Physics quiz question: loop alongside loop with increasing current

Physics 205B Quiz 6, spring semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.27(a)

The current in a circular loop is increasing over time. A square metal loop (with a resistance R) is located alongside the circular loop. The direction of current induced in the square loop is:
(A) clockwise.
(B) counterclockwise.
(C) (No direction, as there is no induced current.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The circular loop is the source of an external magnetic field that creates a flux through square loop, which is the considered the test object. Since the current in the circular loop is increasing, its magnetic field and magnetic flux through the square loop will increase over time, which will create an induced current in the square loop according to Faraday's law.

Everywhere inside of the circular loop, its clockwise current will create a magnetic field that points into the page. Everywhere outside of the circular loop, its current will create a magnetic field that points out of the page. This magnetic field will create a magnetic flux that points out the page through the square loop.

The external magnetic flux through the square loop that points into the page is increasing over time, and from Lenz's law, a current is induced in the square loop that will oppose this change. The square loop will then attempt to create its own magnetic field that points into the page, which means that the induced current in the square loop will be clockwise.

Sections 30882, 30883
Exam code: quiz06Lp4s
(A) : 9 students
(B) : 24 students
(C) : 3 students
(D) : 0 students
(No response: 1 student)

Success level: 24%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.28

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2013
Cuesta College, San Luis Obispo, CA
Section 30882, version 1
Exam code: quiz06h4cK



Section 30882 results

0- 6 :
7-12 :	*** [low = 12]
13-18 :	*********
19-24 :	************* [mean = 20.8 +/- 5.4]
25-30 :	******* [high = 30]

Physics quiz question: loop pulled across magnetic field threshold

Physics 205B Quiz 6, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Example 20.1

A square metal loop (of resistance R) moves at constant velocity. An external magnetic field has uniform magnitude and direction in a certain region, and is zero everywhere outside of this region. The direction of current induced in the square loop is:
(A) clockwise.
(B) counterclockwise.
(C) (No direction, as there is no induced current.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

Since the magnetic flux Φ_B through the square metal loop does not change over time, there will be no induced emf and no induced current in the loop. (Alternatively, positive charges in both the left and the right sides of the loop experience upwards forces, but these induced clockwise and counterclockwise currents will effectively cancel out, leaving no net induced current in the loop.)

Section 30882
Exam code: quiz06LR1r
(A) : 12 students
(B) : 10 students
(C) : 5 students
(D) : 0 students

Success level: 19%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38

Physics quiz question: induced emf in inductor

Physics 205B Quiz 6, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.55

An ideal emf source is connected to an ideal resistor and inductor. The switch is closed at t = 0. Just after t = 0, the __________ side of the inductor is at a higher potential.
(A) left.
(B) right.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer: (B)

Just after the switch is thrown at t = 0, increasing current will flow through the inductor from right-to-left. The resulting induced emf in the inductor will oppose this increasing current, resulting in the right side of the inductor being at a higher potential.

Section 30882
Exam code: quiz06LR1r
(A) : 10 students
(B) : 15 students
(C) : 2 students
(D) : 0 students

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.58

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, spring semester 2012
Cuesta College, San Luis Obispo, CA

Section 30882, version 1
Exam code: quiz06LR1r

Section 30882 results
Quiz 6 results (max score = 30):

0- 6 :
7-12 :	** [low = 12]
13-18 :	*****
19-24 :	************* [mean = 22.0 +/- 4.9]
25-30 :	****** [high = 30]

Physics midterm question: LR circuit current

Physics 205B Midterm 2, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.55

[10 points.] An ideal emf source is connected to an ideal resistor and inductor. The switch is closed at t = 0. Determine whether the resistor or inductor has more current flowing through it just after the switch is closed, or if there is a tie. Explain your reasoning using the properties of currents and potential differences, and Kirchhoff's rules, Ohm's law, Faraday's law and/or Lenz's law.

Solution and grading rubric:

• p = 10/10:
  Correct. Due to Kirchhoff's junction rule applied to this series circuit, both the resistor and the inductor must have the same current flowing through them at any given moment in time.
  
• r = 8/10:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  
• t = 6/10:
  Nearly correct, but argument has conceptual errors, or is incomplete. Says different amounts of current, but at least demonstrates an understanding of Lenz's law.
  
• v = 4/10:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x = 2/10:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y = 1/10:
  Irrelevant discussion/effectively blank.
  
• z = 0/10:
  Blank.
  

Grading distribution:
Section 30882
Exam code: midterm02H3nR
p: 3 students
r: 0 students
t: 3 students
v: 2 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7673):

Physics quiz archive: magnetism, induction

Physics 205B Quiz 6, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Section 30882, version 1

Section 30882 results
Exam code: quiz06eMf0

 0- 6 : 
 7-12 : 
13-18 : **  [low = 18]
19-24 : ***  [mean = 23.6 +/- 4.4]
25-30 : ***    [high = 30]

Physics quiz question: loading LR circuit

Physics 205B Quiz 6, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.55.

Consider an ideal emf source connected to an ideal resistor and inductor. The switch is closed at t = 0. After the switch is closed, the current through the inductor __________, while the current through the resistor __________.
(A) decreases to zero; decreases to zero.
(B) decreases to zero; increases to a finite value.
(C) increases to a finite value; decreases to zero.
(D) increases to a finite value; increases to a finite value.

Correct answer: (D)

The junction rule states that the amounts of current passing through the resistor and inductor must all be identical. From inspection, the current I in all parts of this circuit starts from an initial value of 0, and increases to its limiting value of I = emf/R after a long time, when the inductor is no longer reacting to the change in current, such that the amounts of current through both the resistor and inductor increases to a finite value.

Student responses
Section 30822
Exam code: quiz06eMf0
(A) : 0 student
(B) : 5 students
(C) : 1 student
(D) : 2 students

Success level: 25%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67