Download Exam 2 Physics 195B (3/14/02)

Physics 196 Fall 2012
Exam 3
Some Physics Constants:
Name:
e = 1.6×10-19 C
k = 9.0×109 Nm2/C2
ε0 = 8.85×10-12 C2/Nm2
me = 9.11×10-31 kg
mp = 1.67×10-27 kg
electronic charge
Coulomb constant
Permittivity of free space
Mass of electron
Mass of proton
PART 1 (52 points total. 4 points each.)
1. The life-time of a battery is 150 Ampere - hour . What is its emf if it can deliver a total energy of 2.7MJ ?
(a) 2.0V
(b) 5.0V
(c) 9.0V
(d) 12.0V
2. A proton beam consists of protons of kinetic energy 50.0 MeV . The beam current is 1.8mA . What would
the current be if the energy of the protons becomes 150 MeV ?
(a) 2.6 mA
(b) 3.1mA
(c) 4.7mA
(d) 5.4 mA
3. The electrical resistance of a piece of metallic wire is defined by the ratio of the potential difference across
it over the current through it, if Ohm's law is obeyed. According to this, the resistance can be increased by
(Choose all correct answers)
(a)
(b)
(c)
(d)
increasing the potential difference
increasing the electrical current
reducing the electrical current
none of the above
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4. Find the equivalent resistance between the points a and b in the circuit shown
in which the resistors all have the same resistance of 1.0W .
(a)
(b)
(c)
(d)
8 / 5W
7 8W
21 8 W
13 3 W
5. Refer to the diagram showing part of a circuit. The reading on the ammeter
A1 is 2.0A. What is the reading on the ammeter A2 ?
(a) 6A
(b) 3A
(c) 1 3A
(d) 1 6 A
6. The diagram shows a circuit where appliance can be connected to a 120V
main via a circuit breaker. When identical 100W light bulbs are attached, it is
found that the circuit can handle five of them, but breaks when one more is
added. Which of the following statement(s) concerning the maximum
current of the circuit breaker is (are) false?
(a)
(b)
(c)
(d)
I max < 2.50A
I max > 4.00A
I max < 4.90A
I max > 5.20A
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7. Which of the following statement(s) concerning the emf of a battery is (are) always correct?
(a)
(b)
(c)
(d)
It is the electric potential difference between the terminals
It is the power delivered when a current of 1.0A goes through it
It is the charge delivered in 1.0 second
It is the energy delivered when a charge of 1.0C goes through it
8. The emf of a battery is 12V . When connected to a load resistance of 50W , the potential difference across
the load is 10.0V . What is the internal resistance?
(a) 1.0W
(b) 5.0W
(c) 10.0W
(d) 20.0W
9. A capacitor is charged by a voltage source through a resistor. It takes 90ms for the charge on the capacitor
to reach 1/ 4 of its final value. If the capacitor is replaced by a parallel combination of three identical ones,
and the resistor is replaced by the parallel combination of two identical ones, how long does it take for the
charge on one of the capacitor to reach 1/ 4 of its final value?
(a) 25ms
(b) 37.5ms
(c) 135ms
(d) 225ms
10. Which of the following diagram(s) giving the direction relation of the triplet u, B, F correspond to a
negative charge?
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11. A proton of kinetic energy 100MeV travels in the direction 60° S of E in a region where the magnetic field is
due north and has a strength of 25.0mT . Find the magnitude and direction of the force it experiences.
1.4´10-18 N,
-18
(b) 1.4´10 N,
-18
(c) 2.8´10 N,
-18
(d) 2. ´10 N,
(a)
up
down
up
down
12. Four different copper ions Cu++ ,Cu+,Cu--,Cu- all with the same mass
enter a region with magnetic field pointing out of the paper at the point
O as shown. What is the exit point of the ion Cu++ ?
(a)
(b)
(c)
(d)
13. In a rectangular coordinate system (x, y, z) where coordinates are in meters, a magnetic field of 0.80T exists
pointing in the positive y-direction. A 0.25A current runs from the point (-1, 2, -1) to the point (3, 0, 0) along
a wire joining the two points. The force on the wire due to the magnetic field is
(a) 0.2iˆ + 0.8k̂ (N)
(b) -0.2iˆ + 0.8k̂ (N)
(c) -0.8iˆ + 0.2k̂ (N)
(d) 0.8iˆ + 0.2k̂ (N)
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PART B (48 points)
1. (16 points) Refer to the circuit below where all batteries are ideal.
(a) (5 points) Assign and label with arrows the currents through all branches and write down
Kirchhoff’s loop equations for the two loops indicated when traversed clockwise
(b) (5 points) Find the direction and magnitude of the currents through the resistors and redraw the
circuit diagram with the currents identified.
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(c) (3 points) Find the power delivered by each battery, indicating whether the power is generated or
absorbed
(d) (3 points) What is the potential difference Vb -Va between the points a and b ?
2.
(16 points) Refer to the circuit as shown:
(a) The switch has been connected to B for a long time. It is then
re-connected to A.
(1) (6 points) After 30.0s , find the charge, the voltage, and the energy on the capacitor.
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(2) (2 points) Find the current through the 4.0W resistor at the same time.
(3) (2 points) Find the time rate of change of the energy of the capacitor at the same time.
(b) A long time after the steps in (a) are taken, the switch is connected back to the point B.
(1) (3 points) What is the current running through the 12W resistor right after the connection?
(2) (3 points) At what time after, if ever, is this current equal to 0.50A ?
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3. (16 points) A bare nucleus of helium
(
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He 4 ) enters a half space filled with
magnetic field of 0.25T pointing out of the paper after being accelerated by a
potential difference of 20kV . It enters at the point a and exits at the point b.
(a) (4 points) Find the speed of the nucleus while in the magnetic field.
(b) (4 points) Calculate the distance between the points a and b.
(c) (4 points) Calculate how long the nucleus stays in the magnetic field.
(d) (4 points) Another ion with the same charge as the helium nucleus enters the magnetic field at the same
point a after being accelerated by the same potential difference. It exits the field region at the point c.
The distance bc is 7.4cm . Determine the mass number of the ion. (Hint: use proportionality, how does
the orbit radius depend on mass?)
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