☠ phys. 122: thursday, 29 oct. ☠ written hw 9: due by 2:00 pm. written hw 10: ch. 31 prob. 70,...

☠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-postcurrent up-to-date grades early next week, after exam 2 has beengraded. 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 formidterm grades. Please check your email to see whether you wereaffected by this (and how to fix it).

Clickers: All resistors are equal below.Which circuit has the most current flowingthrough its battery?

a) a b) b c) c d) a and be) They will all have the same current.

Clickers: Rank these resistor networks fromlowest 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 chargedwith a battery to a potential ∆V, and the batteryis then disconnected. What happens to thepotential across the capacitor if the plates are thenmoved 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 allcurrents entering any junction equals the sumof 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 voltagedrops around any closed loop must be zero.

Clickers: Which of the following statementscorrectly 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 voltagewill be across the switchonly d) The battery voltagewill be the same as thatacross C and across R e) The battery voltagewill be the sum of thatacross C and R

Clickers: Which of the following statementscorrectly describes the voltage drops in this circuit,after the switch is closed? (The capacitor isinitially uncharged.)

a) The battery voltagewill be across C only b) The battery voltagewill be across R only c) The battery voltagewill be across the switchonly d) The battery voltagewill be the same as thatacross C and across R e) The battery voltagewill be the sum of thatacross 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 onecan provide the same ΔV? 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 thedirection of current flow! This applies tointernal resistance of batteries as well.

However, a charging battery has the current flowingin the opposite direction, and its terminal voltage willbe larger than the open-circuit (no current) value!

Clickers: The battery below has the internalresistance shown. What will a voltmeter acrossthe battery terminals read?

a) 12 Vb) 9.5 Vc) 14.5 Vd) 0 Ve) 125 A

Clickers: Two unequal batteries,a 9 V and a 6 V one, are connected inparallel with no other circuit elements.What determines the direction currentwill 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 equalto the EMF of battery 3, the current throughR₂ 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-EMFcircuit such as thishas incompatiblevoltages (as wouldbe true here withoutR1 and R2), you mustinclude the resistanceof the wires and thebatteries to make senseout 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 voltageacross it drop with time exponentially. (Here,they grow with time as it's charging.)