Exam next Friday Chapters Closed book Closed notes Calculators OK

Equipotential Lines = Contours of constant V E field points downhill Downhill is always perpendicular to level Conductors at rest are equipotential

Two equal mass, equal + “test charges” are released from points A and B. When they reach A’ and B’=infinity to the right, which is moving faster? A B A’B’ (The big + and - charge are fixed.) Or choose C: they are moving at the same speed > 0 Or choose D: they are both stationary ANSWER: C

Two conductors in proximity form a “capacitor”: they have a capacity to hold a charge Q (+Q on one and -Q on the other) with a voltage difference V. C=Q/V Note: Capacitance C is italicized. Coulomb C is not. (Sorry!) Capacitance is a property of the geometry of the conductors (and depends on whether there is vacuum or material between them, as we will see later.) This is because of linearity (superposition.) If I double the source charges, I will double the field, and double the potential.

If I put ±2 microcoulombs of charge on the sphere and plate, there is a potential difference of 2 V. What is the capacitance? A] 0 B] Cannot determine, since not symmetric C] 1 microfarad = C/V D] 10 6 farads

If I put ±2 microcoulombs of charge on the sphere and plate, there is a potential difference of 2 V. If I put ±4 microcoulombs of charge on the sphere and plate, what will be the potential difference? A] 0 B] 2 V C] 4 V D] 8 V

Examples: 1. Parallel plate capacitor 2. Spherical capacitor 3. Cylindrical capacitor (Gauss’ Law Redux!)