8/8/2019 Joule Equivalency Lab
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Objective:
The objectives of this experiment are to understand the
equivalence of electrical energy
and heat energy, to learn techniques of calorimetry, to learn
how to measure electrical energy,and to measure the Joule
equivalent of electrical energy.
Theory:
Electrical and mechanical energy use the same units of energy.
This is because both weredeveloped by the same principles of energy
and power. Heat energy is measured in quantities
separately defined from the principles of electricity,
magnetism, and mechanics. Due to thisinconvenience, Sir James Joule
developed a constant of proportionality between the two forms
of
energy which is now called the Joule equivalent of electrical
energy. This constant allows for anunderstand relationship between
the two forms of energy.
Power is defined as the rate of doing work, and electrical power
is defined as the amount
of electrical energy being expended per unit of time. Work is
the mechanical energy required tomove an electrical charge through
a potential difference (W = Q x V) therefore power, P, is
given by (P = W / t = V x Q/t) so that power is equal to
potential difference times current.
Heat energy is measured in kilocalories as opposed to mechanical
and electrical energywhich is measured in joules. The change in
heat energy of a material is directly proportional to
the change in temperature of the material and depends on the
type of material and its mass. Thechange of heat energy for a given
change of temperature is equal to mass of the material times
specific heat of the material times the change in temperature (Q
= mcT). When electrical energyis transformed into heat energy, then
the equivalence of the electrical energy and heat energy is
given by multiplying the joule equivalent constant of 4186
Joules/kilocalorie times the change in
heat energy (W = J x Q). Throughout the experiment a constant
current and voltage will bemaintained causing the resistance to
increase and therefore the heat energy. This heat energywill
increase the temperature of a quantity of water and the container
in which it is kept. The
change in heat energy of the container and water will be the sum
of the heat energies of each andwill be given by Q = (mc)(cc)T +
(mw)(cw)T. where mc and mw are masses of the
container and water and cc and cw are specific heats of the
material of the container and water.With our previous equations for
work and energy, we can deduce that our joule equivalence
calculated for the experiment will be given by the following
equation: J = { (VI) / [ (mc)(cc) +(mw)(cw) ]} x [ 1 / (T/t) ]. In
this equation, temperature, T, is a function of time, t.
Data:
See attached.
Conclusions:
