Cuesta College, San Luis Obispo, CA

Solution and grading rubric:
- p:
Correct. Explains why the voltmeter reading will decrease while the ammeter reading will increase as the amount of pencil lead between the contacts is shortened by discussing:- the decrease in the resistance of the pencil lead resistor will reduce the equivalent resistance of the circuit (pencil lead and internal resistance are in series), such that from applying Ohm's law the amount of current passing everywhere through the circuit will increase, resulting in a higher ammeter reading; and
- the voltmeter measures the potential difference of the 6.0 V rise from the emf and the voltage drop I⋅r from the internal resistance, such that an increase in current will result in a lower voltage reading ΔV = +ε − I⋅r.
(May instead discuss how the voltmeter is equivalently measuring the voltage
drop ΔV = −I⋅R across the pencil lead resistor, but must clearly show that the
eight-fold decrease in the resistance (from 8.0 Ω to 1.0 Ω) will be larger than
the corresponding approximate four-fold increase in current (0.65 A to 2.7 A) to
result in a lower voltage reading.)
- r:
Nearly correct, but includes minor math errors. - t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least numerically or qualitatively demonstrates how current would increase, but does not definitely show why voltmeter reading would decrease. - v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at applying Kirchhoff's rules, Ohm's law, and equivalent resistance. - x:
Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Kirchhoff's rules, Ohm's law, and properties of ammeters and voltmeters. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Sections 30882, 30883
Exam code: midterm02GruT
p: 12 students
r: 0 students
t: 8 students
v: 8 students
x: 1 student
y: 0 students
z: 0 students
A sample "p" response (from student 1412):

Another sample "p" response (from student 2643):

A sample "x" response (from student 9319):

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