New trends in physics teaching, v.4; The ... - unesdoc - Unesco

String and tape experiments
Current electricity
We have so far dealt with experiments on mechanics, heat, light and similar topics, but string and
sticky tape does not lend itself so easily to experiments on electricity. The addition of a roll of
aluminium foil to the other simple materials allows one to perform a number of experiments on
static electricity, but the problem of current electricity can only be solved by a source of direct
current (d-c.). Batteries are most often used in schools, the large 1.5 V cell being common. However,
such large cells are expensive, especially if you use one per student. Furthermore, I believe
there is a law, closely related to Murphy’s law, which states that whenever a battery is required
for physical experiments, it is always flat. The reason is, of course, that such batteries are
commonly employed only once or twice a year, and their shelf life is of this order. Furthermore,
students often short circuit the batteries unintentionally for considerable periods of time. I
contend that the best source of d.c., and also low voltage alternating current (a.c.1, is a model
electric train supply. The HO-gauge power supply provides d.c., variable from 0-15V, and is
cheap. Suppliers give reductions for bulk orders, but I have discovered that many high school
students are willing to bring in their personal supplies, having passed through the model train
stage of their development.
The only difficulty with such power supplies is that often they do not give as large an instantaneous
current as a battery (although supplies up to 2.5A are available), but they can be short
circuited more or less indefinitely, and are virtually indestructable, having been designed for the
use by small children. Most important, they never go flat.
1.5
1.3
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a,
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U
U
J 0.9
U
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U a,
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0 0 -7
I I I I 1 I I
0 200 400 600 800 1000 1200
Time / minutes
Figure 28.
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