2007 AP Physics B Circuit Question?

Question

From
http://apcentral.collegeboard.com/apc/public/repository/ap07_physb_frq_kw.pdf
I took the AP Physics B test and was unsure of my answers to question #3, as circuits in general have always been my weak point, although many thrive when learning such a concept. I would truly appreciate it if someone could provide assistance by detailing the generic procedure that should have been followed. I always balk when no specific values are given for each variable.

Answer 1

Start by doing some reduction on the circuit. The parallel resistors to the right have Req as

Req=2*R/3

Next, looking at the current at the node where the three resistors all connect, the only way for current to flow to RC and RB is through RA, so RA has the highest current IA, which is the sum of IB and IC

Next, the voltages across RB and RC are equal. Using v=I*R, the smallest R results in the higghest current, so RC is higher than RB

For the voltages, RB and RC have the same voltage since they are in parallel. Sine the equivalent reistance of the pair is 2*R/3, this is less than RA's voltage.

For c, since I have the parallel as 2*R/3, the equivalent in series with the voltage is
2*R/3+2*R
or
8*R/3

Current in RC. Start by the voltage across RC by summing around the loop:
IN this case one current IA:
The current is related by E=IA*8*R/3
IA=E*3/(8*R)
E=2*R*IA+Vc
where vc is the voltage across RC
substituting in IA
E=2*R*E*3/(8*R)+Vc
simplify
E=E*3/4+vc
Vc=E/4
The current
IC=E/(4*R)

Actually this is not a transient, it is steady state. So the first step is to determine the voltage on VC. In this case the capacitor will stabalize at the same voltage with zero current once charged

The current IA in this case is E/(3*R)
the voltage on RC is E/3 (4 volts)

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Answer 2

There is a circuit with a load on it. There is also a 10 ohm resistor, a voltmeter, and an ammeter. What would happen if we added lots more wire to the circuit? More wire increases the resistance. Your circuit has no voltage or current source, so there is no voltage or current. If I assume you just forgot to mention it, you also have to say where in the circuit you want to measure voltage and current. The circuit is complicated, a voltage source, a 10Ω resistor, another resistor (the load), a voltmeter and an ammeter. There are many different ways they can be connected together. "what would happen if the circuit didn't have a resistor" again, depends on how the circuit is wired. Which resistor, there are apparently two? "don't how a short circuit would affect the entire circuit" depends on where in the circuit the short is applied. edit, and where are the two resistors and how is the ammeter and voltmeter connected to them. You did mention two resistors, actually one resistor and a load.