**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.

- True
- False

**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.

- True
- False

**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?

(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.

**Goal:** Link work and potential

**Source:** 283-465 Interpreting voltage

The potential at two points in space is: V_{1}=200 Volts,

V_{2}=300 Volts. Which of the following statements is true for

moving a point charge, q, from point 1 to 2?

- The work done by an external agent to

move q from point 1 to 2 is positive.- Can’t determine the work

done because you don’t know the direction of V at the two points.- The work done by the electrical force exerted on q in moving it from

point 1 to 2 is: W = -q(100 Volts).

- A
- B
- C
- A and B
- A and C
- A, B, and C
- None of the above

(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.

**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 F_{x} = 0?

- Point 1 only
- Points 2 and 4
- Points 1 and 5
- Points 3 and 5
- Point 3 only
- F
_{x}not equal to 0 anywheree

(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.

**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?

- A
- B
- C
- D
- A & C
- None of the above.
- Cannot be determined

(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.

## 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.