Download Faraday*

Faraday…
• What was his insight about the reason the
current-carrying wire could make the compass
needle move? What did he “invent” in his
explanation? And what’s the connection
between this and the knitting needle we just
observed?
Force at a distance
• Can we see an electric field?
Only its effects
Word description
Picture description; draw the gravitational force or the electric
force at the points. Draw the arrows with the correct relative
lengths.
Draw the following…
Represent with arrows the
gravitational force that the Earth
(the source mass) exerts on a
small object (test mass) at the
points shown.
Represent with arrows the
electric force that the object
with a large negative charge (the
source charge) exerts on a small
object that has a positive charge
(called test charge) at the points
shown.
Represent with arrows the
electric force that the object
with a large positive charge (the
source charge) exerts on a small
positively charged object (test
charge) at the points shown.
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A field can be represented with field lines.
They have the following properties:
• Field lines start on positively charged
objects and end on negatively charged
objects. They follow the direction a
positive charge would move.
• Lines closer together mean a stronger
field. Lines further apart mean a weaker
field.
• Number of lines indicates magnitude of
charge.
• Lines are perpendicular to surface.
• Lines never cross.
• E = F/q
(force per unit of test charge)
• What if I tell you that the electric field strength is not
dependent upon the quantity of charge on the test
charge? How is this possible? After all, the quantity of
charge on the test charge (q) is in the equation for
electric field. So how could electric field strength not
be dependent upon q if q is in the equation?
• Think about Coulomb’s Law: Felec = kq1q2/r2
• Regardless of what test charge is used, the electric field
strength at any given location around the source
charge will be measured to be the same.
Several electric field line patterns are shown in the
diagrams below. Which of these patterns are incorrect?
Explain what is wrong with all incorrect diagrams.
Erin Agin drew the following electric
field lines for a configuration of two
charges. What did Erin do wrong?
Explain.
Use your understanding of electric field lines to
identify the charges on the objects in the
following configurations:
Back to the Van de Graaff:
What do you think the
environment is like
INSIDE the Van de
Graaff’s sphere?
• http://en.wikipedia.org/wiki/File:Faraday_cag
e.gif
http://www.youtube.com/watch?v=qHrBhgwq
__Q
Can the forces exerted by an electric
field do work?
• What evidence do we have?
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PUSH!
So we exert a force over a
distance. What does that
mean?
We did WORK.
Where does that energy go that we input
into our system?
Let’s look at gravitational force.
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X
The same is true for electrical potential. If we put the
charge at a higher electric potential, we do work.
This energy is stored as an increased electrical
potential energy. When it “falls” down to a lower
electrical potential, it can gain kinetic energy
or do useful work.
• Gravitational Potential energy of a mass
depends upon….
• Electrical potential energy of a charge
depends upon…
Magnitude of charge, strength of
electric field, and distance
V = energy / charge = E/q =
Work/q = F∙ d/q
Q = F/E so V = F∙ d ÷ F/E = E∙ d
Fgravity is to gravitational
potential energy as
Felectrical is to voltage
(voltage is same as
potential difference).