25-1) Two parallel metal plates are charged. (A "capacitor"!)
y
+++++++++++++++++++++++++++++++++++++++
E
x
d
+
------------------------------------
PE=0
r
There is a uniform electric field E =-E yˆ between the plates.
A proton, charge q = +e, is lifted by tweezers a distance d.
• What direction is the force on the charge due to the E-field?
GREEN: up
PINK: down
YELLOW: 0
• The work done by the E-field is …
GREEN: +
PINK: YELLOW: 0
PURPLE: Not enough info.
• The work done by the external agent (tweezers) is …
• Change in potential energy is : DPE = +Wext = -Wfield .
If we define PE(proton) = 0 at the bottom plate, then
PE(proton) near the top is...
†
25-2) A positive test charge +q is carefully moved by some
external agent at constant speed
E
between two capacitor plates, as
shown.
+q
The work done by the agent, done by
the electric field, and done by the net
force on the book are:
Agent
E-Field
Net Force
BLUE
GREEN
PINK
YELLOW
+
+
+
+
-
+
0
0
PURPLE:
None of these/wish I knew
25-3) A negative charge -q is moved from position i to
E
position f between the plates of a
charged capacitor.
Did the potential energy (PE)
increase or decrease?
f
Did the voltage (V) at the position
of the test charge increase or
decrease?
PINK:
GREEN:
BLUE:
YELLOW:
PURPLE:
PE increased and V decreased.
PE decreased and V increased.
PE increased and V increased.
PE decreased and V decreased.
None of these.
What if the test charge were positive +q?
i
-q
25-4) A small positive test charge is initially at rest in an
electric field, and is free to move. Which way will the charge
start to move?
BLUE: Moves towards higher potential.
PINK: Moves towards lower potential.
PURLE: Not enough information given.
• What is your answer for a small negative test charge?
25-4B) A charge of +q can be moved from an initial point (i)
to a final point (f) by several different paths.
Which path requires the largest amount of work?
E
(i)
BLUE
+q
PINK
(f)
YELLOW
PURPLE: All paths require the same total work.
25-5) Two test charges are brought separately into the vicinity
of a fixed charge +Q.
First, +q is brought to point A, "r" away.
Second, -q is brought to the same point.
+q
+Q
Situation 1:
A
r
+Q
-q
Situation 2:
r
(Assume that
PE=0 and V=0 at
infinity.)
A
The electrostatic potential, V, at point A is ...
BLUE:
Greater for the +q charge in situation 1
YELLOW: Greater for the -q charge in situation 2
PURPLE: The same for both.
Potential energy of the test charge in situation A is...
BLUE:
Greater than in situation B
GREEN:
Smaller than in situation B
YELLOW: Same for both.
25-6) Two test charges are brought separately into the vicinity
of a fixed charge +Q.
1: +q is brought to point A, "r" away.
2: +2q is brought to point B, "2r" away.
+q
+Q
Situation 1:
A
r
+Q
+2q
Situation 2:
2r
(Assume that PE=0
and V=0 at
infinity.)
B
The electrostatic potential, V at A in situation 1 is ...
BLUE:
greater than
YELLOW: smaller than
PURPLE: equal to
the electrostatic potential at B in situation 2.
The potential energy of the test charge in situation 1 is
BLUE:
greater than in situation 2
YELLOW: smaller than in situation 2
PURPLE: equal to situation 2
25-7) In the figure, there are 4 point charges (same
magnitude, signs shown) arranged in a square.
(Assume potential V=0 at infinity)
At which of the "mid-points" (A, B, C) is the potential 0?
BLUE:
A (only)
GREEN:
B (only)
YELLOW:
C (only)
PINK:
B and C
PURPLE:
None of
these/not enough info
+Q
A
C
-Q
B
+2q
+Q
-Q
25-8) Four charges, all of the same magnitude Q, are arranged
in a square as shown. The two top charges are positive, the
two bottom charges are negative.
The magnitude of the electric field at A is...
GREEN: non-0
A
PINK: 0
+
Q
The voltage at A is...
GREEN: +
PINK: B
YELLOW: 0
+
Q
C
The voltage at C is...
-Q
The voltage at B is...
-Q