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Unformatted text preview: Department of Physics, Lehigh University Physics 21 ~— lntroductory Physics ll Spring 2011
Hour Exam 2 March 30, 2011
Closed Notes 4:10  5:10 PM
Student’s Name Recitation Leader’s Name The test is a multiplechoice examination. First, check to be sure that you have a sixpage
examination, including the equation sheet. Please answer each question by writing the letter
corresponding to your choice in the answer area. Answers marked outside the answer areas will
not be scored. Problem 1: An electron with velocity 1.0 x 104 m/s in the x direction enters a uniform magnetic
field of magnitude 250 mT in the z direction. 1) What is the direction of the magnetic force on the electron?
a) in the x direction b) in the y direction
c) in the z direction d) in the *y direction Answer: 2) What is the magnitude of the force?
a) 4.0 x 1013 N b) 4.0x10'16N
c) 1.6x10'12N 10
d) 1.6x10“15N Answer: 3) How long does it take for the electron to make a complete circle?
a) 1.43 x 1010 s
b) 1.43 x 10"8 s
c) 2.86 x 10‘8 s
d) 2.86 x 10103 Answer: a A 0.1 g wire of length L = 25.0 cm is suspended by a pair of ﬂexible leads in a uniform magnetic
field of magnitude 0.40 T. 4) What is the magnitude of the current required to remove the tension in the supporting leads?
a) 1.96 x 10‘1 A
b) 9.8 x 10'2 A
c) 1.96 ith'2 A 0‘
d) 9.8 x 10'3 A Answer: 5) A magnetic dipole with a dipole moment of magnitude 0.020 HT is located in a uniform magnetic
field of magnitude 60 mT. The angle between the direction the dipole moment and the magnetic field
is 30°. What is the torque on the dipole? a) 1.2 x103 Nm b) 6.0 x10"4 Nm 0) 3.0X10'4Nm B
d) 1.04 x 10'3 Nm Answer: Problem 2: The figure shows the cross section of a long hollow conducting cylinder with inner radius a and
outer radius b. There is a total uniform current i running through the cylinder. 1) What is the magnitude of the magnetic field at a radius r<a? a)B=“°”' b)B=“—°" c)B=~i‘—°i d} 0 21m 21m 2717' 2} What is the magnitude of the magnetic field at a radius a<r<b? ' 2—2 ‘  2 . _
why—r :2) biB=ﬂc “2) clB=ﬂ$ d) Bzﬂi (rat) 21W 132— 2m" bz—a2 21W Zm' (b—a)2 ANSWEi' q, 3) What is the magnitude of the magnetic field at a radius r>b? a)B=“°” b)B=“—°" c)B=‘Lﬂ d) o ZTET 27th 27:13 Answer: C an "(ﬂ
. ' 2
““ f,
t, . r
N
c" 4) For the above figure, what is the total magnetic field at point C? _ﬂ§ _#_oi Lei _#_ai
a)B— 8R b)B _ 4R +2” c)B— 2R d) 0 Answe r: a 5) What is the direction of the magnetic field at point C? aiinto the page b)out of the page c)no direction d)neither a,b, or C Di Answer: wmuwmwwmwm—memu.WM, . WW WW” Problem 3 A circular coil of wire in the plane of the page, with radius r, has a uniform magnetic field B
directed out of the page and passing through it. 1) What is the magnetic flux through the coil?
21) a: r2 B ‘ b) rB
c) r232 (1) 7: r
Answer: a 2) If B w B0 + B1 (t—3)2, where t is the time in seconds, what is the value of the magnetic field at 6
seconds? a) B0 +10 B1
b) 2 B1
C) B0 '1' 9 B;
d) B0 4 Bl
Answer: C
3) What is the value of the induced Electromotive Force (EMF), 8, in the coil at this time?
a) 4 r B0
b) 1: r2 6B1
c) r2 2131
d) r2 B12 Answer: E Q 4) If one loop of the wire has a total resistance of R, what is the magnitude of the current, I, ﬂowing in
the wire?
a) r B R
b) B0 + r B,
c) a R
(1) ER Answer: A 5) What is the direction of the current at time t = 6 S?
3) Counter clockwise.
b) Clockwise.
c) Into the page.
(1) Out of the page. Answer: ’ i MT.» my : Gm j; 2:“, FM
5..3;' «if m “i E H
Tlﬂv’xa) i"? Pragﬁmce a magﬁﬂiﬂ‘iﬁ 1' The. [nowAdi Fw‘i‘i gj‘g ‘ I 3‘ ‘;
WW“ PM’WE 5* i;
L: Problem 4: An RLC circuit is driven by an AC voltage source. The capacitive reactance Xc = 0.1 Q, the inductive reactance XL = 0.25 9, and R = 0.4 Q. The AC source has a voltage amplitude of 10 V and an
angular frequency of 200 rad/s. You may use the axis to draw a phasor diagram which may help to answer the
following questions. 1) What is the capacitance of the capacitor in this circuit?
a) 2000 F
b) 0.05 F c) 5 x 10'41: ' b
d) 20F Answer: 2) What is the inductance of the inductor in this circuit? a) 0.00125 H b) 50 H c) 0.25 H d) 4 H Answer: q
3) What is the total impedance Z of this circuit? a) 0.5315 Q b) 0.7500 9 0) 0.4000 Q (1) 0.4272 Q Answer:
4) What is the angular difference (in degrees) between the phase of the voltage of the power supply,
and the phase of the current in the circuit? a) 90
b) 20.556 0) 32 [9
(1) 41.186 Answer: 5) What is the relationship between the power supply voltage and current?
a) Voltage leads the current
b) Voltage lags the current
0) Voltage and current are in phase with each other
d) None of the above 0 Answer: Problem 4: An RLC circuit is driven by an AC voltage source. The capacitive reactance X; = 0.1 Q, the
inductive reactance XL = 0.25 9, and R = 0.4 Q. The AC source has a voltage amplit de of 10 V and an angular frequency of 200 rad/s. i You may use the axis to draw a phasor diagram which may help to answer the following questions. 1) What is the capacitance of the capacitor in this circuit? a. 2000 F xc = 1/(mc) b. .05 F c. 5 x10‘4 F c = 1/(0)Xc) = 0.05 F d. 20 F Answer: B
2) What is the inductance of the inductor in this circuit? a. 0.00125 H XL: 03L b. 50 H C. 0.25 H L = XL/(D = .00125 H d. 4 H Answer: A
3) What is the total impedance Z of this circuit? a. 0.5 3 15 9 b. 0.7500 Q Z = R2 + (XL — X5)2 = 0.4272 0. 0.4000 (2 (1. 0.4272 9 Answer: D 4) What is the angular difference (in degrees) between the phase of the voltage of the power supply,
and the phase of the current in the circuit? a. 90 _ XL“XC b. 20.556 tanq) — C' 32 (13:20.556 d. 41.186 Answer: B 5) What is the relationship between the power supply voltage and current?
a. Voltage leads the current
b. Voltage lags the current
c. Voltage and current are in phase with each other
d. None of the above Answer: A Y!" >><° >7 0 Voli‘t'ﬁt Jamie; Qtttt’rtn‘i" Problem 5 1) 2) 3) 4) 5) Consider the following statements: I. It is possible to have a purely electric wave propagate
through empty space (that is, a wave made up of an electric field but no magnetic field). II. It
is possible to have a purely magnetic wave propagate through empty space (that is, a wave
made up of an magnetic field but no electric field). a) Only statement I is correct 1)) Only statement II is correct
c) Neither statement is correct
d) Both statements are correct C Answer: A currentwcarryin g loop is at the center of a Gaussian sphere. The sphere completely
encloses the loop and has a radius twice the radius of the loop. What is true at the surface of
the sphere? a) The magnetic field is zero everywhere b) The magnetic field is constant everywhere (but nonzero)
c) The magnetic ﬂux is constant everywhere (but nonzero)
d) The total magnetic ﬂux is zero __..cl_ The uniform electric flux between two capacitor plates is (8.0 X 103 Vm / 53) t3, for t > 0.
What is the correct displacement current? a) (8.0><103A/s3)t3
b) (24.0x103A/sz)t2
c) (2.12x 10'7A1’sz)t2
d) (2.67 ><10‘13A/sz)t2 Answer: C Answer: Calculate the displacement current between the square plates 1.00 cm on a side, of a
capacitor if the electric field is changing at a rate of 3.00 x 106 V I In  s. a) 2.66x10‘9A
b) 2.66x10“7A
c) 2.66x10'5A
2
d) 3.00x 10 A Q Answer: At a given instant, a 2.80 A current ﬂows in the wires connected to a parallel—plate capacitor.
What is the rate at which the electric field is changing between the plates if the square plates
are 1.00 cm on a side? a) 2.48x10'7V/ms
b) 3.16x1015V/m~s
c) 3.16x10'16VIms
d) 3.16x1013V/ms Answer: ‘ i ‘ s p \ h ., ,
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 Spring '07
 Hickman
 Physics

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