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Unit 9 AP Practice Problems ---Magnetism
BEarth = 5 x 10-5 T
1.
2.
3.
*A student holds a bar magnet in each hand. If both hands are brought close together, will the force be attractive or
repulsive if the magnets are held so that (a) the two N-poles are brought close together and (b) an N-pole and an Spole are brought together? (Repulsive, Attractive)
The figure to the right shows five disk magnets floating above each other. The N-pole of the top
most disk faces up. Which poles are on the top-side of the other magnets? ( S, N, S, N)
The figure to the right shows a magnet attracting a nail
to it which, in turn, has attracted many paper clips that are
attached to the nail.
a. Which end of the nail is the N-pole? (the head)
b. Which ends of the three paper clips (from top to bottom) are the N-poles? (the
tops)
c. Which of the items are permanent magnets? (only the bar magnet)
d. Which of the items are temporary magnets? (the nail and the paperclips)
1st and 2nd Right Hand Rules
4. *A long, straight wire has a (+) current traveling East through it. What is the direction of the magnetic field around
the wire? (down in front of the wire, up in back of the wire)
5. *A cross-section of a long, thin wire with a (+) current in it is to the right. The current
is shown as coming out of the page (toward your face). What is the direction of
the magnetic field around the wire? (counter-clockwise)
6.
*What if the cross-section of wire had the (+) current moving into the page (away from your face)? (clockwise)
7.
*A student makes a magnet by winding wire around
a large nail as shown in the figure to the
right. The magnet is connected to the battery as
shown. Which end of the nail, the pointed end or
the head, will be the N-pole? (the pointed end)
8. The diagram to the left shows a current (+) carrying wire wrapped around a piece of
iron to create an electromagnet. The right hand in the diagram is exhibiting the use of the
2nd Right-Hand Rule. What is the direction the (+) current in the wire is moving? (down
in back, up in front---same as #7)
9.
What is the direction of the magnetic field around the coiled loop of
wire in the diagram to the right? (to the left inside of the coil,
exiting the left side of the coil, looping back around and
entering the coil in the right-side; the left side of the coil acts as
an N-pole and the right side of the coil acts as a S-pole of a bar
magnet)
Unit 9 AP Practice Problems
Magnetism
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The Magnitude of the Instantaneous Force of the Magnetic Field (F = BIL sin θ and F = Bqv sin θ)
10. *A straight wire 0.10 m long carrying a current of 2.0 A is at right angles to a magnetic field. The force on the wire is
0.04 N. What is the strength of the magnetic field? (0.2 T)
11. A wire 0.50 m long carrying a current of 8.0 A is at right angles to a 0.40 T magnetic field. How strong a force acts
on the wire? (1.6 N)
12. A wire 75 cm long carrying a current of 6.0 A is at right angles to a uniform magnetic field. The magnitude of the
force acting on the wire is 0.60 N. What is the strength of the magnetic field? (0.13 T)
13. A copper wire 40 cm long carries a current of 6.0 A and weighs 0.35 N. A certain magnetic field is strong enough to
balance the force of gravity on the wire. What is the strength of the magnetic field? (0.146 T)
14. *An electron passes through a magnetic field at right angles to the field at a velocity of 4.0 x 10 6 m/s. The strength of
the magnetic field is 0.50 T. What is the magnitude of the force acting on the electron? (3.2 E -13 N)
15. A stream of doubly-ionized particles (missing two electrons and thus carrying a net positive charge of two elementary
charges) moves at a velocity of 3.0 x 104 m/s perpendicular to a magnetic field of 9.0 x 10 -2 T. What is the magnitude
of the force acting on each particle in the stream? (8.64 E -16 N)
16. Triply-ionized particles in a beam carry a net positive charge of three elementary charge units. The beam enters a 4.0
x 10-2 T magnetic field moving perpendicular to it. The particles have a velocity of 9.0 x 10 6 m/s. What is the
magnitude of the force acting on each particle? (1.73 E-13 N)
17. A force of 5.78 x 10-16 N acts on an unknown particle traveling at a 90 angle through a magnetic field. If the velocity
of the particle is 5.65 x 104 m/s and the field is 3.20 x 10-2 T
a. what charge does the particle carry? (3.2 E -19 C)
b. How many elementary charges does this represent? (2 e)
18. *A proton is moving with a speed of 2.3  105 m/s when it encounters a magnetic field of 0.80 T. The direction of the
magnetic field makes an angle of 50.0° with respect to the velocity of the proton. What is the magnitude of the
magnetic force on the proton? (2.26 E -14 N)
19. A charged particle is fired from a gun with a velocity of 5.2  104 m/s at an angle of 35° with respect to a 0.0045-T
magnetic field. If the magnetic field exerts a force of 0.0026 N on the particle, determine the magnitude of the charge.
(1.94 E -5 C)
3rd Right-Hand Rule
20. *Can the magnetic field do work on a charged particle? (No, the force of B is always perp. to the charge’s velocity)
21. *A proton traveling due west in a region that contains only a magnetic field experiences a vertically upward force
away from the surface of the earth. What is the direction of the magnetic field? (south)
22. *A horizontal, current-carrying wire runs east-west through Earth’s magnetic field. If the + current flows east, in
which direction is the force from the magnetic field acting on the wire? (For this problem, assume the earth’s
magnetic field runs directly in the north-south geographic plane) (up, away from the Earth’s surface)
23. *Picture a magnetic field directed into the plane of the page and a positive charge moving to the right across the paper,
perpendicular to the magnetic field. How should an electric field be oriented so that the force it applies to the charge
could balance the force by the magnetic field? (the electric field should be directed toward the bottom of the
page)
24. A proton traveling west enters the earth’s magnetic field. Which direction will the proton be deflected? (For this
problem, assume the earth’s magnetic field runs directly in the north-south geographic plane) (down, toward the
surface of the earth)
25. An electron traveling due south enters a region that contains a uniform magnetic field that points due west. In which
direction will the electron be deflected? (up, out of the surface of the page)
26. What if the particle in the previous problem was a proton? (down, into the surface of the page)
27. In a television, a beam of electrons from a cathode ray tube in the back of the set approaches the screen while passing
through the deflecting magnets on the surface of the interior of the box. The N-pole is at the top of the box, the S-pole
on the bottom. If you are looking at the screen from the outside, in which direction are the electrons deflected? (to the
left side of the screen as you look at it)
28. A proton traveling due north enters a region that contains both a magnetic field and an electric field. The electric
field lines point due west. It is observed that the proton continues to travel in a straight line due north. In which
direction must the magnetic field lines point? (up, out of the paper)
Unit 9 AP Practice Problems
Magnetism
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Additional (read that as fun) Practice Problems with circles, capacitors, two straight wires and other stuff
29. *A proton is released from the positive plate of a parallel plate capacitor and accelerates to the right toward the
negative plate. The proton then exits the right side of the capacitor through a small hole in the negative plate. The
electric potential of the positive plate is 2100 V greater than the negative plate. Once outside the capacitor, the proton
enters a region of a constant magnetic field of 0.10 T. The velocity of the charge is perpendicular to B, which is
directed out of the page. Find
a. the speed of the proton when it leaves the negative plate of the capacitor (hint, think conservation of energy).
(6.34 E 5 m/s)
b. The radius and direction of the circular path on which the proton moves in the magnetic field. (0.066 m,
clockwise)
30. An electron moves at a speed of 6 E 6 m/s to the right perpendicular to a constant magnetic field that is directed into
the page. The path is a circle of radius 1.3 E –3 m.
a. Draw a sketch showing the electron’s path. (clockwise circle)
b. What is the magnitude of the field? (2.63 E-2 T)
c. Find the magnitude of the electron’s acceleration. (2.58 E 16 to 2.77 E 16 m/s/s depending on rounding)
31. An ionized He atom has a mass of 6.6 E –27 Kg and a speed of 4.4 E 5 m/s. The atom moves perpendicular to a 0.75
T magnetic field on a circular path of radius 0.012 m. Determine if the charge is +e or +2e. (2 e)
32. A beam of protons moves in a circle of radius 0.25 m. The protons move perpendicular to a 0.30 T magnetic field.
a. What is the speed of each proton? (7.19 E 6 m/s)
b. Determine the magnitude of the centripetal force that acts on each proton. (3.45 E -13 N)
33. A charged particle with a charge to mass ratio of q/m = 5.7 E 8 C/Kg travels on a circular path that is perpendicular to
a magnetic field whose magnitude is 0.72 T. How much time does it take for the particle to complete one revolution?
(1.53 E -8 sec/rev)
The strength of the magnetic field some radius away from a current carrying wire
B
0 I
2  r
where μ0 = permeability of free space, 4π x 10-7 T m/A
34. * Suppose you measure the strength of the magnetic field a distance of 1 cm from a 3 A current-carrying wire and
calculate it to be a certain value. What is
a. the strength of the field 2 cm from the wire. (1/2 as much  3 E -5 T)
b. the strength of the field 3 cm from the wire. (1/3 as much  2 E -5 T)
35. What is the strength of the magnetic field 2 cm away from a straight wire of current 0.34 amp? (3.4 E -6 T)
36. *Two wires parallel to one another are separated by 1m. Each carries a current of 3A, but in opposite directions. Find
the magnitude of the magnetic field at a point midway between the wires. (2.4 E -6 T)
37. Repeat the above problem if the current in the wires was moving in the same direction. (zero T)
38. *A long, straight wire with a conventional West to East current of 3.0 A through it has a positive charge (6.5 E –6 C)
moving parallel to it at a distance of r = 0.050 m directly beneath it. The speed of the charge is 280 m/s and is also
directed East. What is the magnitude and direction of the magnetic force exerted on the charge by the current in the
wire? (2.18 E -8 N, up toward the wire)
39. A +6 E –6 C charge is moving with an Eastward velocity of 7.5 E6 m/s parallel to and directly above a long straight,
horizontal wire. The wire carries a conventional current of 67 A in the opposite direction as the moving charge and is
5 E –2 m away from the charge. Find the magnitude and direction of the force on the charge. (1.21 E -2 N, away
from the wire)
40. Two parallel wires are separated by a distance of 0.80 m and carry currents of 25 A and 35 A. Find the net magnetic
field (magnitude and direction) at a point mid-way between the two if the currents are
a. In the same direction. (5 E-6 T)
b. In opposite directions. (3 E -5 T)
Unit 9 AP Practice Problems
Magnetism
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