PASCO TD-8552 ELECTRICAL EQUIVALENT OF HEAT - Manual (Page 11)

012-02833D Electrical Equivalent of Heat
7
Teacher’s Guide
Experiment 1: The Electrical Equivalent of Heat
Notes on Procedure
➀ This measurement is not critical.
➇ It is important that the water temperature is uniform
when you begin. If you use the bulb to bring the
water temperature up to its starting temperature, let
the system rest for a few minutes, then start.
Notes on Calculations
J
e
= 4.184
This is also the conversion between Joules and
calories: 1cal = 4.184J.
The result obtained experimentally should be
within 5% of this value.
Experiment 2: Efficiency of an Incandescent Lamp
Notes on Calculations
It is critical that you change your value of H from
calories to Joules for the efficiency calculations. If
the students have gotten good results for J
e
in the
first experiments (within 5% of 4.184) have them
use that value for the conversion. Otherwise, or if
they haven’t done experiment 1, use
J
e
= 4.184.
Notes on Efficiency
The efficiency will vary depending on the voltage
and the bulb. Generally-accepted values for the
efficienty of incandescent lighting are on the order
of 10-15%. With our test bulb at 11.6V, the effi-
ciency was measured as being 13%.
Notes on Questions
➀ a. Leakage of visible light will have negligible ef-
fect, since most of the output of the bulb is not
visible. In addition, much of the visible light that
escapes is reflected back into the water by the
white inside walls of the calorimeter cup.
b. This is the most significant source of error. Be-
ginning and ending equal distances above and
below room temperature will tend to create self-
cancelling errors.
➁ J
e
is the same as J, since mechanical and electrical
energy are equivalent.
Questions
➀ a. Absorbtion of visible light will decrease the
measured efficiency.
b. Transmission of infrared will increase the mea-
sured efficiency.
c. Conductive and Convective heat losses will in-
crease the measured efficiency.
➁ The bulb is much more efficient as a heater than as
a light source.