## 20101108

### Physics quiz question: ammeter in circuit

Physics 205B Quiz 5, Fall Semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.65

An ideal 12.0 V emf source is connected to ideal resistors and an ideal ammeter, as shown at right. The ammeter reading is:
(A) 1.2 A.
(B) 2.2 A.
(C) 3.4 A.
(D) 8.0 A.

Correct answer: (B)

Writing out the loop rule for the bottom loop, starting from the junction left of the battery:

Summation(delta(V))_loop = 0 = +(12.0 volts) - I*(1.5 ohms) - 0 - I*(4.0 ohms),

where the ideal ammeter does not cause a voltage drop. Solving for the current through the lower loop:

I = +(12.0 volts)/(1.5 ohms + 4.5 ohms) = 2.1818... amps ≈ 2.2 amps.

Response (A) is (12 volts)/(10.0 ohms); response (C) is the current that passes through the battery; response (D) is (12.5 volts)/(1.5 ohms).

Student responses
Section 70856
(A) : 2 students
(B) : 5 students
(C) : 3 students
(D) : 1 student

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00