Tag Archives: Potentials

A2L Item 216

Goal: Hone the relationship between E and V

Source: 283-475 Must V=0 if E=0?

True or false: it is possible to have the electric field be 0 at some
point in space and the electric potential be non-zero at that same
point.

  1. True
  2. False

Commentary:

Answer

(1) Whichever answer students give, ask them to draw a charge
configuration which satisfies their answer. Often this is sufficient to
cause them to change their mind. If appropriate raise for discussion the
case of the interior of a uniformly charged spherical shell.

A2L Item 171

Goal: Reason with electrical potential

Source: 283-470 Lowest voltage at origin

Which of the following charge distributions has the lowest electric
potential at the origin?

Commentary:

Answer

(5) This question serves to motivate a discussion of the
difference between potential energy in a configuration and the
electrostatic potential at a point. It is also important to stress that
potential when point charges are involved presumes that infinity is the
reference point.

A2L Item 150

Goal: Link work and potential

Source: 283-465 Interpreting voltage

The potential at two points in space is: V1=200 Volts,
V2=300 Volts. Which of the following statements is true for
moving a point charge, q, from point 1 to 2?

  1. The work done by an external agent to
    move q from point 1 to 2 is positive.
  2. Can’t determine the work
    done because you don’t know the direction of V at the two points.
  3. The work done by the electrical force exerted on q in moving it from
    point 1 to 2 is: W = -q(100 Volts).
  1. A
  2. B
  3. C
  4. A and B
  5. A and C
  6. A, B, and C
  7. None of the above

Commentary:

Answer

(3) is the best response. Statement A is true only if the charge
is positive. The important thing is to see if students correctly
envision the electric field lines as directed from higher potential to
lower.

Any response involving statement B should be discussed thoroughly as it
indicates confusion between fields and potentials.

A2L Item 147

Goal: Relate force to potential energy function

Source: UMPERG-ctqpe66

The
potential energy function for a certain body is shown at right. For
which points is Fx = 0?

  1. Point 1 only
  2. Points 2 and 4
  3. Points 1 and 5
  4. Points 3 and 5
  5. Point 3 only
  6. Fx not equal to 0 anywheree

Commentary:

Answer

(4) The force is given by the negative of the slope of the
potential function. Points 3 and 5 are equilibrium points, but only 3 is
a stable equilibrium point. Students frequently choose answer #2
thinking that the force is zero at these points.

A2L Item 134

Goal: Link work and potential change

Source: 283-450 Move q, do most work

For
the following situations consider moving a positive charge from very far
away to the origin along the y-axis. For which situation would you do
the most work?

  1. A
  2. B
  3. C
  4. D
  5. A & C
  6. None of the above.
  7. Cannot be determined

Commentary:

Answer

(1) Students indicating #7 because they do not know if the masses
are charged should not be disconfirmed. If students key on magnitude
only they will likely choose answer #5.