Brock University
Physics 1P22/1P92
Winter 2015
Dr. D’Agostino
Tutorial 5 (Chapter 21: Magnetic Forces and Magnetic Fields)
Questions
Purpose: To understand magnetic forces and fields and how they interact with moving charged
particles and electric currents.
1. At the Earth’s equator the Earth’s magnetic field is horizontal and points North. In which
direction is the force exerted by the Earth’s magnetic field on a proton moving
(a) vertically upward.
(b) vertically downward.
(c) horizontally East.
(d) horizontally West.
(e) horizontally South.
(f) horizontally North.
2. Repeat the previous question for an electron.
3. Three particles are moving perpendicular to a uniform magnetic field and travel along circular
paths, as shown in the figure. The particles have the same mass and speed. List the particles
in order of the magnitudes of their charges.
4. How should two long, straight, current-carrying wires be oriented in space so that neither
exerts a magnetic force on the other?
5. How should two current-carrying loops be oriented in space so that neither loop exerts a
magnetic force on the other?
6. How can you produce a magnetic field that is nearly constant over a significant region of
space?
7. How can you produce isolated North and South poles from a bar magnet?
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Problems and Sandbox
Purpose: To understand magnetic forces and fields and how they interact with moving charged
particles and electric currents.
1. Two charged particles move in the same direction with respect to the same magnetic field.
Particle 1 travels 3 times faster than Particle 2. However, each particle experiences a magnetic
force of the same magnitude. Find the ratio |q1 |/|q2 | of the magnitudes of the charges.
2. Suppose that an ion source in a mass spectrometer produces doubly ionized gold ions (Au2+ ),
each with a mass of 3.27 × 10−25 kg. The ions are accelerated from rest through a potential
difference of 1.10 kV. Then, a 0.680-T magnetic field causes the ions to follow a circular path.
Determine the radius of the path.
3. A charged particle moves through a velocity selector at a constant speed in a straight line.
The electric field of the velocity selector has magnitude 4640 N/C, and the magnetic field
has magnitude 0.328 T. When the electric field is turned off, the charged particle travels on
a circular path whose radius is 3.45 cm. Find the charge-to-mass ratio of the particle.
4. A uniform magnetic field points vertically upward. A charged particle can be projected from
a “cannon” with a certain initial speed and with components of velocity East and upwards.
A small target is painted on the South-facing wall of a nearby building, which is within the
region of space that has the magnetic field. The Earth’s magnetic field can be ignored.
(a) The magnetic field strength is 1.2 T, and the target is 1.8 m above the ground. Choose
the initial position of the cannon, the tilt angle of the cannon, and the initial projection
speed of an electron so that the electron hits the target.
(b) Repeat Part (a) in general. That is, obtain expressions for the initial position of the
cannon, the tilt angle of the cannon, and the initial projection speed of an electron so
that the electron hits the target in terms of the magnetic field strength B and the height
H of the target above the ground.
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