Comments on HW20a.
Electric potential energy: (delta)U
= -W
= q(delta)V = qEd
The electric field: E = -gradient of V = slope of V(s)
Analogy of electric field and potentials to topographic maps,
atmospheric
pressure maps, and football.
P20.11 - Given E = 1200 N/C, find the potential difference
between
points A, B, and C in Fig. 20-21.
P20.14 - Where is Ex a maximum in Fig. 20-22?
P20.3: Find the magnitude and direction of the electric
field
inside a cell membrane.
Conservation of energy applies to electrical systems. E =
U + K =
constant
CC20-2: A proton is released from rest in a uniform
electric
field: E = 100 V/m (+x). Where will this charged particle be 1.0
s
later? If the proton is replaced with an electron, where would it
be 1.0
s later? Compare the force, potential, potential energy, and
kinetic
energy for these 2 charges. How much faster is v(e)
compared to v(p)?
P20.8: What is the potential difference between the top
and bottom
of the Washington Monument (h = 555 ft)?
Exercise: An electron is released 1.0 m above the
ground.
Where can we expect to find this electron 1.0 s later?
Dielectric strength is the maximum electric field before
breakdown
occurs in a given dielectric material. Emax
in
air = 3 MV/m.
Demo: Estimate the electric potential on a Van de Graaf generator that produces sparks that are 5
cm long.
The electric potential (voltage) due to a charge configuration
is
defined to be zero at infinity.
For a point charge, V = kq/r
The electric potential energy for a
charge q is
U = qV.
Note that while the electric field is a vector, V and U are
both scalar
quantities.
The principle of superposition applies to V and U, so that the total is
simply
the algebraic sum of individual pairs.
Example (P.20.68): Find the potential required
to
assemble three identical point charges q.
Capacitors have the ability to store and release charge, much
like a
battery, but faster.
Capacitance is the ratio of the charge that can be stored in a capicitor divided by the voltage. C = Q/V
The amount of capacitance is proportional to the area of the capacitor
plates
and inversely proportional to the distance separating the plates. C ~A/d
A dielectric is an insulating
material that
increases the capacitance of a capacitor by reducing the electric field
in the
region between the plates.
C = kCo
Ponderable: What would happen if the
plates of
a capacitor touched?
Note: Just because a capacitor is not charged,
does
not mean that its capacitance is zero. (ref.
CQ20.26
and analogy with volume of a container).
In addition to storing charge, capacitors also store energy:
U =
(1/2)QV = (1/2)CV^2
We can think of this energy as being stored in the electric field, electrical
energy density: u = (1/2)eoE^2
Demo: Display various capacitors
Exercise: Calculate the size of a 1 F parallel-plate capacitor
made from
aluminum foil and paper (d = 0.1 mm)
Demo: 1 F, 5V capacitor and Genecon
Example: How much energy can this capacitor
store? Compare this energy to Ug for
a 1 kg
mass. Would you advise discharging this cap using your tongue or
wet
fingers? How about dry fingers?
Ex. 20-7 - How high could a person be
lifted
with 439 J of energy from a defibrilator?
Demo: Calculate energy stored in a high-voltage capacitor.
Discharge and listen!
High-V, low C cap: E = 0.5(50 uF)(600
V)^2 = 9 J
Low V, high C cap: E = 0.5(1 F)(5 V)^2 = 12.5 J
If the high-voltage capacitor stores less energy,
why does
it "pop" but the low-voltage cap does not?
Demo: Connect three "D" cells in series with 1 F cap and light
bulb to show charging and discharging circuit.
What can happen to a capacitor if its maximum rated voltage is exceeded?
Why should large capacitors be stored with a wire connecting the
terminals?
Problems: 51, 62, 63, 65
Review: Concept Test questions
Warning! - Low resistance!