Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Circuit Question



Tina,
I'll try to answer this with an analogy. PLEASE understand that like
all analogies it will break apart if examined too closely.

Imagine the circuit like a multilevel building. One floor or level for
each resistor (or light bulb) in the circuit, a very large up only
escalator that goes from the ground floor to the top floor that
represents the battery (this can be expanded into multiple escalators for
multiple batteries either in parallel or series). The job of the battery
is to move charges from lower potential energy to higher potential
energy. A "good" battery can do this easily just like a good escalator
can do this without any backlog (a poor battery won't be able to keep up
and will "have to dump people off early). The resistors are the stairs
that only go down in the building. It is the size and type of stairs
that determine how many people can go down per unit of time (the "good"
escalator can easily keep up by supplying more people). The more "open"
the stairs (lower resistance) the more people can go down them per unit
time (the higher the current). The escalator (battery) only make sure
there are enough people (charges) to go down.

This works for series (multiple floors), parallel (multiple stairs) and
network (some floors twice as high as others, some with more stairs than
others). The function of the battery is to change chemical potential
energy into electrical potential energy. The function of the resistors
is to change electrical energy into heat (OOH - Ouch! I mean molecular
kinetic energy) and for light bulbs to change electrical energy into
"molecular kinetic energy" and "electromagnetic energy" or light. In the
building the escalator changes mechanical energy into gravitational
potential energy, the stairs change the gravitational potential energy
into "molecular kinetic energy" (heat in an unorganized form and sound in
an "organized" form). The analogy breaks down at higher voltages because
it is difficult to imagine people going down stairs faster (actually more
people going down at the same speed) because the building is taller.

A 10 watt bulb has less resistance than a 5 watt bulb so more charges per
unit of time go through it (a bigger stairs), transforming more energy
(from electrical to "heat" or "light") per unit time.

Tina Fanetti wrote:

Hello
I am getting ready to teach circuits in the next week. =20

I was looking at a question that has a battery hooked up to a 5 W bul=
b and then the battery is hooked to a 10 W. It then asks which case =
has the greater current and then which light bulb has the greater res=
istance.

Now comes the silly part.

How does the batter "know" that it is supposed to put out more curren=
t for different lighbulbs? Mathematically it makes sense but concept=
ually it doesn't to me.

I know my students are going to ask me this and I am not wanting to s=
ay cause of the little guy that lives in batteries. =20

Doesn't a battery put the same amount of current out regardless of wh=
at is hooked to it?

Thanks
tina

Tina Fanetti
Physics Instructor
Western Iowa Technical Community College
4647 Stone Ave
Sioux City IA 51102
712-274-8733 ext 1429

--
Arlyn DeBruyckere
Science Teacher
Hutchinson High School
1200 Roberts Road
Hutchinson, MN 55350
320-587-2151
mailto:arlynd@hutch.k12.mn.us
http://www.hutch.k12.mn.us
http://www.hutch.k12.mn.us/teacher/ArlynDeBruyckereHS.cfm

HEA Website http://www.angelfire.com/mn3/hea