Chapter 15: Electric Forces and Electric Fields
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ELECTRIC FORCES AND
ELECTRIC FIELDS
PROBLEMS
Electric fields point in the direction of the force on a small positive test charge
1. Two point charges of magnitude 3.0 × 10–9 C and 6.0 × 10–9 C are separated by a distance of 0.30 m. Find the electric
force of repulsion between them.
3. Two point charges of equal magnitude repel each other with a force of 2.00 N when separated by 5.00 cm. Find the
magnitude of the charge on each.
*4. The total charge on two spheres is 600 µC. The two spheres are placed 0.900 m apart and the force of repulsion
between the two is 30.0 N. What is the charge on each sphere?
5. The electric force on a point charge of 5.0 × 10–9 C at some point is 3.8 × 10–3 N in the positive x direction. What is
the magnitude of the electric field at this location?
6. The magnitude of the electric field at a certain location is 500 N/C and the field is directed east to west. (a) Find the
magnitude and direction of the force acting on a proton placed at this point. (b) Repeat for an electron located at the
point.
7. Find the magnitude and direction of the electric field at a distance of 10.0 cm from an electron.
8. The electric field at a distance of 0.800 m from a certain charge is found to have a magnitude of 200 N/C. What is the
magnitude of the charge?
9. What is the strength of the electric field that would give a proton an acceleration equal to that of gravity?
11. A proton is accelerated from rest by an electric field of 400 N/C. (a) Find its acceleration, (b) its speed after 10–8 s, and
(c) its kinetic energy at this time.
12. A proton is shot vertically upward with a speed of 2.00 × 105 m/s in a downward-directed electric field of 500 N/C.
How high will it rise?
14. An object with a net charge of 24 µC is placed in a uniform electric field of 610 N/C, directed vertically. What is the
mass of this object if it "floats" in this electric field?
15. A constant electric field directed along the positive x axis has a strength of 2000 N/C. Find (a) the force exerted on the
proton by the field, (b) the acceleration of the proton, and (c) the time required for the proton to reach a speed of 1.00 ×
106 m/s, assuming it starts from rest.
Answers:
1. F = 1.8 × 10–6 N
3. Q = 7.45 × 10–7 C
5. E = 7.6 × 105 N/C (in +x direction)
*4. Q1 = 4.53 × 10–6 C and Q2 = 5.95 × 10–4 C
6.(a) F = 8.00 × 10–17 N (force is in westward direction)
(b) The magnitude of the force is the same, but the direction is eastward for the electron.
7. E = 1.44 × 10–7 N/C (toward electron)
8.q = 1.42 × 10–8 C
11. (a) a == 3.83 × 1010 m/s2 (b) v = 383 m/s
12. a = 4.79 × 1010 m/s2
9.
E = 1.02 × 10–7 N/C
(c) KE = 1.23 × 10–22 J
H = 4.18 × 10–1 m = 41.8 cm 14.m = 1.5 × 10–3 kg = 1.5 g
15. (a) F = 3.20 × 10–16 N (+x direction)
(c) t = 5.23 × 10–6 s
(b) a = 1.91 × 1011 m/s2 (+x direction)