ELECTRIC FIELD AND MAYBE CHARGE
W2-D1
Answer to a Question• “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)
This WeekToday
◦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
entryWe will see if the system works
properlyClicker Points
◦Correct Answer= 2 points◦Incorrect Answer = 1 point◦No response = 0 points
Let’s see how it works.
123
Click here
Click here
Click here
Click here
Answering Numeric
123
Click here
Answering Numeric
Let’s try it..
Using the A-E buttons, which statement is the most correct?
A. The electric field is a scalar quantity that relates the force on a charge relative to another charge.
B. The electric field is a computational convenience that allows us to normalize Coulomb forces.
C. The electric field is a real scalar property of space.
D. The electric Field is a figment of Dr. Bindell’s mind.
E. None of these are correct.
At a point in space, the electric field is E. A charge is placed at the point.
A. If the charge at the point is doubled, the electric field is doubled as well.
B. If the charge at the point is doubled, the force on the charge that produced the field is doubled.
C. If the charge is doubled, the electric field remains the same.
D. None of these statements are correct.
IMPORTANT
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.
r
x
y
+
-
Note: q1q2 is (-)
origin
SUPERPOSITION!
r1
r points to the position we want to calculate E
r1 points tothe position ofthe charge q1
E2
x
For each charge in the region ..
r1
x
r
1
1
r = r + x
x = r - rx is the vector from the charge to the point of interest.
E(r) 1 1
iq
Thus
Electric Field Mapping
Which Charge Is Bigger?
A B
A A is biggerB B is biggerC Both the sameD You can’t tell from this diagram
ActivityTWO 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/
4x
Prediction
Next TimeWe complete the dipole
discussion.We start some interesting but
simple experiments.