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Chapter 19 Discussion
January-03-15
8:58 PM
Electric Potential Energy and Electric Potential
Reading Review
1: What is gravitational potential energy?
2: What is electric potential energy?
3: What is electric potential? How is it related to electric potential
energy?
4: What is the SI unit for electric potential?
5: What is an electron volt? How is it related to the corresponding SI
unit?
6: Does a superposition principle apply for electric potential?
7: What is an equipotential surface?
8: How can you tell the magnitude of an electric field by looking at its
equipotential surfaces?
9: How can you tell the direction of an electric field by looking at its
equipotential surfaces?
10: Describe the equipotential surfaces for
(a) a point charge.
(b) an electric dipole.
(c) a parallel-plate capacitor.
Discussion Questions
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Discussion Questions
Questions 1 and 2 review work in general; the rest of the questions
deal with electric forces specifically.
1: A particle moves from point A to point B in the figure, while two
constant forces act on it as shown. The magnitudes of the two forces
are equal, but their directions are opposite.
(a) Is the work done by force F1 while the object moves from point A
to point B positive, negative, or zero? Explain.
(b) Is the work done by force F2 while the object moves from point A
to point B positive, negative, or zero? Explain.
(c) Is the net work done by both forces while the object moves from
point A to point B positive, negative, or zero? Explain.
(d) Is the speed of the object greater at point A or point B? Explain.
2: Repeat Question 1 if the directions of the two forces are
unchanged, but the magnitude of F1 is greater than the magnitude of
F2 .
3: A positively charged particle B is released from rest near a
positively charged particle A. Particle A is fixed in place.
(a) Describe the subsequent motion of Particle B. Include a sketch.
(b) Describe how the potential of Particle B changes as it moves.
(c) Describe how the potential energy of Particle B changes as it
moves.
(d) Describe how the speed of Particle B changes as it moves.
(e) Describe how the acceleration of Particle B changes as it moves.
(f) Does the electric force exerted by Particle A on Particle B do
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(f) Does the electric force exerted by Particle A on Particle B do
positive work, negative work, or zero work on Particle B as Particle B
moves? Explain.
4: Repeat Question 3 if both particles are negatively charged.
5: Repeat Question 3 if the particles are oppositely charged.
6: Particle A is positively charged and fixed in place. Particle B is also
positively charged. Particle B is moved in a circular path by some
external forces, and the centre of the circle is at the location of
Particle A.
(a) Does the electric force exerted by Particle A on Particle B do
positive work, negative work, or zero work on Particle B as Particle B
moves?
(b) Repeat Part (a) if both particles are negatively charged.
(c) Repeat Part (a) if the particles are oppositely charged.
7: Particle A is positively charged and fixed in place, and Particle B is
negatively charged. Particle B starts at position 1 with some initial
velocity, and the only force acting on Particle B is the electric force
exerted on it by Particle A. Later Particle B reaches position 2.
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(a) Is the potential higher at point 1, higher at point 2, or the
same at both points? Explain?
(b) Is the electric potential energy of Particle B higher at point 1
or point 2? Explain.
(c) What do you think the path of Particle B is like? It's hard to
say, but can you say anything about the path? For example, do
you think the path is a straight line or curved?
(d) Is the speed of Particle B greater at position 1 or at position
2? Explain.
(e) If you knew the speed of Particle B at position 1, what
minimum additional information would you need to be able to
calculate the speed of Particle B at position 2?
8: Consider the parallel-plate capacitor in the figure.
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(a) Is the electric potential at position 1 greater than, less than, or
equal to the electric potential at position 2? Explain.
(b) If a charged particle is given the right initial velocity at point 1 so
that it reaches point 2, and only the electric force from the capacitor
acts on the particle, will the particle's speed at position 2 be greater
than, less than, or equal to its speed at position 1? Explain.
(c) To calculate the potential difference between points 1 and 2, what
is the minimum information you need?
(d) If you knew the speed of the particle at position 1, what minimum
additional information would you need to be able to calculate the
speed of the particle at position 2?
9: You have a capacitor with capacitance C. You place a dielectric with
dielectric constant 3.1 between the electrodes of the capacitor. Does
the capacitance of the capacitor change? If so, why? If so, exactly
what is the change?
10: You have a capacitor with capacitance C connected to a battery
with voltage V. You place a dielectric with dielectric constant 1.7
between the electrodes of the capacitor. Does the charge on each
plate of the capacitor change? If so, why? If so, exactly what is the
change?
11: You have a capacitor with capacitance C connected to a battery
with voltage V. Once the capacitor is fully charged, you disconnect the
battery. Then you place a dielectric with dielectric constant 1.7
between the electrodes of the capacitor.
(a) Does the charge on each plate of the capacitor change? If so, why?
If so, exactly what is the change?
(b) Does anything change when the dielectric is inserted? If so, what
changes, and why?
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