1. What is the equivalent resistance of the three resistors on in the

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1. 
What is the equivalent resistance of the
three resistors on in the figure at right
A. 3R
D. 2R/3
B. 2R
E. R/3
R
R
C. 3R/2
R
Two resistor R in parallel give Reff = ½ R.
Then, ½ R and R in series à Reff = 3/2 R à correct answer is C
2. 
Which of the following equations applies to the circuit in this
figure
A. I1 + I2 + I3 = 0
C. I1 + I2 – I3 = 0
B.
D.
I1 – I2 + I3 = 0
I1 – I2 – I3 = 0
Consider the node indicated by the red circle in the figure. I1, I2,
and I3 are flowing into the node. No current is flowing out of the node.
Thus I1+I2+I3=0 à correct answer is A
3.  Two copper wires have the same length but the second has half the
diameter of the first. If the resistance of the first is 2 ohms, the
resistance of the second is
A. ½Ω
B. 1Ω
C. ¼ Ω
D. 4 Ω
E. 8Ω
The resistance of a wire is inversely proportional to its cross-sectional
area. The 2nd wire has ½ the diameter, and therefore ¼ of the area.
2/¼ = 8
à correct answer is E
4. 
V1 is the voltage across R1. Which is correct?
Let I be the current. I = V/(R1+R2).
Then V1 = IR1 = VR1/(R1+R2) à correct answer is D
5.  Consider the circuit to the right. The switch S is
closed at time t=0. A very long time later, the potential
drop across the capacitor will be:
A: ε B: 0 C: depends on the value of R
D: depends on the value of C E: depends on the values of R and C
Let VC be the voltage across the capacitor. Kirchoff's law for voltage
says ε = VC + IR. A long time after the switch is closed, the capacitor
is charged and no current flows, ie, I=0. Then the equation above
simply becomes ε = VC
à correct answer is A
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