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.

IMPORTANTIMPORTANT 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 Er1 points tothe position ofthe charge q1

E2

x

For each charge in the region ..

r1

x

r

1

1

r = r + xx = r - r

x 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.