Heat and Temperature
The transfer of energy from a high temperature body to a lower temperature body is known as heat. Heat travels in form of waves and like any other form of energy it has the potential to bring change into the matter it contacts. This process is usually accompanied by the transfer of entropy, the energy per unit temperature that goes unused. Heat transfer is usually generated by three types of thermal interaction, mainly, conduction, convection and radiation. Conduction is mostly related to the transfer of heat in solids, wherein the materials are in direct contact with each other and the heat flows in form of vibration of microscopic particles. Convection is a process of transfer of heat in liquids and gases, in which the heated particles move upwards and the cold and denser particles fall down causing a current; this continuous motion causes the heating. The transmission of electromagnetic waves in space is known as radiation, where there is no existence of the mediums like solids, liquids or gases for the transfer of heat.
Sun is a natural source of heat; this heat energy is called the Solar Energy. The heat energy found on earth in form of natural geysers, rocks and molten rocks in the core of earth is known as Geothermal Energy. The other known sources of heat are burning of fuels like coal, gasoline and natural gases. Mechanical force like friction between two elements is also a source of heat. The exhaust air is one such heat source for the heat pumps. Few more sources are heat generated from Nuclear reactors, electricity and industrial wastes. Industrial wastes examples are the treated or untreated waters from the sewer and the condenser heat from the refrigeration plants.
Heat is a form of energy that is measured using a calorimeter and determined in Joules (J). In various applied disciplines of engineering it is also measured in the British thermal unit (BTU) and the calories (cal). One BTU is equivalent to approximately 252 cal (Weber, 1950). The rate of heat transfer is Watt (W), defined as joules per second.
Heat is further distinguished into Latent heat and Sensible heat. The change of phase due to the presence or absence of heat is called latent heat. There is no change of temperature in this process. Similarly, the change of temperature of an object due to heat is called sensible heat. Then, we have the specific heat, characterized as the amount of energy required by a system to increase its temperature by one degree or the heat capacity per unit mass of the system.
Temperature is the physical property, which states the hotness or coldness of any matter. As stated by (Fiedler, 2007), Temperature is a number stated in degrees A substance having low temperature is referred as cold and the ones with higher temperature is referred as hot. Temperature is measured with the use of a thermometer. The basic International System of Units (SI) unit of temperature is Kelvin (K); however, Celsius (°C) is used for most applications. The United States uses the Fahrenheit (°F) scale for all common purposes.
Various physical properties of material, like the different phases like the solid, liquid, gaseous and plasma, solubility, density, electrical conductivity are temperature dependent. Temperature is important in influencing the rate and degree of chemical reactions. Temperature is also used to determine the thermal radiation of a surface, the example of which is the incandescent bulb. The tungsten filament in the bulb is heated to a certain temperature to produce the required amount of visible light.
Heat is a form of energy, whereas temperature is the quantitative effect of the presence or absence of the heat energy. Temperature is directly proportional to heat. When heat is applied to a substance, its temperature increases; and its temperature decreases with the removal of the heat. The application of heat leads to vibration of particles; with constant application, the vibrations that are caused energize the medium which causes the increase in temperature. The vibration of particles caused is also called the kinetic energy. Heat is only passed on between objects, with the help of different temperatures (Buckley, 2003). Heat is dependent on the size of the substance but the temperature of a given substance, irrespective of its size, remains the same. Heat and temperature are the main components of the three laws of thermodynamics. The first law of thermodynamics is about the conservation of energy, it states, the change in the internal energy of the system is equal to the amount of heat added to it subtracted by the amount of work done by the system. The second law is about the entropy of the system, it states, the entropy or the loss of unused energy of any system which is not in thermal equilibrium constantly increases. The third law states, the entropy of a system becomes a constant as the temperature reaches to an absolute zero. These laws describe as to how the physical quantities like the temperature, energy and the entropy of a system perform under different conditions.
Heat is the measure of the energy in a system and temperature is the measure of the heat content of the system. The relation between heat and temperature is that, the flow of heat is a derivative of the difference in temperatures of two adjacent systems. The heat flow continues till both the systems come in a state of thermal equilibrium. Adding of heat to a system does not always increase the temperature; the temperature increase persists till any matter reaches its melting point and still further, the boiling point. After reaching this point there is a change in the state of the substance. For example, water after reaching 100 °C, adding more heat will turn the water into vapor. In this instance the adding of heat causes the breaking of bonds between the molecules of the substance, which brings about the change of state. This is how the solids transform to liquid on addition of heat and liquids further vaporize to gaseous state.
Heat capacity is the amount of heat energy required to yield a desired change in the temperature of a system. The properties of the substance affecting the heat capacity are mainly, mass and volume of the substance and the average kinetic energy of the particles in the system. The ability of the system to retain heat is also the factor affecting the heat capacity. The greater the mass and volume of the substance, higher will be the heat capacity required.
The kinetic theory explains the matter comprising of large number of particles in a constant motion. This movement of particles is arbitrary in gases, whereas in the condensed matter like solids and some liquids it is much sort of linear and the particles here form a collective arrangement of atoms or molecules known as Phonon or a Quasiparticle. The kinetic energy in the system is caused by the movement of particles at different velocities through Brownian motion. The molecules possess potential energy which keeps them separated regardless of the constant attraction between them. The increase in velocity is due to the collisions of particles to the boundaries of the system. In solids, the molecule movement is restricted to a fixed shape and volume, and cannot change positions. In liquids, the molecules have sufficient energy to prevent the binding forces between them. There is no restriction considering the shape. In gases, the molecules are well apart and practically free from attraction.
The kinetic energy caused by motion of molecules is termed as the heat. The kinetic theory and heat transfer both have a common factor in them, which is well known as motion of molecules. The kinetic energy of a system depends on the temperature of the system. The application of heat to a system increases the kinetic energy in the system by increasing the vibrations in the molecular arrangement of the system. Similarly, this increase of the kinetic energy in the system results in the increase in the temperature of the system. Thus the two components, the kinetic energy and heat are interdependent of each other.
Weber, Robert.L, (1950), Heat and Temperature Measurement, Prentice Hall.
Fiedler, Julie (2007), Learning About Heat and Temperature with Graphic Organizers, The
Rosen Publishing Group.
Buckley, Peter (1991), Macmillan Bookshelf: The Big Heat Level 2, Macmillan Education.