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Exam Two Answers - General Physics II Lab | PHY 012, Exams of Physics

Drake UniversityPhysics

Material Type: Exam; Class: GENERAL PHYSICS II LAB; Subject: Physics; University: Drake University; Term: Summer 2007;

Typology: Exams

Pre 2010

Uploaded on 07/30/2009

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Physics 12 Name:__________________________________
Exam Two
MULTIPLE CHOICE. Circle the one alternative that best completes the statement
or answers the question. Each question is worth 3 points.
1) A vertical wire carries a current straight down. To the east of this wire, the magnetic
field points
A) south.
B) down.
C) east.
D) north.
Use the right hand rule. Point your thumb down. Your fingers curl toward
the south when you look on the east side. The answer is A.
2) A charged particle moves and experiences no magnetic force. From this we can
conclude that
A) the particle is moving at right angles to the magnetic field.
B) no magnetic field exists in that region of space.
C) the particle is moving parallel to the magnetic field.
D) either no magnetic field exists or the particle is moving parallel to the field.
Force on a moving charged particle is qvBsin. If the force is zero, then
either B is zero, or sin is zero. The answer is D.
3) A current carrying loop of wire lies flat on a table top. When viewed from above, the
current moves around the loop in a counterclockwise sense. What is the direction of the
magnetic field caused by this current, outside the loop? The magnetic field
A) points straight down.
B) circles the loop in a clockwise direction.
C) points straight up.
D) circles the loop in a counterclockwise direction.
Use the right hand rule. Inside the loop the field is up and outside it is
down at the tabletop. The answer is A.
4) A stationary proton is in a uniform magnetic field of 0.20 T. What is the magnitude
of the magnetic force on the proton?
A) 1.6 x 10-20 N
B) 1.6 x 10-21 N
C) 3.2 x 10-20 N
D) zero
If the proton is not moving, it experiences no magnetic force. The answer
is D.
July 20, 2007 Page 1
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Download Exam Two Answers - General Physics II Lab | PHY 012 and more Exams Physics in PDF only on Docsity!

Exam Two MULTIPLE CHOICE. Circle the one alternative that best completes the statement or answers the question. Each question is worth 3 points.

  1. A vertical wire carries a current straight down. To the east of this wire, the magnetic field points A) south. B) down. C) east. D) north. Use the right hand rule. Point your thumb down. Your fingers curl toward the south when you look on the east side. The answer is A.
  2. A charged particle moves and experiences no magnetic force. From this we can conclude that A) the particle is moving at right angles to the magnetic field. B) no magnetic field exists in that region of space. C) the particle is moving parallel to the magnetic field. D) either no magnetic field exists or the particle is moving parallel to the field. Force on a moving charged particle is qvBsin. If the force is zero, then either B is zero, or sinis zero. The answer is D.
  3. A current carrying loop of wire lies flat on a table top. When viewed from above, the current moves around the loop in a counterclockwise sense. What is the direction of the magnetic field caused by this current, outside the loop? The magnetic field A) points straight down. B) circles the loop in a clockwise direction. C) points straight up. D) circles the loop in a counterclockwise direction. Use the right hand rule. Inside the loop the field is up and outside it is down at the tabletop. The answer is A.
  4. A stationary proton is in a uniform magnetic field of 0.20 T. What is the magnitude of the magnetic force on the proton? A) 1.6 x 10-20^ N B) 1.6 x 10-21^ N C) 3.2 x 10-20^ N D) zero If the proton is not moving, it experiences no magnetic force. The answer is D.

Exam Two

  1. The force on a current-carrying wire in a magnetic field is the strongest when A) the current is parallel to the field lines. B) the current is at a 60o^ angle with respect to the field lines. C) the current is at a 30o^ angle with respect to the field lines. D) the current is perpendicular to the field lines. Force is current x length x field x sin of the angle between the current and the field. The force is greatest when sin of the angle is one which is when the current and the field are perpendicular. The answer is D.
  2. A velocity selector consists of a charged particle passing through crossed electric and magnetic fields. The forces exerted by these fields are in opposite directions, and only particles of a certain velocity will move in a straight line. In the following, disregard the magnitudes of the fields. In a velocity selector, the particles move toward the east, and the magnetic field is directed to the north. What direction should the electric field point? A) down B) west C) up D) east The magnetic force on a positive charge particle will be upward. So the electric field vector must point down to provide a force in the opposite direction. The answer is A.
  3. As a coil is removed from a magnetic field an emf is induced in the coil causing a current to flow within the coil. This current interacts with the magnetic field producing a force which A) causes the coil to tend to flip over. B) acts in the direction opposite to the coil's motion. C) acts at right angles to the coil's motion. D) acts in the direction of the coil's motion. Lenz’s Law says that the system tries to prevent change. If you are removing a coil from a field, you are lowering the magnetic flux. The induced current will try to increase the magnetic field to prevent it from lowering. The answer is B.

Exam Two

  1. If the inductance and the capacitance both double in an LRC series circuit, the resonant frequency of that circuit will A) decrease to one-eighth its original value. B) decrease to one-fourth its original value. C) decrease to one-half its original value. D) none of the given answers Recall the resonant frequency depends inversely on the square root of the LC product. If we double both L and C, their product goes up by 4. The square root is two. Since the frequency depends on the inverse, the frequency is decreased by 1/2. The answer is C.
  2. In a transformer, if the secondary coil contains more loops than the primary coil then it is a A) step-down transformer. B) step-up transformer. The secondary voltage is Ns/Np times the primary voltage. Since the secondary has a larger N, the secondary voltage will be larger. The answer is B.
  3. In a transformer, the power input A) is equal to the power output. B) can be either larger or smaller than the power output. C) is smaller than the power output. D) is larger than the power output. We have to conserve energy. The answer is A.
  4. According to Lenz's law, the direction of an induced current in a conductor will be that which tends to produce which of the following effects? A) oppose the effect which produces it B) produce a greater heating effect C) enhance the effect which produces it D) produce the greatest voltage Lenz’s Law says that nature doesn’t want to change. The induced current will oppose the change. The answer is A.

Exam Two Problems. Show all work to receive partial credit. Some useful constants are: mass of a proton is 1.67 x 10-27^ kg. mass of an electron is 9.1 x 10-31^ kg. Charge of an electron is 1.6 x 10-19^ coulomb. 1 eV = 1.6 x 10-19^ Joule.

  1. A proton arriving at the earth from the sun (the solar wind) is moving at 1 x 10^7 meters/sec. It encounters a magnetic field line near the equator at almost 90o^ and begins spiraling toward one of the earth’s magnetic poles. The strength of the field at that point in space is 1 x 10-6^ T. What is the magnitude of the magnetic force acting on the proton? (5 points) The magnetic force acts as a centripetal force and causes the proton to spiral toward a magnetic pole. What is the radius of the spiral? (10 points) (Hint: Centripetal force is mv^2 /r) F = qvB F = 1. 6 × 10 −^19 × 1 × 107 × 1 × 10 −^6 F = 1. 6 × 10 −^18 Newton r = mv qB = 1.^67 ×^10 − (^27) × 1 × 107
  2. 6 × 10 −^19 × 1 × 10 −^6 r = 1. 04 × 105 meters
  3. The magnetic field passing through a single loop of wire of area 0.1 m^2 changes from 4 T to 3T in 0.1 seconds.. What is the emf induced in the loop? ( points) emf = − ΔΦ Δ t = − A Δ B Δ t emf = −
  4. 1 × (^) ( 3 − (^4) )
  5. 1 = 1 volt

Exam Two Bonus Question: The transmitter for an FM radio station broadcasting at 100 MHz (10^8 Hz) has an LRC oscillator circuit. If the inductance in the circuit is 1 microhenry, what size capacitor is needed to make the resonant frequency the required value? (20 points) f = 1 2 π 1 LC 4 π^2 f^2 = 1 LC C = 1 4 π^2 f^2 L C = 1 4 × 9. 87 × 1016 × 10 −^6 C = 2. 53 × 10 −^12 = 2. 53 pf

Exam Two MULTIPLE CHOICE. Choose the one alternative that best completes the statement or answers the question.

  1. A
  2. D
  3. A
  4. D
  5. D
  6. A
  7. B
  8. C
  9. D
  10. B
  11. A
  12. C
  13. B
  14. A
  15. A