PLC Activity #7
Practice Exam 1.2
Choose (i) four from problems 1 – 6 and (ii) answer 3 of 5 from problem 7!
Due: see webpage for due date
How to get credit for this activity
First, sign-in at the computer and sign-out when the activity is complete. Show your work
and results to a PLC tutor so they can check your work and initial the signoff sheet. Be
prepared to answer questions about the activity or your results.
Problem 1
(a) What is the magnitude and direction of the electric field at a point midway between a
-8.0 μC and a +7.0 μC charge 8.0 cm apart? (b) Sketch a diagram of the E-field. (b)
What force will a proton experience if placed there?
Answer: 8.4 × 107N/C toward the negative charge, 1.3 × 10-11N
Problem 2
An electron acquires 16.4 ×10-16 J of kinetic energy when it is accelerated by an electric
field from plate A to plate B. (a) Sketch a diagram of the electric potential the electrons
see. (b) What is the potential difference between the plates, and which plate is at the
higher potential? [me = 9.11×10-31, e = 1.6×10-19 C] Answer: 1.03×104 V, B
Problem 3
a. The legend that Benjamin Franklin flew a kite as a storm approached is only a
legend—he was neither stupid nor suicidal. Suppose a kite string of radius 2.00 mm
extends directly upward by 0.800 km and is coated with a 0.500 mm layer of water
having resistivity 150 Ω∙m. If the potential difference between the two ends of the
string is 160 MV (M = 106), what is the current through the water layer? The danger
is not this current but the chance that the string draws a lightning strike, which can
have a current as large as 500,000 A (way beyond just being lethal). Hint: to get the
area of the water, subtract the area of water + string from the area of the string).
Answer: 9.42 mA
b. A coffee cup heater and a lamp are connected in parallel to the same 120-V outlet.
Together, they use a total of 111 W of power. The resistance of the heater is 400Ω.
Find the resistance of the lamp. Answer: 190Ω
Problem 4
Find the (a) equivalent resistance of the circuit, (b) the voltage across and (c)
the current through each resistor, and (d) the current coming from the battery
for the circuit shown.
Answer: 2Ω; 10V, 2.5A; 6.7V, 1.7A; 3.3V, 1.7A; 1.7V, 0.83A; 8.3V, 0.83A;
5.0A
Problem 5
a. A charge of −8.3 μC is traveling at a speed of 7.4 × 106 m/s in a region of space
where there is a magnetic field that lies along the x-direction. The angle between the
velocity of the charge and the field is 52o. A force of magnitude 5.4 × 10-3 N acts on
the charge. (i) Sketch the trajectory of the charge in the magnetic field. (ii) Draw a
diagram of the RHR that explicitly shows the directions of v, B and FB. (iii) What is
the magnitude of the magnetic field? Answer: 1.1 × 10-4 T
b. Particle X of −2.3 μC and mass 3.38 × 10-12 kg moves through a velocity selector at
a constant speed in a straight line. The electric field of the velocity selector is 3.80 ×
103 N/C, while the magnetic field is 0.360 T. (i) What is the speed of particle X when
it exits the velocity selector? (ii) When the electric field is turned off, particle X moves
on a circular path. Determine the radius of the path? Answer: 11,000 m/s, 44.9 nm
Problem 6
A 100-turn, 2.0-cm-diamcter coil is at rest in a horizontal plane. A uniform magnetic field
60° away from vertical increases from 0.50 T to 1.50 T in 0.60 s. (a) Draw a picture of
the situation that indicates Bind, iind and ℇind. (b) What is the induced emf in the coil?
Answer: 26 mV
Problem 7
Use short concise sentences to answer 3 of the 5 following questions using physics
principles. Make sure to explain your reasoning for each answer
a. Over time, atoms “boil off” the hot filament in an incandescent bulb and the filament
becomes thinner. How does this affect the brightness of the lightbulb?
b. Lightbulbs are typically rated by their power dissipation when operated at a given
voltage. Rank the following lightbulbs from smallest to largest resistance when
operated at the voltage for which it's rated? (i) 0.8 W, 1.5V, (ii) 6W, 3V, (iii) 4W, 4.5V,
and (iv) 8W, 6V.
c. In the circuit shown, four identical bulbs are connected to a flashlight battery.
How do the brightness’s of the bulbs compare? Which light bulb has the
greatest current passing though it? Which light bulb has the greatest
voltage? What happens if bulb A is unscrewed? Bulb B? Bulb C? Bulb D?
d. If an electron is not moving, is it possible to set it in motion using a magnetic
field? Explain?
e. A proton is moving near a long, current-carrying wire. (i) When the proton is at the
point (A), in which direction is the force on it? (ii) Point B?