(1) I have completed at least 50% of the reading and study- guide assignments associated with the lecture, as indicated on the course schedule. a) Trueb) False iClicker Quiz

I have a copy of the text "Physics for Scientists and Engineers Vol 2" by Serway, and I have purchased the current course packet and the Formulas & Chapter Summaries booklet from the bookstore. a) Trueb) False iClicker Quiz

I registered my quiz transmitter via the course website (not on the iclicker.com website). I realize that until I do so, my quiz scores will not be recorded. a) Trueb) False iClicker Quiz

I have downloaded and studied my personal Physics 220 homework data sheet and the numeric response guide, and now understand how to correctly submit online homework responses. a) Trueb) False iClicker Quiz

A cat slides down a rubber pole and falls into metal pail A, which rests on a wooden shelf. The impact breaks the shelf, causing metal pails B and C, which were in contact, to separate and fall to the floor. What is the final charge on pail C? a) Positiveb) Negativec) Zero iClicker Quiz

Coulomb’s torsion balance

E 32 E 31

At what position x will the Coulomb forces acting on the charged gray bead be balanced?

E - F + F F = q E Depending on the sign of the charge, the force experienced by a charged particle in an electric field can either point with or against the field.

Fields due to individual point charges

Fields due to multiple charges

The field magnitude is smallest at point: A, B, C?The field magnitude is greatest at point: A, B, C? iClicker Quiz

Electric monopoles We sometimes refer to the total charge Q of an extended charged object as its “monopole moment”. When you back far enough away, any charged object, regardless of its shape, looks like a simple point charge. In the expression for the electric field of a monopole, the vector r originates from the “center of charge”.

Electric dipoles Equal and opposite charges of magnitude Q = |Q + | = |Q  |. r  + points from the negative charge to the positive charge. Like r  +, the dipole moment vector p always points from the negative charge to the positive charge. We now ask “What is the electric field produced by a dipole?” Define dipole moment: p

Moments of a distribution (extra details for purists) Monopole moment (scalar) Dipole moment (vector) Choose origin at the center of charge (like center of mass) Quadrupole moment (polar rank-2 tensor) The electric field of a charge distribution can be expressed as a series expansion involving successively higher moments of the distribution. (If you tell anyone that I mentioned this, I’ll deny it!)

p E Off-axis dipole field Serway 23.6 p

p E On-axis dipole field Binomial approximation +Q+Q-Q-Q E d = 2a p x

Try using the general dipole formula (only for the purists among you) Perpendicular case On-axis case

See interactive applet at

Calculating the electric force/field due to a continuous charge distribution

Find at the center of a hemispherical shell of uniform charge density.

The electricity vs. gravity analogy Newton’s Gravitational Force Law M m Coulomb’s Electric Force Law Q q

E-field perpendicular to conducting surfaces

E =  /  0 = 4πk  Electric field strength at the surface of a charged conductor is proportional to local surface charge density. E-field perpendicular to conducting surfaces