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Physics 121 Practice Problem Solutions 09Magnetic Fields

Contents:

121P09 - 4P, 9P, 16P, 35P*, 38P, 40P, 57P, 59P

Overview What Produces Magnetic Field The Definition of B. Force on a moving Charge Crossed Fields: Discovery of the Electron A Charged Particle Circulating in a Magnetic Field

Cyclotron Frequency

The Mass Spectrometer and Cyclotron Magnetic Force on a Current-Carrying Wire Torque on a Current Loop The Magnetic Dipole Moment Crossed Fields: The Hall Effect (Optional Sect. 28.5)

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PROBLEM 121P09-4P*: An electron that has velocity

moves through the magnetic field B = (0.030 T)i - (0.15 T)j . (a) Find the force on the electron. (b)Repeat your calculation for a proton having the same velocity.

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PROBLEM 121P09-9P*: An electron is accelerated through a potential difference of 1.0 kV and

directed into a region between two parallel plates separated by 20 mm with a potential difference of 100

V between them. The electron is moving perpendicular to the electric field of the plates when it enters

the region between the plates. What uniform magnetic field, applied perpendicular to both the electronpath and the electric field, will allow the electron to travel in a straight line?

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PROBLEM 121P09-16P: An electron is accelerated from rest by a potential difference of 350 V. It

then enters a uniform magnetic field of magnitude 200 mT with its velocity perpendicular to the

field. Calculate (a) the speed of the electron and (b) the radius of its path in the magnetic field.

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PROBLEM 121P09-35P*: A wire of 62.0 cm length and 13.0 g mass is suspended by a pair of

flexible leads in a uniform magnetic field of magnitude 0.440 T (See figure ). What are the

magnitude and direction of the current required to remove the tension in the supporting leads?

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PROBLEM 121P09-38P: Consider the possibility of a new design for an electric train. The engine

is driven by the force on a conducting axle due to the vertical component of Earth's magnetic

field. To produce the force, current is maintained down one rail, through a conducting wheel,

through the axle, through another conducting wheel, and then back to the source via the otherrail. (a) What current is needed to provide a modest 10 kN force? Take the vertical component of

Earth's field to be 10 QT and the length of the axle to be 3.0 m. (b) At what rate would electric

energy be lost for each ohm of resistance in the rails? (c) Is such a train totally or just marginally

unrealistic?

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PROBLEM 121P09-40P: A single-turn current loop, carrying a current of 4.00 A, is in the shape of

a right triangle with sides 50.0, 120, and 130 cm. The loop is in a uniform magnetic field of

magnitude 75.0 mT whose direction is parallel to the current in the 130 cm side of the loop. (a)

Find the magnitude of the magnetic force on each of the three sides of the loop. (b) Show that thetotal magnetic force on the loop is zero.

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Problem 121P09 40 page 2

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Problem 121P09 40 page 3

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Problem 121P09 40 page 4

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PROBLEM 121P09-57P:A proton moves at a constant velocity of +50 m/s along an xaxis, through

crossed electric and magnetic fields. The magnetic field is B = (2.0 mT) . What is the electric field?

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PROBLEM 121P09-59P: An electron moves through a uniform magnetic field given by B = Bx

i +

(3Bx)j . At a particular instant, the electron has the velocity v = (2.0 i + 4.0j ) m/s and the magnetic

force acting on it is (6.4 x10-19 N) k . Find Bx.