Conductors & Electric Fields Class Exercises
AP Physics
Warm Up:
A. Point P is located 12 m north of a 0.8 mC charge and 8 m west of a
–0.5 mC charge.
i) What is the net electric field at point P?
ii) A free proton is placed at point P. What is the electric force acting on the proton?
iii) If the mass of a proton is 1.673 x 10-27 kg, how quickly will the proton accelerate?
B. Two square plates, 50 cm on a side, are laid horizontally, with one suspended 5 cm over the other.
Electrons are moved from one plate to another until one plate has acquired a significant negative charge
and the other plate an equal positive charge. A free electron is placed between the plates and
experiences an upward force of 2.3 x 10-15 N.
i) What is the electric field between the parallel plates?
ii) What is the net charge on each plate?
New Stuff:
1. How will a free charge react to a non-zero electric field at its location?
2. a) How are conductors different from insulators?
b) How do the electrons in a conductor respond when you place it in an external electric field?
3. For static situations, where no charges are moving, what must be true about the electric field inside a
conductor?
4. Imagine that we charge up two parallel plates, 80 cm long and
40 cm wide with a net charge of +/– 450 C, respectively (see
diagram).
a) What is the electric field between the two plates?
b) What will happen when you place a 5 cm x 5 cm x 1 cm
sheet of metal horizontally between the plates?
c) How strong is the net field inside the metal plate after
the charges are done moving? How do you know?
d) How much charge must accumulate on each side of the metal plate before the charges stop moving?
5. A set of square parallel plates, 30 cm on a side and 5 cm apart, are charged until the top plate holds
+ 50 C of charge and the bottom plate holds –50 C of charge.
a) What is the electric field between the two plates?
b) A 10 cm wide, 5 cm long and 2 cm thick piece of copper is placed horizontally between the parallel
plates. How much charge accumulates on the top and bottom of the copper piece?
Now let’s consider a piece of conductor that is not just polarized, but charged.
6. Using conduction, you transfer a charge of 0.5 C onto a 10 cm radius, spherical metal ball sitting on
a perfectly insulating surface.
a) Where are the ball will the excess
b) What would happen if any of the excess
electrons be located?
electrons ended up in a different location?
7. a) What would the electric field around the charged ball in question 6, look like? Why?
b) How strong would the electric field be 5 cm from the center of the ball?
c) How strong would the electric field be 15 cm from the center of the ball?
d) How strong would the electric field be 25 cm from the center of the ball?
8. A 950 nC point charge sits at the center of a thick, spherical, hollow,
conducting shell, as shown in the picture. The situation is allowed to
come to equilibrium.
a) How much charge is on the inner surface of the shell?
b) How much charge is on the outer surface of the shell?
c) Draw the electric field lines on the diagram.
9. You take a 1 cm thick hollow, spherical, aluminum shell with a 25 cm
radius and place a –180 C charge at the center, as shown in the diagram.
a) How much charge accumulates on the inner and outer surface of the
shell?
b) Draw the electric field lines for this situation.
c) What is the electric field strength at a point 10 cm from the center of the sphere?
d) What is the electric field strength at a point 25 cm from the center of the sphere?
e) What is the electric field strength at a point 40 cm from the center of the sphere?
10. When you place a net charge on a spherical, conducting object, the charge spreads evenly over the
surface of the object, as shown in the left hand picture. Would anything change if you placed the charge
on an elongated object, such as that in the left hand picture, instead? Why or why not?