Calorie
A calorie is defined as the amount of energy required to increase the temperature
of a gram water by one degree Celsius. This is equivalent to @J. If you
count your calories when you eat, you are actually counting kilocalories.
That is, the calorie used in measuring food energy is actually one thousand
of the calories we just defined.
British Thermal Unit (BTU).
A BTU is the amount of energy required to increase the temperature of one
pound of water by one degree Fahrenheit. This is equivalent to @ J. For
some reason, the ability of air conditioners to extract energy from air
is always expressed in BTUs.
Energy
It is usually the case that when you add energy to a bunch of atoms they
move faster and get hotter. Similarly, if you remove energy from a bunch
of atoms, they usually move less and get cooler. Because adding heat energy
usually results in a temperature rise, people often confuse heat and temperature.
In common speech, the two terms mean the same: "I will heat it"
means you will add heat; "I will warm it up" means you will increase
the temperature. No one usually bothers to distinguish between these. Adding
heat, however, does not always increase the temperature. For instance, when
water is boiling, adding heat does not increase its temperature. This happens
at the boiling temperature of every substance that can vaporize. At the
boiling temperature, adding heat energy converts the liquid into a gas WITHOUT
RAISING THE TEMPERATURE.
Feedback
Flow and collection are controlled with negative feedback. Many things flow.
Water flows in ditches and pipes, electric charge flows through wires, air
flows over the wings of an airplane, and heat flows from hot to cold. The
rate a which a flow can fill a container depends of the speed of the flow
the effective cross section of the ditch or pipe or wire. Perhaps more interesting
is the principle of negative feedback. In this control process, a changing
condition of some system is regulated to keep it more or less constant.
When the condition reaches some set maximum, the cause of the change is
shut off or reversed.
Heat
You can usually warm something by adding energy. The added energy can be
from light, electricity, friction, a chemical reaction, nuclear reaction,
or any other kind of energy. When first added to a substance, energy might
be concentrated in one atom, but this one will soon bump into others and
spread the energy. Eventually, every atom or molecule in the substance will
move a bit faster. When the added energy is spread throughout a substance,
it is then called heat energy, thermal energy, or, simply heat. All three
terms mean the same thing. Heat is a form of energy, so it has the units
of energy. In the SI system, this is Joules (J). Many other units to measure
thermal energy are in common use. Calories and BTU's are common
heat units.
Heat Change
Heat Energy (Q) The heat energy put into an object by increasing its temperature
(T) is proportional to the temperature change and proportional to the mass
(m) of the object being warmed. The proportionality constant (c), called
the specific heat, depends on the material the object is made of. Heat change
Q = c * m * (T)
Heat Flow
Heat flow is measured as the amount of energy transferred per unit of time.
In SI units this would be measured in Joules per second. A Joule per second
is the same as a Watt (W), so heat flow should be expressed in Watts. Because
thermal energy is often measured in non-SI units, heat flow is often reported
in other units, too. Air conditioners are rated in BTUs per hour. You will
sometimes see ergs per second or calories per second.
Heating Rate
Rate is the change as time (t) passes. Graphs which have time as the independent
variable are standard means of depicting rate. The slope of a "vs.
time" graph tells the rate. Because temperature is proportional to
heat in a sample of material (if it is not freezing or melting) temperature
vs. time graphs are indications of energy change. Rate of heating (or cooling)
r = Q÷t
(Note: Here T is used to represent Temperature, whereas t is used to represent
time. Be sure you keep the UPPER and lower case t's straight. )
Temperature
You cannot measure heat directly, but you can detect its effect on a substance.
Changes in heat can usually be detected as changes in temperature. When
you add heat energy to a substance, it usually warms; when you remove heat
energy it usually cools.
Temperature Units
In the SI system, temperature is measured in degrees Celsius (C). This scale
is defined by: The freezing point of water = 0 C The boiling point of water
= 100 C. The boiling point of water changes somewhat with altitude, so it
is important to add that 100 C is the boiling temperature of water at sea
level. Actually, it is the atmospheric pressure that influences the boiling
temperature, so, for accuracy, an average atmospheric pressure at sea level
is used, taken as a pressure of 760 mm of mercury. The Celsius scale is
commonly used in every country except the U. S. where the old Fahrenheit
(F) scale is used that is defined by: The freezing point of water = 32 F
The boiling point of water = 212 F. There are a total of (212 p; 32)
= 180 Fahrenheit degrees between freezing and boiling, almost twice as many
as in the Celsius scale. That means every Celsius degree represents almost
twice as large a step as a Fahrenheit degree.
