10/19/09
Lab report #2
ELECTRICAL AND MECHANICAL ENERGY
Abstract
Our goal is to investigate the use of capacitors. We decide to calculate how the mechanical
energy transferred to a device powered by a capacitor depends on the capacitance. We will test
our calculation using a laboratory model in which a capacitor provides power for a motor that
drags a block of wood across a table. We will calculate how far the block will move as a function
of the capacitance of the capacitor, the efficiency of the system, and other properties of the block
and table. We assume that we know the initial voltage on the capacitor.
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Before we will start the experiment we will have to test the motion of the block over the track to
find the friction coefficient using the following equation:
friction
F
mg
μ
=
The force of friction over time will be equal to work, or energy, that is done on the block due to
the friction. Let us use the workenergy theorem to write an equation for the net work done on the
block:
The potential energy of an ideal system is:
We use equations (1) and (2) to write the amount of energy transferred from the capacitor to the
block, during the entire process, as a function of the distance the block slides and properties of the
block and track.
(3)
2
1
2
CV
mgx
=
We can define efficiency for this situation. Since the friction of the block with the track is not
negligible, some of the potential energy is lost to the heat due to friction, and we have to redefine
equation (3)
Where (
1
∈<
)
And therefore:
In order to sketch a graph of the distance the block slides as a function of the capacitor’s
capacitance we assume constant efficiency (later on we will see if the assumption is correct), and
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 Fall '09
 Mueller
 Physics, Capacitance, Conservation Of Energy, Energy, Force, Power, The Trial, Ffriction

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