Heat & Temperature

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.


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.


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.

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. )

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.

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