Heat

Heat may be defined as energy in transit from a high temperature object to a lower temperature object.

Some Characteristics of Heat and Mathematical Equivalent

•    Heat is also defined as the transfer of kinetic energy from one medium or object to another, or from an energy source to a medium or object.

•    The heat transfer can occur in three ways: radiation, conduction, and convection.

•    The standard unit of heat in the International System of Units (SI) is the calorie (cal).

•    One calorie is defined as the amount of energy transfer required to raise the temperature of one gram of pure liquid water by one degree Celsius, provided the water temperature is higher than the freezing point and lower than the boiling point.

•    Sometimes the kilocalorie (kcal) is specified as a unit of heat; 1 kcal = 1000 cal. This is the also called diet calorie.

•    The amount of heat contained in a body depends upon the mass of the body.

•    If W is work performed and Heat produced is H, then W/H = J or W = JH, where J is a constant called mechanical equivalent of heat. The value of J is 4.186 Joule / Calorie. This means if 4.186 Joule of work is done, 1 Calorie of heat is consumed.

Temperature- Temperature is a measurement of the average kinetic energy of the molecules in an object or system and can be measured with a thermometer or a calorimeter. It is a means of determining the internal energy contained within the system.

Temperature Scales- Several temperature scales exist. In America, the Fahrenheit temperature is most commonly used, though the SI unit Centrigrade (or Celsius) is used in most of the rest of the world. The Kelvin scale is used often in physics, and is adjusted so that 0 degrees Kelvin is absolute zero.

Relation between temperatures on different scales
C – 0 / 100 = F – 32 / 180 = R – 0 / 80 = K – 273 / 100 = Ra-492 / 180

Freezing Point  of Mercury is -39 0C. So as to measure temperatures below this temperature alcohol thermometer is used. Freezing Point of alcohol is -115 0C.

Range of different thermometers

Mercury Thermometer- -30 0C to 350 0C
Constant Volume Thermometer- From -200 0C to 500 0C with H2, below -200 0C up to -268 0C with He; above 1000 0C up to 1600 0C with N2 and bulb of glazed porcelain.
Platinum Resistance Thermometer- -200 0C to 1200 0C
Thermocouple Thermometer- from -200 0C to 1600 0C

Total Radiation Pyrometer - A pyrometer which focuses heat radiation emitted by a hot object on a detector (usually a thermopile or other thermal type detector), and which responds to a broad band of radiation, limited only by absorption of the focusing lens, or window and mirror.

Specific Heat- Specific heat is the amount of heat required to change temperature of one kilogram of a substance by one degree. Specific heat may be measured in kJ/kg K.

Specific Heat Capacity of Some Common Materials

 Substance Specific Heat - cp - (cal/gramoC) (J/kgoC) Air, dry (sea level) 0.24 1005 Asphalt 0.22 920 Bone 0.11 440 Ice (0oC) 0.50 2093 Granite 0.19 790 Sandy clay 0.33 1381 Quartz sand 0.19 830 Water, pure 1.00 4186 Wet mud 0.60 2512 Wood 0.41 1700

Thermal Expansion- When a body is heated its length, surface area and volume increases. This increase in length, area and volume of the body due to rise in temperature are measured in terms of coefficient of linear expansion or linear expansivity (α), coefficient of superficial expansion or superficial expansivity (β) and coefficient of cubical expansion or cubical expansivity (γ).

Relation between α, β and γα : β : γ = 1 : 2 : 3 or β = 2 α or γ= 3α

Anomalous Expansion of Water- Water shows unusual expansion. If we take a cube of ice at -5°C and heat it, it expands till ice starts melting. During melting its temperature remains 0°C but its volume decreases. If heat is continuously supplied to water at 0°C, it further contracts up to 4°C and then it starts expanding. Thus water has its minimum volume and maximum density at 4°C. This unique property of water is called anomalous expansion of water.

The anomalous expansion of water helps preserve aquatic life during very cold weather. When temperature falls, the top layer of water in a pond contracts, becomes denser and sinks to the bottom. A circulation is thus set up until the entire water in the pond reaches its maximum density at 4°C. If the temperature falls further, the top layer expands and remains on the top till it freezes. Thus even though the upper layer are frozen the water near the bottom is at 4°C and the fishes etc. can survive in it easily.

Conduction - Heat conduction is the flow of internal energy from a region of higher temperature to one of lower temperature by the interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space.

Convection - Convection is the transfer of internal energy into or out of an object by the physical movement of a surrounding fluid that transfers the internal energy along with its mass. Although the heat is initially transferred between the object and the fluid by conduction, the bulk transfer of energy comes from the motion of the fluid. Convection can arise spontaneously (or naturally or freely) through the creation of convection cells or can be forced by propelling the fluid across the object or by the object through the fluid.

Radiation - Heat radiation is the transfer of internal energy in the form of electromagnetic waves. For most bodies on the earth, this radiation lies in the infrared region of the electromagnetic spectrum.

Kirchhoff’s Law

According to Kirchhoff’s Law, the ratio of emissive power to absorptive power is same for all surfaces at the same temperature and is equal to the emissive power of black body at that temperature.

Kirchhoff’s Law signifies that good absorbers are good emitters.

Newton’s Law of Cooling

Newton's Law of Cooling states that the rate of change of the temperature of an object is proportional to the difference between its own temperature and the ambient temperature (i.e. the temperature of its surroundings).

The total radiant energy emitted E per unit time by a black body of surface A is proportional to the fourth power of its absolute temperature.

E ∝ T4

or E =  σAT4      where  σ = Stefan's constant

For a body which is not black body

E =  ε σAT4     Where ε   = emmisitivity of the Black Body.

Wien's Law

At a constant temperature T, when wavelength ƛ  is increased , the energy emitted E, first increases, reaches a maximum and then decreases i.e at a particular temperature the spectral radiancy Εƛ is a maximum at a particular wavelength  ƛm.

Change of Stat

Any material may remain stay in any one of the three states; solid, liquid or gas. To change the material from one state to another is called change of state. For this either substance is heated or heat is drawn from the substance. Change of state happens at a fixed temperature.

 Term For Change Of State Phase Change Fusion (melting) Freezing Vaporization (boiling) Condensation Sublimation Deposition Solid to Liquid Liquid to Solid Liquid to Gas Gas to Liquid Solid to Gas Gas to Solid

Fusion – The process through which a substance is changed from solid state to liquid state is called fusion. Fusion takes place at a fixed temperature called Melting Point (MP).

Freezing- The process by which a substance is changed from liquid state to solid state is called freezing. Freezing takes place at a fixed temperature called Freezing Point.

Vapourization –  The process through which a substance is changed from liquid state to vapor state is called vapourization. Vapourization takes place by two methods- 1. Evaporation and 2. Boiling.

Evaporation- The change of a liquid into a vapor at a temperature below the boiling point. Evaporation takes place at the surface of a liquid, where molecules with the highest kinetic energy are able to escape. When this happens, the average kinetic energy of the liquid is lowered, and its temperature decreases.

Boiling- Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure.

Condensation-Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition.

Latent Heat- Heat absorbed or released as the result of a phase change is called latent heat. There is no temperature change during a phase change, thus there is no change in the kinetic energy of the particles in the material. The energy released comes from the potential energy stored in the bonds between the particles.

Sublimation: Sublimation is the term for when matter undergoes a phase transition directly from a solid to gaseous form, or vapor, without passing through the more common liquid phase between the two. It is a specific case of vaporization.

The most well known example of a material that undergoes sublimation is dry ice, or frozen carbon dioxide.
Relative Humidity - Relative humidity is the ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.