Physics 204, Spring 2012, Second Midterm Practice Exam – One page of notes
will be permitted
If you need the charge −e and mass me of the electron, use e = 1.6 × 10−19 C
and me = 9 × 10−31 kg. For Boltzmann’s constant, use kB = 1.38 × 10−23 J/K.
1. How long does it take a 1 µF capacitor to discharge through a 10 kΩ resistor?
(A) 0.01 s; (B) 1 msec; (C) 10 µs (D) 10−7 s
2. The number of resistors in a series circuit is increased. What will be the
effect on the circuit?
(A) The current will decrease and the power dissipated will decrease; (B) The
current will decrease and the power dissipated will increase; (C) The current will
increase and the power dissipated will decrease; (D) The current will increase
and the power dissipated will increase.
3. A 2 µF capacitor will break down if more than 100 V is applied to it. What
is the maximum energy it can store?
(A) 10−4 J; (B) 5 × 10−3 J; (C) 10−2 J; (D) 5 × 10−1 J.
4. Even if a system can’t lower its energy in a certain process, the process may
take place spontaneously, and is most likely to take place if
(A) The change in entropy is positive and temperature is low; (B) the change
in entropy is positive and temperature is high; (C) the change in entropy is
negative and temperature is high; (D) the change in entropy is negative and
temperature is low.
5. A capacitor is charged to potential difference V . A particle with charge q
starts from rest at one capacitor plate and is accelerated toward the other plate,
a distance d away. Halfway to the other plate it collides with a second particle.
What is its kinetic energy when it collides?
(A) |qV |; (B) |qV |/2; (C) |qV |/d; (D) 2|qV |/d
6. A battery supplying voltage V drives a current through a resistor R immersed
in a fluid of mass m and specific heat c. After time t the temperature change
∆T is
V2
V 2t
V 2 mc
(A) Vmt
2 Rc ; (B) Rtmc ; (C) Rmc ; (D)
Rt
7. If a 1 µF capacitor is charged to 10 V , then discharged through a 1000 Ω
resistor, the maximum current is
(A) 10 mA; (B) 1 mA; (C) 0.1 mA; (D) not enough information to tell.
8. A battery like a Daniell cell is constructed at room temperature with copper
at both electrodes and copper sulfate in both half cells. The only difference in
the half cells is that the copper sulfate solution is three times more concentrated
1
in one than the other. If copper is deposited at the electrode where the solution
is more concentrated, and dissolved at the other electrode, with 2 electrons
flowing through the wire connecting them to complete the half-reactions, then
the entropy change for the full reaction is
(A) kB ln(3); (B) kB ln(3/2); (C) 0; (D) −kB ln(3/2)
9. A wire with a linear mass density 10 g/m can just be levitated by the
magnetic force in a 1000 Gauss magnetic field (0.1 T) if it carries a current
(A) 0.01 A; (B) 0.1 A; (C) 1 A; (D) 10 A.
10. A horizontal current-carrying wire can be levitated (supported against gravity) by a magnetic field that is
(A) up; (B) horizontal and perpendicular to the wire; (C) along the wire in
the direction of the current; (D) along the wire in the direction opposite to the
current.
Homework-like problems:
1. A horizontal 100 eV electron beam, directed south, enters a region of constant
magnetic field 0.2 T, created by a permanent magnet having north pole piece
above and south pole piece below, so that the field is vertical.
(a) Which direction is the electron beam deflected?
(b) What is the radius of curvature of the beam while it is in the field?
(This is just the radius of the circle it would move in if the field region were
large enough to completely confine the beam to a circular orbit.)
2