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Unformatted text preview: General Physics ' . _
Some Practice Problems for Chapter 21 1. An electron moveswith a Velocity of S Mm / s in the X}? planeat an
angle of 30.degrees to the x axis and 60 degrees to the y axis. A uniform magnetic ﬁeld of 1.5 T is in the positive y direction. Findthe force on the electron,.'both._magnitude and direction.  2. A longwire parallel to the x axis carries a current of 14 A flowing in
the positive X direction. There is a uniform magnetic ﬁeld of 8000 G in
the positive y direction. Find the force per unit length on thewire. 3.'An electron of kinetic energy 25 X loo16 Joules moves in circular orbit in'a uniform magnetic field of 0.2 T.
Find the radius. of the orbit. I F. ', a! run . " ' ' I .
Find tlieﬁperiod of the motion. 4. An alpha particle of charge +26 and mass 6.65 x IDA2'7 kg moves in a
circular"path of radius 0.5 m in a magnetic ﬁeld of 1.4 T. '  . Find the period. W
Find the Speed of the alpha particle. _ Find the Kinetic eneiay m theralpha particle. _ Find the frequency of the alpha particle’s motion.
IT ' ' 5. A beam of protons moves along the x axis in the positive x direction with a speed of 10 km / 5 through a region of crossed ﬁelds.
If thereis a magnetic ﬁeld of magnitude 0.8 T in the positive y direction,
find the magnitude and direction of the electric field such that the beam is undeﬂected.
.._"ns v‘lw Would a beam'oI electrons traveling at the same speed be deflected by
these crossed ﬁelds? If so? in what direction? 6. A velocity selector has a magnetic ﬁeld of magnitude 0.2 T perpendicular to an electric field of magnitude 0.4 MV/ 111.
What kinetic energy must a proton have to pass through undeﬂected? ‘ _ . :v r I ‘J
What kinetic energy must an electron have to pass through undeflected? _ 7. A long straight wire carries a current of 60 A that is directed
perpendicularly into the plane of your paper. The wire is positioned at
the origin of a set of x—y axes. Find the magnitude and direction of the magnetic ﬁeld due to this wire at the location (2 m, 0 m). '8. The lengths of two 10ng straight wires both run perpendicularly to the
plane of your paper. One is placed at (—6cm, 0cm) on a set of Xy axes and the other at (+6em, 0cm]. The current in each wire has a magnitude of 20A.
If the currents in each Wire are ﬂowing into your paper, find the magnitude and direction of’the net magnetic ﬁeld at (—3ern, 0cm).
h  . \ a . c _. .
Ii the current in the wire at ( 6cn'ii', 'O‘CIn) ﬂows into your paper, and the
current in the other wire flows out of your paper ﬁnd the net magnetic ﬁeld at (Ocm 0cm). At (00111, 8cm). 9.F11'1Cl the magnitude of the force per unit length exerted by one wi1e 011
the other in problem 8. 10 As shown, three long parallel straight wires pase through the corners
of an equilateral triangle of side 10.0 cm. The length of each of the wires runs'perpendicuiarly to the plane of your paper. The currents ﬂow as shown, each of magnitude 15 A.
Find the force per unit length on the upper wire. Also, give the direction of this force. 4
. Find the net magnetic ﬁeld experienced by the upper wire due to the two
lewer Wires (magnitude and direction]   11. You are given the wire as shown. The two vertical segments are 5 cm
long" each and the three horizontal segments are 25 cm long each. A
current of 7.5 A that exists in the wire runs from bottom to top. Find the
net force on the bottom most horizontal segment of this Wire. _ 12. What magnetic ﬁeld would be needed to keep an electron (mass —
9.1 1x10A 31 kg) m a circular path of 5 cm radius at a speed of 3 X 10A5 m/s? What 13 the magnetic moment of the electron in this path? ,....I‘ I ‘2: 'l‘ " ' I _ .7 r .,
If'youb wanted to Re eep this electron going in this same exact motion, but
you no longer wished to use the above magnetic ﬁeld, what charge would you put at the center of this electron’ s circular path? 10. A mass spectrograph uses a velocity selector consisting of parallel
plates separated by 2.0 mm and having a potential difference of 160 V.
The magnetic ﬁeld crossed between these plates is 0.42 T. Beyond this
. velocity selector in the mass spectrograph is a magnetic ﬁeld of 1.2 T.
Find the separation distance of the peaks on the spectrograph’s
photographic ﬁlm for the ions, U238 (q=+1 and mass = 3.95 X IDA—25 kg)
and U235 “PH and mass = 3.90 X IDA—25 kg). . 14. A very long straight wirecaries a current of 20 A. An electron is 1 cm
from the centerof the wire and is moving with a speed of 5 X 10A6 m/ s. _
Find the force on the electron when it moves: a. directly away from the wire, in a path perpendicular to the Wire’s
length. “ b. parallel to the wire in the direction ”of the current. [J _:.r _c._ perpendicular to the wire and tangent to a circle around the wire. ll ...
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 Spring '08
 Matilisky
 Physics, Work

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