = potential difference
 Zero for voltage is arbitrary since
we can only measure PE
+
+
+b
+
+
+
+
+
+
+
a -
Electric Potential and Potential
Difference
 Since electric potential (V) = PE/q
then PE = PEb  PEa = qVba
 if an object with charge q moves
through a potential difference Vba
its potential energy changes by an
amount qVba
 electric potential difference is a
measure of how much energy an
electric charge can acquire in a
situation and also a measure of
how much work a charge can do
+
+
+b
+
+
+
+
+
+
+
a -
Electric Potential and Potential
Difference
 Accelerating a charge; PE =
qV = KE so

v = (2qV/m)
 since KE = ½ mv2
+
+
+b
+
+
+
+
+
+
+
a -
Relationship between Electric
Potential and Electric Field
 In a uniform electric field (parallel plates) to move a
charge:
 W = qV = Fd = qEd (since F = Eq)
 so V = Ed or
 E (electric field) = V/d
Equipotential Lines
 Equipotential lines—graphic representation of
electric potential
 Potential the same on lines so it takes no work to
move charges along the lines
 Always perpendicular to field lines (diagram p.507)
 Continuous lines, never end
 A conductor must be entirely at the same potential
in the static case or electrons would accumulate at
its surface
Equipotential Lines
 Equipotential lines—
graphic representation of
electric potential
 Always perpendicular to
field lines (diagram
p.507)
The Electron Volt; Unit of Energy
 Electron Volt (eV)—used to measure very small
energies (electrons, atoms, molecules, etc.)
 Energy acquired by a particle carrying the charge
of one electron as a result of moving through a
potential difference of 1 volt
 1 eV = 1.6 x 1019 J (qe = 1.6 x 1019 C)
 electrons accelerated through potential difference
of 10V loses 10V of PE and gains 10V of KE
Electric Potential due to Point
Charges
 V = kQ/r where Q = point charge, r = distance
between point and test charge and k = ?
 V represents the absolute potential since the V at r = 
equals zero
 So V Q and V 1/r but E1/r2 (remember E = kQ/r2)
Electric Dipoles
 electric dipole--two equal point charges (Q) of
opposite sign separated by a distance
 dipole moment--the product of charge times length
(Ql)
 polar molecules—molecules that have a dipole
moment
Capacitance
capacitor—a device that can store electric charge
 consists of two conducting objects placed near
each other but not touching
 widely used in electronic: camera flash, surge
protectors, energy backups, memory for binary
code (RAM)
 often consists of two parallel plates (of area A, and
separation d) rolled together with an insulating
material between them
 symbol : —||—
Capacitance
 Capacitors
 Leydon Jar
Capacitance
capacitor—a device that can store electric charge
(diagram)
Capacitance
Amount of charge acquired by a given capacitor
Q = CV
 where:
 Q = amt. of charge (C)
 V = potential difference (V)
 C = capacitance of capacitor (constant of
proportionality dependent on properties of capacitor)-units farad (F) = Coulombs/Volts
Capacitance
For a parallel plate capacitor:
 C = oA/d where:
 o = permittivity of free space = 8.85 x 1012 C2/Nm2
(remember)
Dielectrics
Dielectric—the insulating sheet between the plates of a
capacitor
 Serves several purposes:
 Allows higher voltages to be applied without charge
passing the gap, dielectrics break down less readily than
air
 Allows plates to be closer together, the closer the plates
are the larger the capacitance of the capacitor (WHY?)
Dielectrics
For a parallel plate capacitor:
 C = KoA/d where:
 K = dielectric constant (Table 17-3)
 Since C = oA/d then
  = Ko where:
  = the permittivity of the material
Dielectrics
Molecular Description of dielectrics
 With air between plates; only plates of capacitor have a
potential difference
Dielectrics
Molecular Description of
dielectrics
 With a dielectric; molecules
of dielectric can line up in
electric field of capacitor
plates causing a net negative
side by positive plate and a
net positive by negative plate
(even though charges do not
move in dielectric material—
insulator)
Dielectrics
Molecular Description of
dielectrics
 A positive test charge within
the dielectric does not feel
the full force of the electric
field of the capacitor so it
takes a greater potential
difference between the two
plates of the capacitor to
cause it to move in the
dielectric
Storage of Electric Energy
 A charged capacitor stores electric energy
 The net effect of charging a capacitor is to remove
charge from one plate and add it to another (using a
source of electricity—battery)
Storage of Electric Energy
 A capacitor is not charged instantly—it requires
time and work to do this and this increases with
increasing charge on plates (?)
 If the work were constant then the work required
to charge a capacitor would be W = QV
 But since it is not we deal with the average voltage
(1/2 of Vf + VI) so
 W = Q Vf/2 (why?)
Storage of Electric Energy
Internal energy stored in a capacitor is:
U = 1/2QV where:
 V is potential difference between plates
 Since Q = CV
 then: U = ½ Q2/C
 and since C = oA/d & V = Ed
 then
U = 1/2oE2 A/d
Storage of Electric Energy
Energy density (u)—energy per unit volume
 u = energy/volume = 1/2 oE2
Cathode Ray Tube
Read section 17-10 and know this:
 What is a CRT?
 What is thermionic emission?
 What is a cathode?
 What is an anode?
 Explain how a cathode ray tube works.
 What is an oscilloscope?
 Explain how an electrocardiogram measures
heart function.
Extra Credit: Find out about LCD, LED and
Plasma screens (for TV’s)
Cathode Ray Tube
 Explain how
a cathode
ray tube
works.
Cathode Ray Tube
 What is an
oscilloscope?
The Electrocardiogram
 Explain how an electrocardiogram measures heart
function.