Tag Archives: Voltage

A2L Item 238

Goal: Reason with Faraday’s law

Source: 283-720 Faraday’s Law

For which of the following is there an induced emf?

  1. A conducting rod is pulled on conducting
    rails that are placed in a uniform magnetic field directed into the
    page.
  2. A conducting loop moves through a uniform magnetic field
    directed into the page.
  3. A conducting loop rotates in a uniform
    magnetic field directed into the page.
  4. A conducting loop moves
    in a magnetic field produced by an infinite current-carrying wire.
  1. A only
  2. A and B only
  3. A and C only
  4. A and D only
  5. B and C only
  6. A, B, and C
  7. A, C, and D
  8. All of them
  9. None of the above

Commentary:

Answer

(3) Viewing the various cases using the Lorentz force law helps students
understand why current flows in those loops experiencing a change of
magnetic flux.

A2L Item 221

Goal: Reason regarding circuits

Source: 283-560 Circuit voltages

Consider the circuit below. How does the voltage difference between
points A and C compare to the voltage difference between points A and D?

  1. VAC < VAD

  2. VAC = VAD

  3. VAC > VAD

  4. Cannot be determined

Commentary:

Answer

(2) It is interesting to find out how many students recognized that the
comparison did not require the use of Ohm’s law. To gauge student
understanding it is interesting to ask questions such as; does any
current flow in the wire between C and D? If C and D are at the same
potential, why does any current flow?

A2L Item 219

Goal: Reason regarding capacitors

Source: 283-545 Adding capacitors in parallel

A capacitor having, C1, is connected to a battery until
charged, then disconnected from the battery. A second capacitor,
C2, is connected in parallel to the first capacitor. Which
statements below are true?

  1. Charge on C1 decreases.
  2. Total charge on C1 and C2 is the same as the original Q.
  3. The total energy stored in both capacitors is the same as the
    original U stored in C1.
  4. The potential (Voltage) across C1 decreases.
  5. All of the above.
  6. Only 1, 2, and 3 are true.
  7. Only 1, 2, and 4 are true.

Commentary:

Answer

(7) Statement #3 is the hardest for students to reason about. This is
most easily decided as incorrect if the two capacitors are taken as
equal.

A2L Item 217

Goal: Reason regarding capacitors and dielectrics.

Source: 283-535 inserting a dielectric changes a capacitor

A capacitor with capacitance C is connected to a battery until charged,
then disconnected from the battery. A dielectric having constant
κ is inserted in the capacitor. What changes occur in the charge,
potential and stored energy of the capacitor after the dielectric is
inserted?

  1. V stays same, Q increases, U increases
  2. V stays same, Q decreases, U stays same
  3. V increases, Q decreases, U increases
  4. V decreases, Q stays same, U decreases
  5. None of the above
  6. Cannot be determined

Commentary:

Answer

(4) It should be clear to students that the charge cannot change. Most
students recognize that capacitance increases when a dielectric is
inserted into a capacitor. The issue then becomes whether they
appreciate the relationships between C, Q, V and U.

A2L Item 218

Goal: Reason regarding capacitors

Source: 283-540 Adding capacitors in series

A capacitor, C1, is connected to a battery until charged, and
then disconnected from the battery. A second capacitor, C2,
is connected in series to the first capacitor. What changes occur in
capacitor C1 after C2 is connected as shown?

  1. V same, Q increases, U increases
  2. V same, Q decreases, U same
  3. V increases, Q decreases, U increases
  4. V decreases, Q same, U decreases
  5. None of the above
  6. Cannot be determined

Commentary:

Answer

(5) All quantities remain the same. Some students may consider the
capacitors to be connected in parallel despite the figure.