Name
University of Alabama
Department of Physics and Astronomy
PH 106-4 / LeClair Fall 2008
Exercise 4: Current
1. A “free” electron and a “free” proton are placed in an identical electric field. Which of the following statements are true?
Check all that apply. Note that the electron mass is 9.11 ×10−31 kg, and the proton mass is 1.67 ×10−27 kg.
Each particle is acted on by the same electric force and has the same acceleration.
The electric force on the proton is greater in magnitude than the force on the electron, but in the opposite direction.
The electric force on the proton is equal in magnitude to the force on the electron, but in the opposite direction.
The magnitude of the acceleration of the electron is greater than that of the proton.
Both particles have the same acceleration.
2. Two isolated identical conducting spheres have a charge of qand −3q, respectively. They are connected by a conducting
wire, and after equilibrium is reached, the wire is removed (such that both spheres are again isolated). What is the charge
on each sphere?
3. When we power a light bulb, are we using up charges and converting them to light?
Yes, moving charges produce “friction” which heats up the filament and produces light
Yes, charges are emitted and observed as light
No, charge is conserved. It is simply converted to another form such as heat and light.
No, charge is conserved. Moving charges produce “friction” which heats up the filament and produces light.
4. In semiconductors such as Si, the number of carriers is not fixed, it depends on e.g., temperature. For a certain sample of
Si, the number of carriers doubles but their drift velocity decreases by 10 times. By how much does the sample’s resistance
change?
5. An electric current of 1mA flows through a conductor, which results in a 150mV potential difference. The resistance of
the conductor is:
6. The resistance of a 150 W, 115 V light bulb is 88 Ω when the light bulb is at its operating temperature. What current
passes through the light bulb when in operation?
7. How many electrons per second does the current above correspond to?
