# Physics 2145 Spring 2015 Test 2 (4 pages)

```Physics 2145
Spring 2015
Name:_________________ March 18, 2015
e = 1.602 x 10-19 C
πΌ=
π₯π
series: πππ = ∑π ππ
V
parallel:
1
πππ
π‘
ππΆ
Discharging
π = π0 π −
Charging
π = ππ (1 − π −
πΏ
π = ππ΄
1
= ∑π π
)
πΌ = πΌ0 π −
π = πΌπ =
π2
π
= πΌ2π
π
πΆ=π
π
πΌ = πΌ0 π −
π‘
ππΆ
(4 pages)
Total Score:_________/120_
I=R
π₯π‘
Test 2
π‘
ππΆ
π‘
ππΆ
ππΆ = ππΆ0 π −
π‘
ππΆ
ππΆ = ππΆπ (1 − π −
π‘
ππΆ
)
____1.(5) An ideal voltmeter has _______ resistance. An ideal ammeter has ______ resistance.
A) zero, zero
B) zero, infinite
C) infinite, zero
D) infinite, infinite
____2. (5) The potential difference across a length of wire is increased. Which of the following
does not increase as well?
A) Electric field in the wire B) Power dissipated in the wire
C) Resistance of the wire
D) Current through the wire
____3. (5) Current flows through a resistor which consists
of two segments of equal lengths and equal radii. Their
resistivities differ, with ο²2 =β ο²1. If the power dissipated
in segment 1 is P1, how big is the power dissipated in
segment 2, P2 ?
A) P2=β P1
B) P2=P1
C) P2=3 P1
L
L
ο1
ο2
D) P2= 6 P1
____4. (5) The time constant of an RC circuit is the time it takes
A) for the capacitor to be completely charged.
B) for the current to reach its maximum value.
C) for the current to decrease to 37% of its initial value.
D) for the current to drop to zero.
Physics 35 Test 2 Spring 2015
1
5) (20) A sodium ion channel in a cell membrane has the shape of a cylinder with a diameter of
0.30nm and a length of 5.0nm. It is filled with intracellular fluid of a resistivity 0.60 Ωm.
a)(5) Calculate the resistance of the ion channel.
b) (5) The potential difference across the channel is 80mV. Calculate the current through the
channel.
c)(5) Calculate the power dissipated in the channel.
d) (5) How many sodium ions (Na+) pass through the channel if the channel opens for 2.0ms?
Physics 35 Test 2 Spring 2015
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6. (40) For the resistor circuit shown R1 =3.0 Ω, R2 = 4.5 Ω, R3 = 9.0 Ω, and R4 = 12.0 Ω. The
battery provides a potential difference of V0 = 24 V.
(a) 20 points) Find the equivalent resistance.
V0
R4
R3
R1
R2
b) (20) Calculate the potential drop across each of the resistors and the current through each
resistor. Put the answers in the table below.
V1=
I1=
V2=
I2=
V3=
I3=
V4=
I4=
Physics 35 Test 2 Spring 2015
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7) (40) A capacitor of capacitance C= 4 μF has been charged so that the potential difference
between its plates is 100 V. The capacitor is then connected to a 5k Ω resistor as shown. The
R = 5 kο
switch S is closed and the capacitor begins to discharge.
a)(5) Calculate the time constant of the circuit.
S
b) (5) Determine the charge on the capacitor
when the switch is closed at t=0.
C
c)(5) Calculate the current through the resistor immediately after the switch is closed.
d)(10) Calculate the potential difference between the capacitor plates 10 ms after the switch is
closed.
e) (15) Calculate the time after which the charge on the capacitor has decreased to one fifth its
maximum value.