H A&S 222d
Introduction to Energy and Environment:
Life Under the Pale Sun
P.B.Rhines, M Ewert
SCIENCE CORE: PHYSICS OF ENERGY, continued
We are beginning to fill in the list of ‘forms of energy’ and to give examples of the
..of energy present in familiar objects.
The kinetic energy in a 1 kg.
rock moving at 1 meter/sec is ½ Joule.
As shown earlier, Ch. 2 sec. 2.11, a flowing river can
contain a large kinetic energy by virtue of its size.
..its mass (although there it makes sense to
consider the power flowing past an observation point on the river, as well as the kinetic energy
per kg. of water). We will see again that thermal energy is ‘rich’, or concentrated and chemical
energy is even more so.
THERMAL ENERGY AND THE HEAT ENGINE
Thermal energy…’heat’…can be transformed into mechanical energy or vice-versa.
move by radiating light or invisible, infrared radiation. But some aspects of thermal energy are
Earlier we introduced the 1
law of thermodyamics, and mentioned the idea of an
engine that produces mechanical energy by converting some thermal or chemical energy. If you
squeeze a gas it becomes warmer…instantly.
This happens because ‘squeezing’ means
exerting a force and changing the volume of gas.
Recall that exerting a force on a
moving object requires
in the amount:
work = force x distance traveled….Joules
and this corresponds to:
power = force x velocity of the moved object
This is equal to the change in energy of the moving object.
In our case the object is the
gas, and compressing it requires work, and the work goes into internal heat energy of the
gas: it warms up.
Thinking of a gas like air as billiard-ball like molecules flying around,
bouncing off each other and bouncing off the walls of their container, you can imagine
that squeezing the gas could make the molecules move faster.
We can illustrate this by
squeezing the air in a spherical glass vessel, measuring the increase in pressure, and
seeing the temperature rise (for example, by the change in color of a liquid crystal
When you hit a tennis ball with a racquet, the speed of the ball is greatly
increased. Now think of a billiard ball bouncing off a wall.
If no energy is lost (the collision is called ‘elastic’), then
the speed of the ball perpendicular to the wall after rebounding, U
, is the same as before rebounding, U
, yet in the
opposite direction: U
If, instead, the wall is moving steadily toward the ball before the collision, then the ball’s
rebound speed will be greater, U
+ 2 U
We can see why by imagining that we are riding with the moving
wall. In this frame of reference, the ball bounces simply, conserving its velocity which is U
both before and after
An observer not riding on the moving wall thus sees speeds U
before and after the
In the 1
Law of Thermodynamics, that is the thermal energy equation,