PHYS 155 Midterm 2 Tutorial
Example 1: Photoelectric Effect
A phototube with a zinc cathode is created
and connected to a variable voltage supply.
The work function for zinc is 4.31 eV.
A. What is the lowest frequency of light that
will release photoelectrons from the zinc
surface? Assume the voltage supply is
supplying no voltage.
B. If photons of wavelength 225nm are
incident on zinc, what is the maximum
kinetic energy of the ejected
photoelectrons?
C. What minimum stopping voltage is
required to keep emitted photoelectrons
from reaching the anode?
Note: h = 6.626 x 10-34 Js
Example 1: Photoelectric Effect Answers
A. 1.042 x 1015 Hz
B. 1.20eV
C. 1.20V
Note: h = 6.626 x 10-34 Js
Example 2: Resistivity
A tungsten wire (ρ = 5.6 x 10-8 Ωm) of length L has a cross-sectional area of
0.600mm2.
A. Plot the power P dissipated in the wire as a function of length L from
L=0m to L = 5m.
B. What is the power dissipated by the wire at L = 1.5m? What is the
current through the wire?
C. What would happen to this plot if the radius of the wire was decreased
by a factor of 2?
Example 2: Resistivity Answers
A. P = 8.678571/L
B. P = 5.7857W; I = 6.43A
C. Plot is scaled down
by a factor of 4
P = 2.169643/L
Example 3: Mesh Analysis
Write current equations for the following circuit with a maximum 3 current
loops. What happens to the current through the 2Ω resistor if the 1V
battery is changed to a 10V battery?
Example 3: Mesh Analysis Solution
-4V = 3I1 + 0I2 + 0I3
5V = 0I1 + 2.8I2 – 0.8I3
-3V = 0I1 – 0.8I2 + 0.8I3
Example 4: Superposition
Take the simplified circuit from Example 3 and prove the independence of
the 1V battery from the 2Ω resistor using superposition. Also, find the
current through the 4Ω resistor.
Example 4: Superposition Solution
Current through 4 Ω resistor = 1.11A
1V battery is independent because the rest of the circuit is short circuited
when the other batteries are short circuited.
Example 5: RC Transient Properties
Consider the following circuit and answer the questions:
A. Switch 1 and 2 closed: find charging time to 85% of supply voltage, total
charge and energy at 85%. Switch 1 becomes open when charge hits 85%
while Switch 2 stays closed: find discharge time to 10% of supply voltage.
Example 5: RC Transient Properties
B. Switch 1 and 2 are closed for a long time. Switch 1 and 2 are then opened. Switch
2 and 3 are then closed at the same time. What is the time it takes for the voltage
across the capacitors to each 18V? What is the instantaneous current flow at this
time?