Download Name: Class Period: Classwork: Electric field February 13, 2017

Name: ______________________________________________________ Class Period: ______________
Classwork: Electric field
February 13, 2017
Multiple choice. On the space, write the letter that corresponds to the correct answer.
_____1. The electric field created by a moving negative charge
A. points toward the charge.
B. points away from the charge.
C. describes concentric circles about the direction of motion.
D. points in the direction of motion
_____2. Comparing the electrostatic force and the gravitational force we can say that
A. both have the same dependence on distance, both involve attraction and repulsion but the
gravitational force is stronger.
B. both have the same dependence on distance, both involve attraction and repulsion but the electrostatic
force is stronger.
C. both have the same dependence on distance, the electrostatic force can be either attractive or repulsive
while the gravitational force is only repulsive, and the electrostatic force is weaker.
D. both have the same dependence on distance, the electrostatic force can be either attractive or
repulsive while the gravitational force is only attractive, and the electrostatic force is stronger.
_____3. Imagine four equal positive charges, q, placed on points of a circle of radius r at the 3, 6, 9 and 12
o’clock positions. The magnitude of the electric field at the center of the circle is given by
A. 4kq/r2.
B. 14 kq/r2.
C. kq/4r2.
D. zero.
_____4. Three equal negative charges are placed at three of the four corners of a square. The direction of
the electric field at the remaining corner of the square is
A. along a side of the square toward one of the charges.
B. along a side of the square away from one of the charges.
C. along the diagonal connecting this corner and another charge, away from the other charge.
D. along the diagonal connecting this corner and another charge, toward the other charge.
_____5. The number of electric field lines passing through a unit cross sectional area is indicative of:
A. field direction.
C. field strength.
B. force direction.
D. sign of the charge
_____6. A free electron is in an electric field. With respect to the field, it experiences a force acting:
A. parallel.
C. perpendicular.
B. anti-parallel (opposite in direction).
D. along a constant potential line.
Solve the following problems. Show your work to earn the credit. Each problem is worth 5 points: 1 point
– correctly label ALL given, 2 points – write the correct equation and plug-in the correct values, 2 points –
correct numerical answer, with correct sign, unit or direction.
1. An electric field of 260000 N/C points due west at a certain spot. What are the magnitude and
direction of the force that acts on a charge of -6.50 x 10-6 C at this spot?
2. A +3.00 μC test charge is placed in a 20,000 N/C field. What force does it experience?
3. A test charge experiences an electric field of 400,000 N/C at a point 1.00 cm away. What is the
magnitude of the charge producing the field?
4. Suppose you want to determine the electric field in a certain region of space. You have a small
object of known charge and an instrument that measures the direction of the force exerted on an
object by the electric field.
a. The object has a charge of +20.0 C and the instrument indicates that the electric force
exerted on it is 40.0 N, due east. What are the magnitude and direction of the field?
b. What are the magnitude and direction of the electric field if the object has a charge of -10.0
C and the instrument indicates that the force is 20.0 N, due west?
5. A small drop of water is suspended motionless in air by a uniform electric field that is directed
upward and has a magnitude of 8480 N/C. The mass of the water drop is 3.50 x 10 -10 kg. What is
the magnitude and sign of the charge on the drop of water?
6. A proton and an electron are moving due east in a constant electric field that also points due
east. The electric field has a magnitude of 8.00 x 104 N/C. Determine the magnitude of the
acceleration of the:
a. proton and the
b. electron.
7. At a distance of 1.0 meter from an isolated point charge the electric field strength is 100 N/C. At
what distance (in m) from the charge is the electric field strength equal to 50 N/C?
8. The magnitude of the electric field 300 m from a point charge Q is equal to 1,000 N/C. What is the
charge Q (in C)?
9. A neon sign includes a long neon-filled glass tube with electrodes at each end across which an
electric field of 20,000 N/C. is placed. This large field accelerates free electrons which collide with
and ionize a portion of the neon atoms which emit red light as they recombine. Assuming that
some of the neon ions (mass 3.35 x 10-26 kg) are singly ionized (i.e. have charge of 1.6x10-19 C) and
are accelerated by the field, what is their acceleration?
10. Two point charges, +8.0 nC and +2.0 nC are separated by 6.0 m. What is the magnitude of the
electric field midway between them?
11. Two point charges, +4.0 C and -1.0 C, lie on the x-axis. If the +4.0 C lies at the origin while the
-1.0 C is at x = 1.0 m, at what point (or points) on the x-axis is the net electric field zero?
1979B7. Two small spheres, each of mass m and positive charge q, hang from light
threads of lengths l. Each thread makes an angle with the vertical as shown above.
a. On the diagram draw and label all forces on sphere I. b. Develop an expression for the charge q in terms of m, l, , g, and the Coulomb's
law constant. 1981B3. A small conducting sphere of mass 5 × 10–3 kilogram, attached to a string of
length 0.2 meter, is at rest in a uniform electric field E, directed horizontally to the right
as shown above. There is a charge of 5 × 10–6 coulomb on the sphere. The string makes
an angle of 30° with the vertical. Assume g = 10 meters per second squared.
a. In the space below, draw and label all the forces acting on the sphere. b. Calculate the tension in the string and the magnitude of the electric field. c. The string now breaks. Describe the subsequent motion of the sphere and sketch on the
following diagram the path of the sphere while in the electric field.