Phys. 122: Thursday, 17 Sept.

HW 3 returned: in front. (One had no name...) Written HW 4: due by 2:00 pm. Written HW 5: ch. 26, problems 2, 17, 22, 36, 48, and 54.

Due a week from Tues. (if exam 1 Thurs.). Mast. Phys.: assignment 3 due Tuesday. Reading: Should have finished ch. 26 (skipping 26.7);read ch. 27 for next week. Exam 1: will be over material we've covered in chapters 23,

20, 21, and 22, in one week. Study guide and sample formula sheet are available from our class web page. Sample exam problems also available (but skip electric force problems there). Remember: only “dumb” calculators allowed!

In SI units,k is usually written as

The two charges must be different! Otherwise,all charges would fly apart. SI charge unit =“Coulomb” or “Coul” or just “C”.

k=1

(4 πε 0)

[k = 9 x 10⁹ N m²/C²]

Electric Forces: another type of force in Nature

When there are more than two charges, simply add the forces due to each charge pair.

This should remind you of the gravitational field! The main difference is that “charge” replaces “mass” here.

Do not try to use the electric field DUE TOcharge q in this definition! Charges onlyfeel forces due to other charges.

Useful to remember: Electric fields point in theforce direction that a positive charge would feel.

Since total force is the vector sum of theforces, so is the total electric field the sumof the electric fields.

Example: find the electric field on the zaxis due to a uniform ring of total charge Qand radius a sitting in the (x,y) plane.

A Conductor is an object whichhas plenty of charge, and the chargeis free to flow (not stuck in place).

The opposite type of object is called an insulator.

Conductors always have zero electric field in their interior (in equilibrium)!

Examples of conductors: metals, ionized water, plasmas (like inside a fluorescent light).

Clickers: An uncharged conductingsphere is placed in the electric field dueto a point charge q. The total electricforce on the sphere will be...

(a) directed toward the point charge(b) directed away from the point charge(c) perpendicular to the line to the point charge(d) zero(e) Not enough information given to decide.

A rod attracts a positively charged hanging ball. The rod is

A. Positive.

B. Negative.

C. Neutral.

D. Either A or C.

E. Either B or C.

QuickCheck 25.2

Slide 25-38

Clickers: Two conductors are placed inthe same background electric field. One hasrounded ends and the other is sharp (pointy).Which will have the strongest field near it? a) They will be the same if the background E was the same for each b) The rounded one will have a stronger field c) The pointy one will have a stronger field d) Neither one will have any field outside it e) It depends upon how dry or moist the air around is

Clickers: A non-conductor is near acharged conductor. It will...

(a) not feel any force whatsoever(b) feel a repulsive force(c) spin rapidly, remaining in place(d) be initially attracted to the conductor, then repelled once it touches it(e) certainly never happen in the real world

Metal spheres 1 and 2 are touching. Both are initially neutral.a. The charged rod is brought near.b. The spheres are separated.c. The charged rod is then removed.Afterward, the charges on the sphere are:

A. Q1 is + and Q2 is +.

B. Q1 is + and Q2 is –.

C. Q1 is – and Q2 is +.

D. Q1 is – and Q2 is –.

E. Q1 is 0 and Q2 is 0.

QuickCheck 25.4

Slide 25-60

Clickers: It's good to get into anautomobile in a lightning storm whenno other shelter is available. Why?

(a) The rubber and air in the tires are good insulators(b) The metal roof and frame are good conductors(c) The air is a better insulator out of the rain (inside the car)(d) Being in an enclosed space prevents agoraphobia and panic(e) ... so you can listen to the radio (for weather updates, and Justin Bieber).

Faraday Cages

The use of a conducting box, or Faraday cage, to exclude electric fields from a region of space is called screening.

Slide 27-103

Example: E for an infinite charged rod