Refrigerator

Refrigerator

A refrigerator is a cyclically operating device which absorbs energy as heat from a low-temperature body and rejects energy as heat to a high-temperature body when work is performed on the device. The objective of this device is to refrigerate a body at a low temperature. Usually, it uses the atmosphere as the high-temperature reservoir.

Figure 1

Refer to figure 1. Let and represents the amount of energy absorbed as heat from the low-temperature reservoir and the energy rejected as heat to the high-temperature reservoir respectively, Let W be the work done on the device to accomplish the task.

(1)

Therefore,

                 

and

(2)

              

(3)

Heat engine and the refrigerator (/heat pump) can be represented as shown in Figure 1.

The efficiency of a heat engine is given by

(4)

since  (heat) transferred to the system cannot be completely converted to work in a cycle. Therefore is less than unity. A heat engine can never be 100 efficient. Therefore., there has always to be a heat rejection. Thus a heat engine has to exchange heat with two reservoirs, the source and the sink. This experience leads to the proposition of the second law of thermodynamics which has been stated in several different ways.

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Intensive and Extensive properties with examples

PROPERTIES OF SYSTEMS

A property of a system is a characteristic of the system which depends upon its state, but

not upon how the state is reached. There are two sorts of property :

1. Intensive properties. These properties do not depend on the mass of the system.

Examples: Temperature and pressure.

OR

An extensive system property depends upon the total amount of material in the system. Mass, volume, internal energy, heat contents, free energy, entropy, and heat capacity are all extensive properties.

• Mass: This gives the idea of how much of the initial matter was contained in the system and how much is left after the process is complete.
• Volume: This gives an idea of the matter's dimension and what will be the final dimension after the process is over.
• Internal energy: It is the total energy contained in to create the thermodynamic system but excludes the energy to displace the system’s surroundings. It has two major components of kinetic energy and potential energy due to the movement of particles and the static electric power of the atoms contained in them.
• Heat contents: Under a given pressure, the heat content or Enthalpy is a measure of the total energy of a thermodynamic system. It includes internal energy which is required to create a system and establish its volume and pressure.
• Free energy: It is the energy in the physical system which can be converted into work.
• Entropy: It is a thermodynamic property that is used to determine the energy available for useful work in a thermodynamic process.
• Heat capacity: Heat capacity or thermal capacity is the measurable physical quantity that gives an idea of the amount of heat required to change a substance’s temperature by a given range.

2. Extensive properties. These properties depend on the mass of the system. Example :

Volume. Extensive properties are often divided by the mass associated with them to obtain the intensive

properties. For example, if the volume of a system of mass m is V, then the specific volume of

matter within the system is V

m = v which is an intensive property.

OR

An intensive property is defined as a property that is independent of the amount of material in the system. Density, molar property, surface tension, viscosity, specific heat, thermal conductivity, refractive index, pressure, temperature, boiling point, freezing point, and vapor pressure of a liquid are all intensive properties.

• Density: The density of a material is defined as the ratio between its volume and the matter contained in or mass.
• Molar property: Molar property mainly consists of the detailing of molar volume, molar energy, molar entropy, and molar heat capacity, and all these are quantified from the point of moles of the substance involved.
• Surface tension: It is a property of a liquid surface that helps in resisting any kind of external force applied to it.
• Viscosity: It is a measurable internal quantity of a fluid that resists its flow.
• Specific heat: It is the amount of heat per unit mass required to raise the temperature by one degree Celsius.
• Thermal conductivity: Thermal conductivity (λ) is the intrinsic property of a material that relates to its ability to conduct heat.
• Refractive index: The measure of the speed of light in a medium is referred to as the refractive index of that medium.
• Pressure: It is the perpendicular force acting per unit area on the surface of an object.
• Temperature: It is the property of the matter which quantitatively expresses the coldness or hotness of a substance.
• Boiling point: It is the temperature of the substance at which the vapor pressure of the liquid equals environmental pressure.
• Freezing point: It is the temperature at which a liquid composition solidifies under a given pressure.
• Vapour pressure of a liquid: It is defined as the equilibrium pressure above its liquid resulting due to the evaporation of liquid

3. Specific property: An extensive property expressed per unit mass of the system 

Example- specific energy, specific entropy

4: Molar property: The ratio of extensive property to mole number is known as the molar property

A thermodynamic system depending upon interactions between system and surroundings can be classified as an open system, closed system, or isolated system.

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Heat | गर्मी

Heat

Heat- Heat is energy. Sun is the natural source of heat energy

Definition:- Heat is the form of energy which produces the sensation of hotness or coldness.

Or

Heat energy of a system is defined as the sum molecules of the body.

Heat energy is also accurately called thermal energy or simply heat.  It is a form of energy transfer among particles in a substance (or system) by means of kinetic energy. In other words, heat is transferred from one location to another by particles bouncing into each other.

In physical equations, the amount of heat transferred is usually denoted by the symbol Q.

Units of Heat-

The SI unit for heat is a form of energy called the joule (J). Heat is frequently also measured in the calorie (cal.), which is defined as "the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius." Heat is also sometimes measured in "British thermal units" or Btu.

Sign Conventions for Heat Energy Transfer

Heat transfer may be indicated by either a positive or negative number. The heat that is released into the surroundings is written as a negative quantity (Q < 0). When heat is absorbed from the surroundings, it is written as a positive value (Q > 0).

A related term is heat flux, which is the rate of heat transfer per unit cross-section area. Heat flux may be given in units of watts per square meter or joules per square meter.

Measuring Heat

Heat may be measured as a static state or as a process. A static measure of heat is temperature. Heat transfer (a process that occurs over time) may be calculated using equations or measured using calorimeter. Calculations of heat transfer are based on variations of the First Law of Thermodynamics.

Example of Heat: A very hot cup of coffee is placed on the table. After a certain time we notice that both the cup and coffee are cold because the surrounding room temperature is the only 260C, Hence the heat flows from a hot cup (900C) to the surroundings. If you wait patiently for a long duration of time the coffee will be cooled to room temperature.

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Thermal reservoir

Thermal reservoir

A thermal energy reservoir (TER) is defined as a large body of infinite heat capacity, which is capable of absorbing or rejecting an unlimited quantity of heat without suffering appreciable changes in its thermodynamic coordinates. The changes that do take place in the large body as heat enters or leaves are so very slow and so very minute that all processes within it are quasi-static.

The thermal energy reservoir (TER) from which heat Q1 is transferred to the system operating in a heat engine cycle is called the source. The thermal energy reservoir TER to which heat Q2 is rejected from the system during a cycle is the sink.

A typical source is a constant temperature furnace where fuel is continuously burnt, and a typical sink is a river or sea or the atmosphere itself.

A mechanical energy reservoir (MER) is a large body enclosed by an adiabatic impermeable wall capable of storing work as potential energy (such as a raised weight or wound spring) or kinetic energy (such as a rotating flywheel). All processes of interest within an MER are essentially quasi- static. An MER receives and delivers mechanical energy quasi - statically.

Thermal reservoir

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Enthalpy | Specific heat

Enthalpy:-
Enthalpy is a property which is defined as the sum of inter-molecular energy and the product of pressure and volume of a system. It is denoted by symbol ' H '.
                            H=U+pV
Enthalpy H is an extensive property. The corresponding intensive property, the specific enthalpy h, is defined as
                           h=u+pv

Specific heat:-
The specific heat of a substance is defined as the amount of heat required to raise a unit mass of the substance through a unit rise in temperature. The symbol ' C ' will be used for specific heat.
Therefore,   C= Q/m.∆t J/kg k

Specific heat at constant volume:-
specific heat at constant volume is defined as the change of specific inter-molecular energy with temperature at constant volume.

                                              

Specific heat at constant pressure:-

specific heat at constant pressure is defined as the change of specific

enthalpy at constant pressure.

                                              

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