Pasco TD-8552 ELECTRICAL EQUIVALENT OF HEAT Manuel d'utilisateur télécharger pdf (Page 11)

012-02833D Electrical Equivalent of Heat

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

= 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

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

= 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

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.

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