Physics 9: Static Electricity
A.
Electric Nature of Matter (16-1 to 16-4)
1. subatomic particles
a. summary chart (Coulomb, C, unit of charge)
particle
Proton
Neutron
Electron
mass (kg)
1.67 x 10-27
1.67 x 10-27
9.11 x 10-31
charge (C) +1.60 x 10-19
0
-1.60 x 10-19
b. + and – attract (+ and + or – and – repel)
c. attraction between protons and electrons holds
matter together/apart explains Fs, Ff, Fn, etc.
2. conductors and insulators
a. metals are conductors because valence electrons
are loosely held by individual atoms and can travel
from atom to atom (excess charge on a conductor
spreads out on the outside surface)
b. non-metals are insulators because valence
electrons are restricted to individual atoms
(excess charge on an insulator stays localized)
c. metalloids (semiconductors) are intermediate
between metals and nonmetals
3. adding static charge to insulators
a. rubbing dissimilar insulators together transfers
electrons from one to the other
b. excess electrons = negatively charged
deficient electrons = positively charged
c. total charge is unchanged (conservation of charge)
4. adding static charge to conductors
a. conduction: charged conductor contacts neutral
conductor; electrons flow until equal electric
potential (water tanks analogy)
b. polarization through induction
1. charge conductor approaches an isolated neutral
conductor but no connection is made
2. neutral conductor develops a separation of
charge or polarization
c.
induction
1. neutral conductor is connected to
the earth by a grounded wire (a)
2. charged conductor approaches the
neutral conductor, but doesn’t touch, so
that electrons flow away from the
charged conductor and down into the
earth (b)
3. connection to earth is removed and
excess charge spreads evenly over
surface (c)
B.
Electric Force and Field (16-5 to 16-9)
1. electric force, Fe = k|qQ|/r2 (N)
a. charges Q and q or Q1 and Q2 in (C)
b. k = 1/(4єo) = 9.0 x 109 N•m2/ C2
c. єo = 8.85 x 10-12 C2/N•m2
1. permittivity of free space
2. measures the degree that an electric field is
"permitted" in a vacuum
d. r is distance between Q's from center to center
e. charge sign is not included in calculation, but is
used to determine direction of Fe
f. similar to Fg = -GMm/r2, except electric force can
attract (–) or repel (+)
Name __________________________
2.
electric field, E = k|Q|/r2 (N/C)
a. Michael Faraday's concept is useful to explain
forces at a distance, where space around –Q is
curved inward so that +q is drawn in as if it were
pulled (no physical contact is necessary)
+q
r
-Q
b.
c.
electric field lines (lines of force)
1. lines directed into cone show direction of field
2. away from +Q and toward –Q
3. perpendicular to the conductor's surface
4. (concentric rings are equipotential lines, V,
which are discussed in the next section)
oppositely charged plates
edge effect
E=0
E is constant (except edge effect) E kQ/r2
E = 0 inside a conductor (hollow or solid;
charged or uncharged)
solving E and Fe from multiple charges problems
(determining the magnitude and direction of the solid
line at point p)
1.
2.
3.
E1 E2
• 
+Q1




4.





p
–Q2
determine E1and E2, at position, p, due to Q1 and Q2:
E = k|Q|/r2 (don't include charge sign in calculation)
determine direction of E1 and E2 (+), –))
add vectorily
o same direction: Etot = E1 + E2
o opposite direction: Etot = E1 – E2
o right angles: Etot = (Ex-tot2 + Ey-tot2)½, tan = Ey/Ex
determine the electric force on q at point p
o magnitude, Fe = |q|Etot
o direction

+q: Fe is in the same direction as E

–q: Fe is in the opposite direction as E
solve x where E = 0 problems
D
Q1
Q2
location is closer to weaker charge
like charges (x < D), unlike charge (x > D)
(|+Q1| > |+Q2|): kQ1/x2 = kQ2/(D – x)2; solve for x
(|+Q1| > |–Q2|): kQ1/x2 = kQ2/(x – D)2; solve for x
math shortcut: take the square root of both sides
before cross multiplying
C.
Electric Potential Energy and Voltage (17-1 to 17-5)
1. potential energy between point charges, Q and q
a. Ue = kqQ/r (J)
b. Ue = 0, when Q and q are infinitely far apart
c. scalar quantity
d. sign of charge is included in calculation
e. analogous to gravitational potential energy
1. Ug = -GMm/r (Ug = 0 when r = )
2. negative sign in Ug to convert positive M and
m into negative Ug
2. electric potential (also called "potential" or "voltage")
due to point charge Q
a. V = kQ/r (V)—volt
b. is relative to where V = 0
1. infinitely far from a point charge (theoretical)
2. practical to set earth as V = 0 ("ground")
c. letter V has three uses (confusing!)
1. voltage (electric potential) = V
2. potential difference, V = V
3. volt = V
d. scalar quantity (include sign in calculation)
3. solving V and Ue from multiple charges problems
(determine the value of the dashed line at point p)
V1 + V2
•
+Q1



4.


5.
p
–Q2
determine V1 and V2 at position p, due to Q1 and Q2:
V = kQ/r (include charge sign in calculation)
add the voltages together, Vtot = V1 + V2
determine electric potential energy for charge q at
point p: Ue = qVtot
solve x where V = 0 problems
D
+Q1
–Q2
only meaningful for unlike charges (between charges
and closer to weaker charge)
(|+Q1| > |–Q2|): kQ1/x = kQ2/(D – x); solve for x
cathode ray tube
a. device for accelerating a beam of electrons
(cathode rays)—video monitors
b.
c.
design
1. high voltage used to charge two plates
2. electrons accelerate from cathode (– plate) to
anode (+ plate)
3. anode has small opening into evacuated tube
calculating the velocity of the electrons: |Ue| = K
 |qe(Vhigh – Vlow)| = ½mev2
me = 9.11 x 10-31 kg
qe = 1.6 x 10-19 C
Capacitor (17-7 to 17-9)
1. battery connected to parallel conducting plates fills
plates with opposite, but equal, charges
a. Vplates = Vbattery
b. charge can't pass directly from one plate to the
other so plates remain charged when battery is
removed
c. stores charge and energy
2. electric field, E = V/d (NOT kQ/r2)
a. solid arrows
b. constant
c. V/m = N/C
3. equipotentials, V
a. all points at the same V
b. dashed lines
c. perpendicular to E
d. decrease evenly with d
e. analogous to elevation
lines on contour map
4. capacitance, C =єoA/d (F)
a. "capacity" to store charge
b. єo = 8.85 x 10-12 C2/N•m2
c. A = common area of plates
d. d = distance between plates
e. farad, F = C/V
5. stored charge, Q = CV
a. charge increases as voltage increase
Q
slope = C
Work = Area under line = Uc
V
potential energy, Uc = ½QV
Steps
Algebra
W = ½bh
start with W = area under line
substitute Q for h and V for b
W = ½QV
Uc = ½QV
substitute Uc for W
Uc = ½CVV = ½CV2
substitute CV for Q
Uc = ½C(Q/C)2 = ½Q2/C
substitute Q/C for V
6. variable capacitor
a. connect/disconnect voltage source (battery)
1. connected: voltage V is constant
2. disconnected: charge Q is constant
b. increase/decrease area A or distance d
Capacitance
Battery
Variable
Connected
Disconnected
Capacitor
C = єoA/d
Q =C x V
Q = C x V






A












d






7. uses
a. RAM memory (charged = 1, uncharged = 0)
b. computer keyboard (variable capacitor—d)
c. camera flash (energy/charge storage)
d. heart defibrillator (energy/charge storage)
constant
b.
constant
D.