Heat and Temperature
[caption id="attachment_59756" align="alignnone" width="287"]
physics[/caption]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.
Thermal Expansion
Thermal expansion is a phenomenon that takes place in solids, liquids, and gases. Almost all substances expand when their temperatures increase, unless they are constrained in some manner. Examples include the heating of air in a hot air balloon, which makes the balloon expand and rise, and mercury in a thermometer, which rises in response to heat. Metal rods are used in a variety of applications also. For example, metal rods or strips that are used as expansion joints at the ends of bridge sections account for the expansion of steel bridges in hotter weather. The amount of expansion that occurs and how we predict it depends on the substance. For example, a solid metal rod generally expands linearly and increases in length, while liquids and gases experience an increase in volume. In all three cases, thermal expansion occurs in response to an increase in temperature and useful devices take advantage of this concept.
Thermodynamics
Thermodynamics is the study of heat and its transformation to mechanical energy. There are four laws of thermodynamics, but we only concentrate on the two principal laws here: the first law and second law.
The first law says that the change in internal energy of a substance equals the work done on it plus the heat transferred to it. Mathematically, we use the equation:
delta U = work + Q
Internal energy is the sum of the kinetic and potential energies of all the atoms and molecules within a substance. The significance of the first law of thermodynamics is that there are two ways to increase the temperature of a substance:
1) By exposing it to another substance that has a higher temperature and
2) By doing certain kinds of work on the substance
Friction and compression of gases are two examples of ways to increase temperature by the work method. Pistons in internal combustion engines take advantage of this concept. Air is compressed in a cylinder by the piston, which raises the temperature to almost twenty-seven times the temperature of the uncompressed state.
The second law says that heat cannot be transferred from a colder body to a hotter body without work being done by an outside agent. Stated another way, no device can be built that will repeatedly extract heat from a source and deliver mechanical energy without ejecting some heat to a lower-temperature reservoir. The perfect example is the heat engine, which is discussed later in this lesson.
Heat Transfer
Heat transfer occurs by three mechanisms: conduction, convection, and radiation.
- Conduction is the transfer of heat between atoms and molecules in direct contact
- Convection is the transfer of heat by movement of the heated substance itself, such as by currents in a fluid
- Radiation is the transfer of heat by way of electromagnetic waves
.jpg)