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

New Trends in Physics Teaching IV
length with sticky tape, as shown in figure 6a. Fasten the top end to a sheet of card or paper
using tape. Mark the position of the bottom end very carefully. Now, you must use the cup to
pour the hottest water available through the lower straw. To do so, make a little funnel by folding
one end of the third straw, as shown in the figure, so that it wil fit into the top end of the
lower straw. Slice the top end off the third straw diagonally to make it easier to pour through.
Record how much the lower end of the straw shifts by making a pencil mark. Qualitatively, it is
easy to see how the hot straw expands against the cold one, as shown in figure 6b, pushing both
straws into a bow shape.
It is also interesting to study the result quantitatively. Boiling water wil be at 100°C but if it
is not available, measure the temperature using a thermometer. Measure the length of the straw
L and the distance it moves, d. Then, the radius to which the straws bow, r, is given by 2rd = L2.
If the centre of the straws is separated by a distance a, and the hot straw expands an amount
x, then
x=(r+a)0-r0
where 0 = L/r
x = aL/r = 2 ad/L
The coefficient of linear expansion CY is given by the ratio
Hence,
CY = expansion/(original length X increase in temperature)
= x/LT
where Tis the temperature increase.
d = -aL2
1
T/a
2
The coefficient is roughtly for the type of plastic of which straws are often made, so a
temperature rise of 50°C where a is 0.5 cm and L is 20 cm moves the bottom end of the straw
2 cm which is easily measurable. Heat loss and other problems generally gave rise to a low
measured value for the coefficient.
A drinking straw thermometer
It is difficult for students to understand the concept of temperature without a thermometer.
Here is a simple experiment requiring only sticky tape, a straw and a little water, which demonstrates
Charles’ law as well as giving the temperature.
Fold the end of a straw over two or three times as shown, and fasten it with sticky tape
(figure 7).
Fill the open end of the straw with about 5 cm of water (it may be easier to put the water in
first, before sealing the other end). If you place the closed end in your mouth, you can see that
the expanding hot air forces the water out. Remove the straw from the mouth, and notice how
the air moves the water back up the tube (to its original position) as it cools. Now squirt cold
water over the straw, or place it in a cold drink. The water wil move way back.
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