☠  Phys. 122: Thursday, 29 Oct. ☠ Written HW 9: due by 2:00 pm. Written HW 10: ch. 31 prob. 70, and ch. 32, probs. 26, 34, 58, 60, and 68. Due in one week. Mast. Phys.: assign. 6 due (moved to) Tuesday.. Reading: Finish ch. 32 by Tuesday (may skim sects. 32.5,32.9). Midterm Grades: breakdowns have been posted to Canvas; grades there represent values from Oct. 16. I will re-post current up-to-date grades early next week, after exam 2 has been graded. Last day to change grade options (to pass/fail or withdraw) is Wednesday, Nov. 4. Clicker registrations: for some of you have not gone through; for those students an average of unclaimed clicker scores was used for midterm grades. Please check your to see whether you were affected by this (and how to fix it).

Clickers: All resistors are equal below. Which circuit has the most current flowing through its battery? a) a b) b c) c d) a and b e) They will all have the same current.

Clickers: Rank these resistor networks from lowest to highest total resistance. a) a, b, c, d b) b, c, d, a c) c, d, a, b d) b, d, c, a e) a, c, d, b

Clickers: A parallel-plate capacitor is charged with a battery to a potential ∆ V, and the battery is then disconnected. What happens to the potential across the capacitor if the plates are then moved farther apart? a) It goes down (but not to zero). b) It goes up. c) It does not change at all. d) It goes down to exactly zero. e) It switches sign (from positive to negative).

Kirchhoff's Junction Rule: the sum of all currents entering any junction equals the sum of all currents leaving!

You can't keep going downhill (in voltage) in a full loop! (Unless you live inside an M. C. Escher drawing.)‏ Formally, this is known as “Kirchhoff's Loop Rule.”

Kirchhoff's Loop Rule: The sum of voltage drops around any closed loop must be zero.

Clickers: Which of the following statements correctly describes the voltage drops in this circuit, before the switch is closed? (The capacitor is initially uncharged.) a) The battery voltage will be across C only b) The battery voltage will be across R only c) The battery voltage will be across the switch only d) The battery voltage will be the same as that across C and across R e) The battery voltage will be the sum of that across C and R

Clickers: Which of the following statements correctly describes the voltage drops in this circuit, after the switch is closed? (The capacitor is initially uncharged.) a) The battery voltage will be across C only b) The battery voltage will be across R only c) The battery voltage will be across the switch only d) The battery voltage will be the same as that across C and across R e) The battery voltage will be the sum of that across C and R

The fluid (water) analogy... updated Electrical Thing Charge Voltage Electric Field Capacitor Current (in a wire)‏ Resistor Battery (source of EMF)‏ Fluid thing Fluid (water)‏ Pressure Pressure difference Water tank Current (in a pipe)‏ Pipe filled with sand Pump (can make the fluid flow “uphill”)‏

Why ever use the BIG battery, if the small one can provide the same Δ V? The answer is internal resistance: the larger battery has less, and so it can provide more current at the same voltage.

Rule for resistors: The voltage drops in the direction of current flow! This applies to internal resistance of batteries as well. However, a charging battery has the current flowing in the opposite direction, and its terminal voltage will be larger than the open-circuit (no current) value!

Clickers: The battery below has the internal resistance shown. What will a voltmeter across the battery terminals read? a) 12 V b) 9.5 V c) 14.5 V d) 0 V e) 125 A

Clickers: Two unequal batteries, a 9 V and a 6 V one, are connected in parallel with no other circuit elements. What determines the direction current will flow? a) The battery voltages b) The batteries' internal resistances c) The wire's actual resistance d) The capacitance of the wires e) On days like today, I do. (I rule!)

Clickers: if the EMF of battery 2 is equal to the EMF of battery 3, the current through R ₂ will be... a) upward b) downward c) zero d) alternating e) dependent upon the EMF of battery 1

Example: Using Kirchhoff's circuit rules If a multiple-EMF circuit such as this has incompatible voltages (as would be true here without R 1 and R 2 ), you must include the resistance of the wires and the batteries to make sense out of the diagram.

Clickers: Ohms ( Ω ) times Farads (F) equals... a) Megatons b) Amps c) Coulombs d) Kilometers times Volts e) Seconds

Example: A resistor and capacitor together

For a discharging capacitor, charge and voltage across it drop with time exponentially. (Here, they grow with time as it's charging.) ‏