W2-D1

• “Was this a typical class”? (Friday)
• My answers:
– No – it was a bit screwed up.
– Usually there would be some hands on experiments but for this topic (E-Field) there really aren’t any.
• There is for the next topic
– Sometimes it will be solving problems in a team atmosphere.
– Sometimes, like next Monday, we don’t do anything at all! (Labor Day)

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Today
◦ Complete E field and discuss (briefly) the dipole.
◦ More talking than usual.
◦ Clicker time (easy)
Wednesday
◦ We will begin a serous discussion of charge and how it moves around.
Friday
◦ Quiz on the past two weeks of work through the end of Wednesday’s class
Next Monday
◦ Labor Day – no classes

Let’s try those new fangled clicker-2 thingys …

I will quickly review numeric entry
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We will see if the system works properly
Clicker Points
◦ Correct Answer= 2 points
◦ Incorrect Answer = 1 point
◦ No response = 0 points
Let’s see how it works.

Answering Numeric
123
Click here Click here
Click here
Click here

Answering Numeric
Click here
123

Using the A-E buttons, which statement is the most correct?
A.
B.
C.
D.
E.
The electric field is a scalar quantity that relates the force on a charge relative to another charge.
The electric field is a computational convenience that allows us to normalize
Coulomb forces.
The electric field is a real scalar property of space.
The electric Field is a figment of Dr.
Bindell’s mind.
None of these are correct.

At a point in space, the electric field is E. A charge is placed at the point.
A.
B.
C.
D.
If the charge at the point is doubled, the electric field is doubled as well.
If the charge at the point is doubled, the force on the charge that produced the field is doubled.
If the charge is doubled, the electric field remains the same.
None of these statements are correct.

IMPORTANT drawing convention: We place the tail of the electric field vector at the place where the field has that (vector) value. The electric field is everywhere, and has different (vector) values at different locations. So to be clear we always place the tail at the location corresponding to that vector.

y r x
+
Note: q
1 q
2 is (-)

SUPERPOSITION!
r
1 points to the position of the charge q
1 x
E
2 r
1 r points to the position we want to calculate E origin 

each
r
1 q i r x
E(r)
1 x = r - r
1 x is the vector from the charge to the point of interest.
1
Thus




A B
A A is bigger
B B is bigger
C Both the same
D You can’t tell from this diagram

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
TWO CHARGE THUNKER
◦ two equal and opposite charges
◦ a few antennas
◦ A few volunteers.
The E-Field Worksheet
◦ A – Prediction
◦ B - Verification via
 http://physics.weber.edu/schroeder/software/EField /

x 
4
Prediction

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
We complete the dipole discussion.
We start some interesting but simple experiments.