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F r o n t l i n e
Lighting accounts for a relatively large fraction of
annual energy consumption in many industrialized
nations. One way to reduce this consumption is to
replace traditional lighting with alternative sources,
such as LEDs. Since the use of such alternatives is
sure to increase in the future, an understanding of
how these devices conserve energy is important for
students of physics.
This article presents two circuits to show how
traditional and LED lighting compare. The supplies
required for these demonstrations include a
spring-loaded circuit board with sockets (or another
means of connecting the circuit elements), three
0.9 W incandescent bulbs, three green LEDs, wire
leads and a 9 V battery (or DC power supply).
In the first circuit three incandescent bulbs are
connected in series with a 9 V source. In the second
circuit the three LEDs and three bulbs are wired
alternately in series (figure 1). The LEDs must be
forward biased for the demonstration to work. The
circuits should be connected before the beginning
of the class, except for a final lead that serves as a
switch. During class, images of these circuits can
be projected onto a screen, or students can connect
the circuits themselves and make observations and
measurements.
As the first circuit is shown, students should be
asked the following questions:
What do you observe when the three bulbs are
connected in series with the 9 V source?
What are the measured current and calculated
power for this series circuit?
Measured values for voltage and current are shown
in table 1 for our set-up.
Students can then observe the second circuit and
answer three additional questions:
What differences do you see when the three
LEDs are added to the circuit?
What is the measured current in the second
circuit?
How do the power requirements compare for
the two circuits?
Once students have responded to these questions,
we usually discuss their answers in a large group
setting. There are helpful ways to direct these discussions,
an analysis of which is given below.
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In the first circuit the three bulbs are brightly
illuminated when connected in series. While in
operation, a potential drop of approximately 3 V
is present across each bulb. The series current is
measured at approximately 0.31 A, so the power
required for each bulb is 0.93 W (from P = VI).
Thus the total power supplied by the battery is
2.8 W (9 V × 0.31 A, or three bulbs × 0.93 W).
In the second circuit, the LEDs are brightly illuminated,
whereas the bulbs show no illumination.
This drastic change clearly indicates a decrease in
the power required to operate the circuit. The reasoning
here is that the battery serves as a constant
voltage source so that the power delivered by the
source is the product of the constant voltage multiplied
by the current supplied. From a measured
current of approximately 62 mA, the total power
delivered by the source is 0.56 W, compared with
2.8 W for the incandescent bulbs.
The power requirements for the second circuit
can be reduced further by decreasing the applied
voltage with a variable voltage source (as shown in
figure 1), since LEDs typically require only a few
milliamps for nominal illumination. However, the
L i g h t i n g
LEDs provide ‘green’ energy
Figure 1. Circuit board with three bulbs and three
LEDs wired alternately in series to a 9 V power
source. The LEDs are brightly illuminated while
the bulbs show no illumination. The multimeter
indicates a series current of 0.062 A.
F r o n t l i n e
goal here is to use readily available supplies, so a
9 V battery is best for general classroom use.
Further discussion may focus on why the two
devices require such vastly different currents for
operation. To handle these questions, instructors
should focus on how light is generate

 

lol if you follow the link... im pretty sure i dont work for them or get any money from them, why would i promote them except for the fact that my names in it?

 

sounds like you got the same kinda curve we got going for almost all of my engineering classes. Seems like all of our professors write exams that would take them 40 minutes to do and only give us 50 mins.

 

I remember getting a 27% on my first Thermo Dynamics class at the University of Illinois and it was an open book test! The class average was around 24%.

 

I hated thermo. I had never completely bombed a test before that class. I should have failed but D get degrees!

 

ya i have looked into internships. my dad works at rockwell collins and hopefully i will be able to get one ther well see though.

 

ahh Physics.. thats too complicating... go Comp. Sci

 

Ok this is a beast homework problem.

A crate of mass m1 on a frictionless inclined plane is attached to another crate of mass m2 by a massless rope. The rope passes over an ideal pulley so the mass m2 is suspended in air. The plane is inclined at an angle = 36.9°. Use conservation of energy to find how fast crate m2 is moving after m1 has traveled a distance of 1.4 m along the incline, starting from rest. The mass of m1 is 10.2 kg and the mass of m2 is 18.4 kg.

I'm trying to figure out what types of energy each have. I believe m2 only have gravitional potential energy but I'm unsure of what if m1 has kinetic energy. I know they both have to have grav potential. Right now I've concluded that that the Work for m2 = Work for m1.

I'm sitting here in the dark, infront of my computer, with my cal, playing rock and metal. Extreme Physics!

 

So I think the total work for m1 is 8.574J. Knowing that I dont think it matters what kinds of work m1 has done as long as the total work that m2 does is equal to 8.574J