Download PHYS 221 Midterm Practice Summer 2012

PHYS 221 Midterm Practice
Summer 2012
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A charged, metal object has a cavity inside it into
which is placed two charges, one of Q and the other
of −3Q. There is a net charge of −Q on the object
itself. What is the net charge on the outside surface
of the metal object?
Consider a insulating sphere of radius R centered on the origin with total charge Q uniformly distributed throughout the volume of the sphere. What
is the electric field E inside the sphere at a distance
r from the origin? (i.e. r < R)
HINT: Vsphere = 43 πr3 and Asphere = 4πr2 .
(a) 0
(a) E = 0
(b) −Q
(b) E = kQ/r2
(c) E = kQr3 /R2
(c) −3Q
(d) E = kQr/R3
(d) −5Q
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Consider two charged spheres, one with charge +2 C
and the other with −2 C. A proton is at the point
halfway between the spheres. What is not zero?
One electrons (-e) and two protons (+e) are placed
an equal distance r away from the origin in the configuration shown. If another proton were placed at
the origin, what is the electric force felt by that proton.
(a) the potential energy of the proton
(b) the work to move the proton from infinity to
that point
(a) F = 0
(c) the force on the proton
(b) F = ke2 /r2
(c) F = −ke2 /r2
(d) F =
(d) all of the three above are zero
4ke2 /r2
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A 5 C charge moves through a region of constant
electric field. If the electric field is 20 N/C and the
net displacement of the electron is 3 m, what is the
change in the potential energy of the electron?
(Assume that the displacement is in the direction of
the field.)
(a) 1 J
Three parallel wires carry a current of I1 = 3 A, I2 =
2 A, and I3 = 1 A, respectively. They are arranged as
shown. If r = 2 m and L = 5 m, what is the magnitude
of the net force on the bottom wire?
(b) 12 J
(c) 33 J
(d) 300 J
(a) 0 N
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(b) 5 × 10−7 N
A resistor R2 is placed in series with another resistor R1 .
How does the resulting equivalent resistance Req compare
with the individual resistances?
(c) 2 × 10−7 N
(d) 3 × 10−7 N
(a) Req > R1 and Req > R2
(b) Req < R1 but Req > R2
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(c) Req < R1 and Req < R2
(d) Not enough information given
An experiment needs a magnetic field of 2 T. I decide
to build a solenoid with a length of 5 cm and a radius of
2 mm. I can supply 10 A to the solenoid. What is the
total length of wire I need to build my solenoid?
(Assume the solenoid is comparitavely long and treat
each turn of the wire as a seperate loop.
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(a) 400 m
(b) 100 m
(c) 300 m
(d) 200 m
Consider the circuit shown. If ε = 1.5 V, R1 = 3 Ω,
and R2 = 5 Ω, what is the magnitude of the current
through R1 ?
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(a) 0.50 A
(b) 0.80 A
A 1.5 V battery, a 100 Ω resistor, a 8 H inductor, and a
switch are all connected in series. The switch is initially
open. After the switch is closed, how long will it take for
the current in the circuit to reach 50% of its maximum
value?
(c) 0.30 A
(d) 0.19 A
(a) 160 ms
(b) 80 ms
(c) 55 ms
(d) 40 ms
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A series LC circuit in a radio is in resonance with AM
1200 kHz. If the radio station is changed to AM 600
kHz, by what factor must the capacitance be multiplied
to again achieve resonance?
An electron and a proton are each released from rest in the
same uniform electric field. The electron moves a distance
delectron and the proton moves a distance dproton as
each particles kinetic energy increases by 1.6 eV. How do
delectron and dproton compare?
(a) 4
(a) delectron > dproton
(b) 2
(b) delectron = dproton
(c) 1/2
(c) delectron < dproton
(d) 1/4
(d) The answer depends on the direction of the electric
field
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Three loops of wire, one circular, one rectangular, and
one square, are made from identical lengths of wire. If
the loops are in the same increasing magnetic field perpendicular to the plane of the coils, which loop has the
least induced emf?
(a) the square one
(b) the rectangular one
(c) the circular one
(d) All three would have the same emf induced
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An electron is released such that its initial velocity is from
right to left across this page. The electrons path, however, is deflected in a direction toward the bottom edge of
the page due to the presence of a uniform magnetic field.
What is the direction of this field?
(a) from bottom edge to top edge of the page
(b) into the page
(c) from right to left across the page
(d) out of the page
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The unit of electric current, the ampere, is equivalent to
which of the following?
(a) V/Ω
(b) V/s
(c) Ω · m
(d) V · Ω
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Answers
(1) D
(2) C
(3) C
(4) C
(5) D
(6) A
(7) B
(8) A
(9) B
(10) C
(11) A
(12) B
(13) D
(14) A
(15) B
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