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Section: ______
Name: ______________________________
Section Instructor: ______________________________
Assignment 3: Charges in Motion & Electric Circuits
Concepts:
electric current
current density
charge carriers and drift speed
resistance and resistivity
Ohm’s law
energy and power in electric circuits
“pumping” charges and emf
calculating and measuring voltage and current
simple multiloop circuits (series, parallel)
RC
circuits
Reading in text:
Chapter 26: Sections 15, 7.
Chapter 27: Sections 19.
Assignment:
Solutions posted on Friday, February 16.
Quiz may be given on or after
Tuesday, February 20.
1. In the lower atmosphere of the Earth there are negative and positive ions, created by
radioactive elements in the soil and cosmic rays from space.
The density of singly charged
negative ions is
550
3
/ cm , while the density of singly charged positive ions is 620
3
/ cm .
In a
certain region, the strength of the
fair weather atmospheric electric field
is 120 V/m, directed
vertically downward, and the measured conductivity is
27 10
14
./
(
)
×
−
Ω
m .
(a) What is the direction of drift of each kind of ion?
(b) What is the direction of the total electric current?
Do the motions of the two kinds of ions
produce electric currents that add or subtract?
Please explain.
(c) What is the electric current density
?
(d) What is the ion drift speed
, assuming it to be the same for both kinds of ions?
(e) If this current density were the same over the entire surface of the Earth, what would be
the total electric current
carried by these atmospheric ions?
(f) Does the charge transfer due to this current tend to enhance or diminish the atmospheric
electric field?
Please explain.
2. A common flashlight bulb is rated at 0.30 amp and 3.0 volts (the values of the current and
voltage under operating conditions).
(a) What is the bulb's resistance when lit?
(b) What electric power is dissipated into light and thermal energy by the bulb?
(c) If the resistance of the bulb's tungsten filament at room temperature (20
°
C) is 1.2
Ω
, what
is the filament's temperature when the bulb is lit?
Please note any assumptions that you
make.
3. The “kilowatthour” (kWh) is a standard unit for measuring electric energy delivered by your
electric power company and commonly appears on household electric bills.
1 kWh is defined
as the total energy delivered in 1 hour at a steady rate (power) of 1 kilowatt (kW).
Hence, the
name.
[Note: “kilo” means
10 .]
3
(a) Show that
13
.
6
1
0
6
kWh
joules (J)
=×
.
(b) If a jelly donut contains chemical (food) energy equal to about 250 kcal or about
1 million J, how many jelly donuts worth of food energy is equivalent to 1 kWh?
(c) If you leave a 100 W light bulb on continuously for 1 day (24 hours), how many kWh of
electricity does this bulb use?
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 Spring '07
 FULBRIGHT, R
 Physics, Charge, Current, Electrical Circuits

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