The Definitions of Heat Transfer

                                                                     The Definitions of Heat Transfer

    Heat is the form of energy that is transferred between two substances at different temperatures. The

direction of energy flow is from the substance of higher temperature to the substance of lower

temperature. Heat is measured in units of energy, usually calories or joules. Heat and temperature

are often used interchangeably, but this is incorrect.

    Temperature is the measure of hotness or coldness of matter. Stated another way, temperature is

the average kinetic energy per molecule of a substance. Temperature is measured in degrees on the

Celsius (C) or Fahrenheit (F) scale, or in kelvins (K). In simplest terms, temperature is how hot or cold

an object is, while heat is the energy that flows from a hotter object to a cooler one. For example, the

temperature of a cup of coffee may feel hot if you put your hand around it. It is hot because heat from

the coffee is transferred to the cup.

    Heat energy, or thermal energy is the energy of a substance or system in terms of the motion or

vibrations of its molecules. The faster the molecules in a substance move, the more heat energy they

have. You can transfer heat energy from one object to another, and in fact, a heat transfer will occur

naturally whenever a hotter object is in contact with a colder one; heat will always move from the hotter

object to the colder one. For example, if you were to put your warm hands against a cold metal pole,

the skin on your hands would feel cold. That's because the heat from your hands transfers to the

cooler metal. Your hands feel colder because some of your heat has been transferred to the metal.

    When you add heat energy to a substance, one of two things can happen: either the temperature of

the substance will increase, or the phase (or state) of the substance will change. Temperature is the

average kinetic energy of the molecules in a substance. So it's pretty obvious why temperature might

increase - heat is the motion of molecules, so if you get the molecules in a substance to move faster,

then they'll have a higher temperature. But what about phase change? 

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