Transcript Slide 1

Journal 3/3/17
Electric fields are similar to gravity. But how are they
different?
Objective
To learn how the electric
field is different from gravity
Tonight’s Homework
pp 445: 11, 12, 15, 16, 19
Notes on Electric Shielding / Capacitors
Let’s start today with a setup. Imagine we have
a hollow metal sphere. This sphere is charged
with a bunch of negative charges. Inside, we
have a positive charge. Let’s draw the field lines
for this setup.
Notes on Electric Shielding / Capacitors
Let’s start today with a setup. Imagine we have
a hollow metal sphere. This sphere is charged
with a bunch of negative charges. Inside, we
have a positive charge. Let’s draw the field lines
for this setup.
Notes on Electric Shielding / Capacitors
Let’s start today with a setup. Imagine we have
a hollow metal sphere. This sphere is charged
with a bunch of negative charges. Inside, we
have a positive charge. Let’s draw the field lines
for this setup.
Notes on Electric Shielding / Capacitors
You’ll notice that outside the sphere, we have a
lot of lines pointing in to the negative charges.
It doesn’t seem like the positive charge has had
any effect outside the sphere.
Notes on Electric Shielding / Capacitors
Let’s do another setup that’s similar. This time
we place our positive charge outside the sphere.
Where do the field lines inside go? (draw them!)
Notes on Electric Shielding / Capacitors
Let’s do another setup that’s similar. This time
we place our positive charge outside the sphere.
Where do the field lines inside go? (draw them!)
We can’t! There is no way to draw the lines
inside so they don’t
exist!
Notes on Electric Shielding / Capacitors
There are two conclusions we can draw from
these examples:
1) From a distance, a charged metal shell will
appear exactly the same as a single point
charge.
2) If we have a metal shell, the area inside and
the area outside are completely independent
of each other electrically. Charges on the
inside cannot be detected outside and vice
versa.
Notes on Electric Shielding / Capacitors
This leads us to what we saw in our lab a few
days ago. If we create a metal cage around an
object, we get this same effect. Any charges we
place inside can’t “communicate” with the
outside.
The first person to really realize the potential of
this was Michael Faraday in 1836. He coined
these “Faraday Cages”.
These cages have tons of use. The FBI
headquarters has metal lining the entire
building. This prevents any spy devices from
seeing computer information from outside.
Notes on Electric Shielding / Capacitors
Let’s look at one last setup for today. What kind
of electric field do we get in this example where
we have 2 charged sheets of metal?
Notes on Electric Shielding / Capacitors
Let’s look at one last setup for today. What kind
of electric field do we get in this example where
we have 2 charged sheets of metal?
You’ll note that our electric field between the
sheets is constant. It’s uniform. If we pack more
and more charge on to something like this, we
can create a device called a capacitor.
Notes on Electric Shielding / Capacitors
Not THAT kind of capacitor.
(If none of you get this, then Mr. C. is officially getting
old)
Notes on Electric Shielding / Capacitors
Capacitors have the ability to store electrical
energy. How?
As we add charge to the plates, the electric
potential between them increases. We’re making
a steeper “hill to valley” of potential.
We measure capacitance as charge over
voltage. How much charge we have and how
strong of a potential it creates.
The more potential we can create with less
charge, the more efficiently we’re storing
electrical energy.
Equations
Potential on
Parallel
Plates
E V=E●d
V V: Potential difference in Volts
E: Electric field strength in N/C
d: Distance between the plates in
meters
S This equation tells us how strong of a
potential difference we can find
between two charged, parallel plates of
metal.
Equations
Capacitance
E C=q/V
V C: Capacitance in Farads (1 C/V)
q: The strength of the charge on one
plate in Coulombs.
V: The potential difference between the
plates in volts.
S This equation tells us how efficiently
electrical energy is stored between two
charged plates.
Exit Question
Do you think a stronger capacitor could be made with 3
charged plates instead of 2?
a) yes
b) no