240 Physics Fall 2003: Exam #2
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Form #1
Instructions 1. 2. 3. 4. Fill in your name above This will be a 1.5 hour, closed book exam. The exam includes 20 questions. You may use a calculator, please do not share calculators You may use two 3"x5" note cards (one from the first exam, plus one new one) with notes and equations you think may be useful. You can write on both sides of the card if you like. 5. You will be asked to show your University student ID card when you turn in your exam.
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Table of constants: 0 = 8.85 x 10-12 C2/Nm2 k = 1/(40) qelectron=-1.6 x 10-19 C qproton=1.6 x 10-19 C melectron=9.1 x 10-31 kg mproton=1.67 x 10-27 kg 0 = 4 x 10-7 Tm/A G = 6.67 x 10-11 Nm2/kg2
1: Which of the following is equivalent to the quantity 1000 Ohms * 20 microFarads? A) B) C) D) E) 20000 Joules/second 0.02 kilogram*meters/second 0.0002 volts 0.02 seconds 20000 Newtons/Coulomb
2: What is the equivalent resistance of the resistor network seen by the battery shown in the figure? A) B) C) D) E) 5R/2 5R/3 2R 11R/5 R/2
R R V 2R 2R
3: Three identical light bulbs are connected in a circuit as shown. Each of the bulbs has a resistance of 30, and the battery supplies a voltage of 30V. How much power does light B consume? A) B) C) D) E) 30W 60W 10W 20W 50W
A
B
C
4: A mass spectrometer accelerates a singly charged ion to a velocity of 4.5x105 m/s before injecting it into region with a constant 0.400 Tesla field. A `singly charged ion' has a charge equal in magnitude to the electron charge. If it bends to the left in a circular path with a radius of 25 cm, what is the mass of the ion? A) B) C) D) E) 2.6x10-27 kg 9.1x10-31 kg 3.6x10-26 kg 8.3x10-25 kg 3.3x10-24 kg
5: Five ions, labeled A-E, all enter a region of uniform magnetic field with the same velocity. Which ion lands at position 3?
6: In Oersted's laboratory a compass needle is aligned with a constant magnetic field. This field has a magnitude of 6.0x10-6 T, and points in the yunit direction. A wire, extending up and down in the zunit direction, passes through the table 30 cm from the compass, as shown in the figure. What current must he pass through this wire to make the compass point 45 to the right of the yunit direction? You may treat the wire as infinite.
45
A) B) C) D) E)
9.0 A in the -zunit direction 6.3 A in the -zunit direction 6.3 A in the zunit direction 12.6 A in the -zunit direction 8.4 A in the -zunit direction
30 cm
yunit xunit wire
7: A circular loop with radius r lies flat in the xy plane. If I look down the z axis I see a current i circulating in the counterclockwise direction in the loop. The loop is immersed in a magnetic field B = (Bxxunit + Byyunit + Bzzunit). What is the magnitude of the magnetic torque felt by the loop? A) B) C) D) E) ir2(Bx2 + By2 + Bz2)1/2 ir2(Bx2 + By2 + Bz2)3/2 ir2(By2 + Bz2)1/2 ir2(Bx2 + By2 )1/2 ir2(Bx2 - By2 )1/2
z i y x
8: A capacitor is charged by a battery until the charge on the capacitor is Q. It is then disconnected from the battery, leaving the charge in place. A dielectric material with dielectric constant K is now slowly inserted into the capacitor. How does the energy stored in this capacitor change in this process? A) B) C) D) E) The energy stored increases by a factor K The energy stored increases by a factor K2 The energy stored decreases by a factor 1/K The energy stored decreases by a factor 1/K2 The energy stored remains unchanged
9: A three way light bulb has two filaments tungsten filaments with resistances 50 and 100 connected in series. The resistors are connected to three terminals as indicated in the figure. The light switch determines which two of the three terminals are connected to a potential difference of 120V at any given time. Which two terminals should you connect to make the light bulb shine most brightly? A) B) C) D) E) A and C B and C C and B A and B A and C
A
50
B
100
C
10: What is the equivalent capacitance between points A and B for the group of capacitors shown in the figure? A) B) C) D) E) 14.1F 9.5F 21.0F 26.3F 12.4F
8.2F A 22F 15F B
11: You find a 20V battery on the shelf and want to determine its internal resistance. To find out, you connect a 200 resistor across the terminals of the battery and measure a current of 65mA. What is the internal resistance of the battery? A) B) C) D) E) 12 28 36 108 8
12: An ion with a negative charge of 4.8x10-19 C and a mass of 6.6x10-25 kg has a velocity of 3x106 m/s. This particle enters a magnetic field, pointing out of the page, with magnitude 3.5x10-3T. The particle travels in a semicircle, eventually leaving the region with the magnetic field. What is the kinetic energy of the particle when it is at the point shown, half way through this semicircle? A) B) C) D) E) 3.0x10-12 J 6.8x10-11 J 4.4x10-11 J 1.7x10-11 J 5.5x10-11 J
B
13: Consider a flat rectangular conductor connected to a battery as shown. The conductor is embedded in a magnetic field coming out of the page. The electric potential on the conductor is larger on the left than on the right. Which of the following statements is true? A) B) C) D) E) The charges which flow feel a magnetic force into the page The charges which flow feel a magnetic force to the right The charges which flow in this conductor must be negative The electric field in the conductor points to the left The charges which flow in this conductor must be positive
V
14: A high voltage power line carries a current of 110A at a location where the Earth's magnetic field has a magnitude of 6x10-4 T and points north and dips down 32 below the horizontal (toward the ground). Calculate the magnitude of the magnetic force on a 250 meter segment of wire in which current flows east. A) B) C) D) E) 8.7 N 16.5 N 14.0 N 4.3 N 0.87 N
15: We might make a simple model for the hydrogen atom in which the electron orbits the proton in a circle just like the Moon orbits the Earth. This orbiting electron travels with speed 2.2x106 m/s in a circle with radius 5.3x10-11 m. To us this orbiting electron looks like a little current loop. In this model, what is the magnetic moment we would associate with this current loop? A) B) C) D) E) 5.2x10-23 Am2 4.2x10-21 Am2 5.4x10-23 Am2 4.0x10-26 Am2 9.4x10-24 Am2
16: Four long parallel conducting wires, arranged in a square, carry the same current I in a direction perpendicular to the page. In which of the following arrangements is the magnitude of the magnetic field at the center of the square largest?
A C
B
E
D
17: A rectangular loop of area A carries a current Iloop in the direction shown in the figure, forming a magnetic dipole with dipole moment =IloopA. This loop is placed next to a long straight wire which carries a current Iwire. Which of the following statements correctly describes what happens? A) The loop feels a torque pulling side A into the page and pushing side B out of the page. B) The loop feels a torque pushing side A out of the page and pulling side B into the page C) The loop feels no net force or torque D) The loop feels a net force pulling it toward the wire E) The loop feels a net force pushing it away from the wire
Side B Iloop Side A Iwire
18: Consider the set of light bulbs, powered by a battery, shown in the figure. With the switch open, only bulbs A and B are lit. When the switch closes, how does the circuit change? A) B) C) D) E) Bulbs A and B get brighter Bulb A gets brighter, B remains the same Bulb A remains the same, B gets brighter Bulb A and B get fainter Bulb A gets brighter, B gets fainter
A
V B C
19: Which of the following statements about voltmeters and ammeters is correct? A) To function properly, a voltmeter must have very small internal resistance B) To function properly, a voltmeter must be placed in series with the circuit element it is meant to measure C) To function properly, an ammeter must have a very large internal resistance D) To function properly, an ammeter must be placed in series with the circuit element it is meant to measure E) To function properly, a voltmeter should be placed in parallel with the ammeter it is being used with
20: A cylinder of material 0.5 m long and 8 cm in diameter is connected across the terminals of a 9 V battery and a current of 3.5x10-3 A is seen to flow. What is the resistivity of this material? A) B) C) D) E) 3.9x10-4 m 227 m 26 m 9.4x10-2 m 3.1x10-3 m