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14
MECHANICAL EQUIVALENT OF HEAT
(ELECTRICAL METHOD)
with solar power optimization competition
OBJECT
To measure the mechanical equivalent of heat by an electrical method.
APPARATUS
Heating coil, calorimeter and stirrer, DC voltmeter and ammeter, switch, connecting wires, temperature
sensor (brass or plastic), resistor boxes (1 kiloohm step, 100 ohm and 10 ohm step), distilled water, ice,
balance, cork, computer temperature and time program.
THEORY
Potential difference is defined as work per unit charge given by the expression
V = W/Q
(1)
where W is the work done in moving the charge Q through the circuit.
In the practical (mks) system V is expressed in volts, W in joules, and Q in coulombs.
Equation (1) may be written as
W = VQ
(2)
And recalling that
I = dQ/dt
or
Q
t
I
(3)
V = I R
(4)
we may rewrite equation (1) using these substitutions as
W = VQ = VIt = IR(I)t = I
2
Rt
(5)
This equation indicates that if an electric current of 1 ampere is sent through a coil of wire of resistance 1
ohm for 1 second, then 1 joule of work has been expended in maintaining this current.
This energy is
transformed into heat.
If the coil is submerged in a calorimeter containing water, the number of calories of heat produced can be
obtained by the usual method of calorimetry.
In a circuit containing only resistance, a direct proportion
exists between the expenditure of the electric energy W and the heat H developed.
This fundamental law is
represented by the conservation of energy equation
W = JH
(6)
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where J is the proportionality factor called the mechanical equivalent of heat, or the number of joules
required to produce one calorie of heat.
By referring to the experiments on calorimetry you will derive an expression for H, the heat developed, so
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 Fall '09
 JohnJames
 Power, Heat

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