Upcoming Schedule

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Upcoming Schedule
Sep. 22 Exam 1 Ch. 16, 17 Sep. 24 18.1-18.4 Sep. 26 18.6, 18.7, 18.10
Sep. 29 boardwork Oct. 1 boardwork Quiz 3 Oct. 3 Free Day
Oct. 6 19.1-19.2 Oct. 8 boardwork Oct. 10 19.3-19.5 Quiz 4
Life is uncertain. Eat dessert first.UMR
Professor Emeritus D. M. Sparlin
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Chapter 18 Electric Currents
18.1 The Electric Battery
Read this section. There will not be exam
problems on it. There could be exam questions on
it.
Howstuffworks shows how batteries work.
So does energizer.com.
Looks like lots of nasty chemical things going on.
Ugh!
Ugh! Chemistry!
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18.2 Electric Current
Connecting wires (and/or lamps, etc.) to a
battery permits electric charge to flow. The
current that passes any point in the wire in a
time ?t is defined by
OSE I ?Q/?t,
where ?Q is the amount of charge passing the
point.
One ampere of current is one coulomb per second.
Because electric charge is conserved, the current
at any point in a circuit is the same as the
current at any other point in the same circuit at
that instant in time.
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We use this symbol for a battery (the short line
is negative)
Dont try that at home! (Why not?)
The current is in the direction of flow of
positive charge.
Opposite to the flow of electrons, which are
usually the charge carriers.
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current
electrons
An electron flowing from to gives rise to the
same conventional current as a proton flowing
from to -.
Conventional refers to our convention, which is
always to consider the effect of charges.
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Hey, that figure you just showed me is
confusing. Why dont electrons flow like this?
current
electrons
Good question.
Electrons want to get away from - and go to .
Chemical reactions (or whatever energy mechanism
the battery uses) force electrons to the
negative terminal. The battery wont let
electrons flow the wrong way inside it. So
electrons pick the easiest paththrough the
external wires towards the terminal.
Of course, real electrons dont want anything.
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Example 18-1
A steady current of 2.5 A flows in a wire for 4.0
min. (a) How much charge passed through any point
in the circuit?
This is the symbolic answer.
Minutes are not SI units! So convert minutes to
seconds.
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(b) How many electrons would this be?
This is a piece of cake so far!
Dont worry, it gets better, especially in
chapter 19.
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18.3 Ohms Law
It is experimentally observed that the current
flowing through a wire depends on the potential
difference (voltage) causing the flow, and the
resistance of the wire to the flow of
electricity. The observed relationship can be
written
Is this V the same thing that we saw in Chapter
17?
OSE V I R,
and this is often called Ohms law.
Ohms law is not fundamental, so it is not
really a law in the sense of Newtons Laws. It
only works for conductors, and things that
conduct electricity do not necessarily obey Ohms
law.
The unit of resistance is the ohm, and is equal
to 1 Volt / 1 Ampere.
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Example 18-3
A small flashlight bulb draws 300 mA from its 1.5
V battery. (a) What is the resistance of the
bulb?
V I R
R 1.5 / 300x10-3
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(b) If the voltage dropped to 1.2 V, how would
the current change?
V I R
I 1.2 / 5.0
(If its this easy now, does that mean Ill pay
later?)
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Every circuit component has resistance.
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Resistors are often intentionally used in
circuits. The picture shows a strip of five
resistors (you tear off the paper and solder the
resistors into circuits).
The little bands of color on the resistors have
meaning. Here are a couple of handy web
links http//www.dannyg.com/examples/res2/resisto
r.htm http//xtronics.com/kits/rcode.htm
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You light me up. http//jersey.uoregon.edu/vlab/Vo
ltage/ http//jersey.uoregon.edu/vlab/Voltage/volt
1.html
18.4 Resistivity
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R ?L / A,
?
A
L
The longer a wire, the harder it is to push
electrons through it.
The greater the resistivity, the harder it is
to push electrons through it.
The greater the cross-sectional area, the
easier it is to push electrons through it.
Resistivity is a useful tool in physics because
it depends only on the properties of the wire
material, and not the wire geometry.
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Resistivities range from roughly 10-8 ?m for
copper wire to 1015 ?m for hard rubber. Thats
an incredible range of 23 orders of magnitude,
and doesnt even include superconductors (we
might talk about them some time).
Example 18-4
Suppose you want to connect your stereo to remote
speakers. (a) If each wire must be 20 m long,
what diameter copper wire should you use to make
the resistance 0.10 ? per wire.
R ?L / A
A ?L / R
? (d/2)2 ?L / R
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(d/2)2 ?L / ?R
d 2 (1.68x10-8) (20) / ? (0.1) ½
d 0.0021 m 2.1 mm
In the spirit of not skipping steps, you are
welcome to show all units!
(b) If the current to each speaker is 4.0 A, what
is the voltage drop across each wire?
V I R
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V (4.0) (0.10)
V 0.4 V
Example 18-5
A wire of resistance R is stretched uniformly
until it is twice its original length. What
happens to its resistance?
Hint the volume of wire material stays the same.
Hint R ?L / A.
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Resistivity depends on temperature (see equation
18-4), but I dont plan on testing you on this
part of the section (all of page 536).
A couple more links. Sorry about the shockwave
you have to go through on the first one. Too bad
they both seem to be dead now. http//www.article1
9.com/shockwave/oz.htm http//www.sciencejoywagon.
com/physicszone/lesson/07electr/resist2/