Download Electric and Magnetic Fields Review Questions

Ch. 7 and Ch. 8 review
1. The electrostatic force between two small charged objects is 5.0  10–5 N. What effect would each
of the following changes have on the magnitude of this force, considered separately?
(a) The distance between the charges is doubled.
(b) The charge on one object is tripled, while the charge on the other is halved.
(c) Both of the above changes occur simultaneously.
2. Calculate the force between charges of 5.0  10–8 C and 1.0  10–7 C if they are 5.0 cm apart.
3. Two equal charges of magnitude 1.1  10–7 C experience an electrostatic force of 4.2  10–4 N. How
far apart are the centres of the two charges?
4. What is the electric field intensity 0.50 m away from a small sphere with a positive charge of
1.6 10–8 C?
5. The electric field intensity in the region between two parallel plates is 400 N/C. If the plates are
connected to a battery with a potential difference of 90 V, what is the separation of the plates?
6. Calculate the magnitude of the electric field in a parallel plate apparatus whose plates are 5.0 mm
apart and have a potential difference of 300 V between them.
7. What potential difference would have to be maintained across the plates of a parallel plate
apparatus, if they are 1.2 cm apart, to create an electric field of intensity 1.5  104 N/C?
8. In a Millikan type experiment, two horizontal plates are 2.5 cm apart. A latex sphere of mass
1.5  10–15 kg remains stationary when the potential difference between the plates is 460 V, with the
upper plate positive.
(a) Is the sphere charged negatively or positively?
(b) What is the magnitude of the electric field intensity between the plates?
(c) Calculate the magnitude of the charge on the latex sphere.
(d) How many excess or deficit electrons does the sphere have?
9. An electron is released from rest adjacent to the negative plate in a parallel plate apparatus. A
potential difference of 500 V is maintained between the plates, and they are in a vacuum. With what
speed does the electron collide with the positive plate?
10. What potential difference would accelerate a helium nucleus from rest to a kinetic energy of
1.9  10–15 J? (q = +2e, for a helium nucleus)
11. What is the strength of the magnetic field 15 cm from a long, straight conductor carrying a current of
100 A?
12. What is the magnetic field strength in the core of a solenoid 5.0 cm long, with 300 turns and a
current of 10 A?
13. What is the magnetic field strength in the core of a coil 10 cm long, with 420 turns and a current of
6.0 A?
14. A coil 8.0 cm long is composed of 400 turns of wire, and produces a magnetic field of strength
1.4  10–2 T in its core. What is the current flowing through the coil?
15. What is the force between two parallel conductors 2.0 m long, carrying currents of 4.0 A and 10.0 A,
that are 25 cm apart in a vacuum?
16. What is the magnetic force between two parallel wires 50 m long and 10 cm apart, each carrying a
current of 100 A in the same direction?
17. A proton, of mass 1.67  10–27 kg, moves in a circle in the plane perpendicular to a uniform 1.8 T
magnetic field. If its radius of curvature is 3.0 cm, what is its velocity?
18. A singly ionized
ion, of mass 3.9  10–25 kg, is accelerated through a potential difference of
1.0  105 V.
(a) What is its maximum velocity?
(b) What is the radius of the path it would take if injected at 90° into a 0.10 T uniform magnetic field
at this velocity?
19. An electron, with mass of 9.1  10–31 kg and charge of 1.6  10–19 C, is accelerated to a velocity of
4.0  106 m/s, then enters a uniform magnetic field of 5.0  10–3 T at an angle of 90° to the field.
(a) What is the radius of the circular path it follows?
(b) Through what potential difference was the electron accelerated?
20. Charged spheres A and B are fixed in position, as shown, and have charges of +4.0  10–6 C and –
2.5  10–7 C, respectively. Calculate the net force on sphere C, whose charge is +6.4  10–6 C.
21. What is the magnitude and direction of the electric field at point Z in the diagram below, due to the
charged spheres at points X and Y?
Ch. 7 and Ch. 8 review: Answer Section
1. (a)
(b)
(c)
2. ANS:
3. ANS:
4. ANS:
5. For parallel plates:
6. ANS:
7. ANS:
8. ANS:
(a) The electric force must be up, to balance the downward force of gravity. Since the upper plate is
positive, the latex sphere must be charged negatively, to be attracted to the upper plate and
repelled by the lower plate. The electric field is downward, giving an upward force on a negative
charge.
(b) For a parallel plate apparatus:
This field is directed downward.
(c) When the sphere is balanced,
(d)
There is an excess, since the charge was negative.
9. ANS:
10. ANS:
11. ANS:
12. ANS:
13. ANS:
14.
15. ANS:
16. ANS:
17. ANS:
18. ANS:
(a)
(b)
19. ANS:
(a)
(b)
Note: For a magnetic field pointing into the page, the electron would move in a clockwise circle in
the plane of the page. Of course, a positive particle would move in a counterclockwise circle in the
same field.
20. Since all three charges are in a straight line, we can take the vector nature of force into account by
assigning forces to the right as positive. Sphere C has forces acting on it from spheres A and B.
For the force from sphere A:
And for the force from sphere B:
The net force acting on sphere C is the sum of
21.
and