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

New Trends in Physics Teaching IV
How energy is taught
In the first period, examples of the ‘warming capability’ of bodies are given: a warmer body
warms a colder one; burning oil, coal, wood or gas warms the stove; an electric heater or iron
connected to the mains supply wil warm up; bodies in the sunshine wil be warmed; a moving
hammer makes a nail warmer when it strikes it; a revolving wheel moved by the spring of a toy
car can warm up a thermometer against which the wheel is allowed to rub; another thermometer
is warmed by the body of a person and that person’s body is warmed as food is consumed. It is
clear to the pupils that all these things have a ‘warming capability’.
In the second period, the pupils investigate the factors on which the ‘warming capability’ of a
body may depend. They use beakers containing 0.1 kg of water at room temperature, at 40°C
and at 60°C and another with 0.2 kg of water at 40°C. They determine the final temperatures
when they dip the colder and the warmer beakers into water at the intermediate temperature.
They conclude that the larger the mass, the greater the ‘warming capability’. The teacher then
declares that the different ‘warming capabilities’ of bodies Carl be characterized by the ‘energy’
and the descriptions of the observations wil be phrased in sentences using the new word ‘energy’.
The energy of the fuel burnt is proportional to the mass and at the end of this lesson the pupils
learn to designate different kinds of energy - elastic, kinetic, internal and chemical.
Whether the body possessing energy can cause only a temperature rise and how the energy of
bodies taking part in interactions changes wil be examined in the third period. Sunshine melts
ice, heat cooks, eggs harden and some plastics soften in boiling water. A moving ball may break
a window, the electric mains supply wil light lamps and causes electric motors to rotate. From
such examples the pupils can conclude that bodies having energy can not only heat other bodies
but can also cause many other changes. By analyzing the interaction of a cold body entering
warmer surroundings, or the collision between two balls or between a hammer and a nail, discussion
leads us to conclude that, during the interaction, the states of the interacting bodies change
and that, whilst the energy of one body increaases, the energy of the other decreases. The energy
gains are equal to the energy losses. These experiences lead to the concept of the function of
state.
In the fourth period, the pupils meet the concept of mechanical work. With the help of experiments,
they notice that the internal energy of a body can be changed by friction. Halving a cork
and pressing the two halves on the bulb of a thermometer whilst turning the pieces of cork round
causes the thermometer to show a temperature rise. This is a result of friction. The pupils notice
that when the cork turns twice, the path over which the friction acts is twice as long; the temperature
rise is also twice as much. And so on. When the pieces of cork are pressed together more
tightly, the temperature rise is greater for the same number of turns. The temperature rise and
the change in the internal energy depend on the force and the frictional path. The changes can
be calculated from the distance, s, and the force component parallel to it, F, through the relationship
AE = F s. A change of energy always accompanies the performance of work.
The energy of fields is studied by the pupils in the fifth period. On the basis of the observations
that an electric field can change the state of motion of light bodies or particles and that the field
of a rubbed plastic rod makes a small glow-discharge lamp flash as it is moved along the rod, the
pupils readily deduce that the electric field possesses energy. However, the electric field itself
changes as well; no light can be seen when the glow-discharge lamp is moved a second time. A
magnetic field can change the motion of a magnet or of a moving piece of iron. The pupils have
already seen that the motion of a piece of iron in a magnetic field wil change the field, and it
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