Physics final exam problem: shunted ammeter readings

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA


A 4.5 V emf source is connected to two light bulbs (each with different resistances), two ammeters, and a switch. All of these components are ideal. Discuss why the ammeters have unequal readings while the switch is closed, and have equal readings after the switch is opened. Show your work and explain your reasoning using the properties of ammeters, Kirchhoff's rules and Ohm's law.

Solution and grading rubric:

• p:
  Correct. Explicitly explains/demonstrates that the ammeters in the closed-switch circuit have unequal readings, and the ammeters in the open-switch circuit have equal readings by:
  1. applying Kirchhoff's junction rule to both circuits; and/or
  2. explicitly calculating the currents that flow through each of the ammeters in both circuits.
• r:
  Nearly correct, but includes minor math errors. For the closed-switch circuit, may instead compare the different amount of currents flowing through each light bulb, instead of through each ammeter.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x:
  Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying Kirchhoff's rules, Ohm's law, and properties of ammeters.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 12 students
r: 14 students
t: 5 students
v: 3 students
x: 2 students
y: 2 students
z: 1 student

A sample "p" response (from student 1157):