Announcements Reminders: No lectures next week: Feb 15 th and Feb 17 th Quiz II: March 3rd, Ch 25,26, most of 27. {tentative}

Exam Grades Median: 85.5 % {high B+} Mean: 82.8% {low B+} Stdev: 13.2% Questions: or ask in office hrs

Exam Histogram

Exam Comments Most Common issues: Field does NOT depend on the magnitude of the test charge, but does on the distance. The force from a field on a charge depends on the sign of the charge. Not all oscillatory motion is simple harmonic motion! On each exam, there should be: Score, Exam Grade Overall Grade The overall grade is what you would have now. If there are two grades this indicates that you are at an interface.

Capacitor Two conductors carrying charges of equal magnitude but opposite sign form a capacitor. +Q -Q A parallel plate capacitor is a particularly common type: +Q-Q Capacitors have a voltage across them

Capacitor: Field Lines +q -q Perfect parallel plate capacitors have a uniform field. And of course a potential difference

Capacitors and Charge Capacitors store charge Capacitance is measure of how well it stores charge Capacitance is a constant for a given capacitor. Farad (F) is C/V, the unit of capacitance Typical capacitances are in  F or pF ranges.

Capacitors and Charge Capacitance is a constant for a given capacitor. The voltage (i.e. potential difference) used is this equation is positive. C depends only on the geometry of the capacitor! Note: the material doesn’t matter (except in between the plates} If I change the voltage (for a given geometry), I change the charge. If the charge changes, than the voltage must have changed

Finding Capacitance a b +Q+Q-Q-Q So we need to find the field the find the voltage (use Gauss law, withthe sphere as the gaussian surface )

Finding Capacitance a b +Q+Q-Q-Q

Finding Capacitance: isolated sphere a b +Q+Q-Q-Q a b +Q+Q-Q-Q The isolated sphere is the limit as b-> infinity of the concentric spheres

A Parallel Plate Capacitor d area A Dielectric constant 

Circuits: First glance wire (conductor) capacitor switch battery +– We will learn how to combine these elements and others as the class progresses. +– This is what is right here!

Batteries An ideal battery creates a voltage difference between the two sides +– E A battery can be used to charge a capacitor, and maintain a constant voltage.

Combining Capacitors wire (conductor) capacitor switch battery +– In Series: C1C1 C2C2 C3C3 C1C1 C2C2 C3C3 In Parallel: Same Voltages Voltages are the same across each capacitor Charges are the same on each capacitor

Each of the Capacitors above has a capacitance of 12  F. What is the combined capacitance of the whole system? A)12  FD) 36  F B)6  FC) 4  F Combining Capacitors

For complex problems, capacitors may be combined successively in pairs. 4  F 8  F 6  F 12  F6  F

Each of the Capacitors above has a capacitance of 12 pF. What is the combined capacitance of the whole system? A)12 pFC) 8 pF B)4 pFD) 20 pF