HEAT AND THERMODYNAMICS

HEAT AND THERMODYNAMICS

Heat Energy and Thermodynamics
In this topic we are going to discuss about heat energy, temperature and its measurement, transfer of heat energy and the effects of heat on matters. It is important that you take out time and read our previous topics in physics because it will help understand what we are going to deal with here.

We will talk about heat and thermodynamics in this topic. We know that word heat but in physics it is important to understand what it means. Heat by definition is a means by which energy is transferred from a hot object to a cooler object.

Remember that objects are made up of matter and matter consists of atoms and particles. Particles that move around in an object possess kinetic energy. Increase in temperature increases the kinetic energy of a particle.

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Let's think of a real life situation that demonstrates energy transfer in form of heat. Imagine if we place a cup of hot tea on top of a table. Let's also assume that the temperature of the tea is 90 degrees Celsius. At that temperature, it is impossible for someone to attempt to drink the tea because it can burn your mouth.

But what happens if the hot cup of tea is left on the table for a long time? Yes I know you guess right. The cup of tea will lose some heat energy which will in return lower the temperature to equal the temperature of its surrounding.

This simple example shows that energy is transfer from the tea which is at a higher temperature to the surrounding which is at a lower temperature.

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We normally talk about a system and its surrounding when we are discussing heat energy and heat transfer. In our case in this example we will regard the cup of tea as our system while the environment where the cup of tea is placed as the surroundings.

This brings us to how we can measure the temperature of an object.

Temperature
Temperature is the measure of the average kinetic energy of the particles in a sample of matter, expressed in terms of units or degrees designated on a standard scale. It is important to note that the higher the temperature of an object the higher the tendency of that object to transfer heat and via-visa.

Temperature of an object can be measured with an instrument called a thermometer. There are three types of thermometers – Celsius thermometer, kelvin thermometer and Fahrenheit thermometer. Each of the thermometers can be used to measure temperature even though they all have different scales.

One of the confusing aspects of heat and temperature is that quite often most students usually interchanged the meaning of heat and temperature as the same. Understanding the difference between heat and temperature is very important in physics.

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One outstanding difference between heat and temperature is that heat is a form of energy while temperature is not.

Secondly, temperature can be measured with a device called thermometer while heat cannot be measured directly with any kind of device.

Heat Transfer
In our initial study of heat, we mention that energy is transfer from a hot object to a cooler object; we will proceed to learn the different ways by which energy can be transferred from one system to another. Note also that the internal energy of a substance is generated from the motion of its individual atoms or molecules.

Heat transfer can take place through the following processes- conduction, convention and radiation. We will go further to discuss each and every one of them in details.

Conduction
Conduction is a form of heat transfer that involves the flow of the internal energy from a region of higher temperature to region with lower temperature without any motion of the material as a whole.

A simple example of heat transfer by conduction is when you heat one edge of a metal on a fire; the increase in temperature of the region in contact with the fire will increase the kinetic energy of its particles. The particles will experience a higher speed and will randomly collide with other particles in the cooler region.

As these particles collide with one another, the particles with higher kinetic energy will transfer some of its energy to particles with lesser kinetic energy. This will continue to happen until there is a thermal equilibrium.

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Convention
Convection is a form of heat transfer in liquids and gases where by the atoms or molecules of a heated substance moves up to the upper region due to decrease in density and the atoms or molecules in the upper region moves to the lower region. This form of circulation will continue until there is a thermal equilibrium.

Radiation
Radiation is quite different from conduction and convention since it does not use any medium for transfer of energy. Radiation is the transfer of heat by means of electromagnetic waves. A simple example of radiation is the heat from sun.

Latent Heat
The energy required to change a gram of a substance from the solid to the liquid state without changing its temperature is commonly called its "heat of fusion". This energy breaks down the solid bonds, but leaves a significant amount of energy associated with the intermolecular forces of the liquid state.

Change of State
Thermodynamics
Thermodynamics is a branch of natural science concerned with heat and its relation to energy and work. It defines macroscopic variables (such as temperature, internal energy, entropy, and pressure) that characterize materials and radiation, and explains how they are related and by what laws they change with time.

Thermodynamics describes the average behavior of very large numbers of microscopic constituents, and its laws can be derived from statistical mechanics. It is also the subject of the relation of heat to forces acting between contiguous parts of bodies, and the relation of heat to electrical agency.

The four laws of thermodynamics define fundamental physical quantities (temperature, energy, and entropy) that characterize thermodynamic systems. The laws describe how these quantities behave under various circumstances, and forbid certain phenomena (such as perpetual motion).

Laws of Thermodynamics
The four laws of thermodynamics are:

• Zeroth law of thermodynamics: The zeroth law in its wider sense establishes a notion of internal thermodynamic equilibrium of a system. In a narrow sense, the law states that if two systems are both in thermal equilibrium with a third system then they is in thermal equilibrium with each other. This law helps define the notion of temperature.

• First law of thermodynamics: The first law establishes a notion of internal energy for a thermodynamic system. Heat and work are forms of energy transfer. The internal energy of a thermodynamic system may change as heat or matter is transferred into or out of the system or work is done on or by the system. All the energy transfers must be accounted for to see that there is strict conservation of the total energy of a thermodynamic system and its surroundings. The law implies that perpetual motion machines of the first kind, which would do work without using the energy resources of a system, are impossible.

• Second law of thermodynamics: An isolated physical system, if not already in its own internal state of thermodynamic equilibrium, spontaneously evolves towards it. In an isolated physical system, there is a tendency towards spatial homogeneity. In particular, when an isolated physical system reaches its own internal state of thermodynamic equilibrium, its temperature is spatially uniform. When work is done on or by a thermodynamic system, a certain amount of that energy is lost to inefficiency, related to the difference between the energy level of the input and the output. This loss is described by the notion of entropy, which is often used to express the law. Some of the loss is due to friction when work is done, and some of it may be due to the relaxation of the system towards spatial homogeneity. The law says that these two mechanisms occur always and inevitably. The law implies that perpetual motion machines of the second kind are impossible.

• Third law of thermodynamics: There are various ways of expressing the third law. They derive from the statistical mechanical explanation of thermodynamics. They refer to ideally perfect theoretical models of physical systems. A common expression of the law states that no practicable means can bring a physical system to an exactly zero absolute thermodynamic temperature.

Initially, the thermodynamics of heat engines concerned mainly the thermal properties of their 'working materials', such as steam.

This concern was then linked to the study of energy transfers in chemical processes, for example to the investigation, published in 1840, of the heats of chemical reactions by Germain Hess, which was not originally explicitly concerned with the relation between energy exchanges by heat and work.

Chemical thermodynamics studies the role of entropy in chemical reactions. Also, statistical thermodynamics, or statistical mechanics, gave explanations of macroscopic thermodynamics by statistical predictions of the collective motion of particles based on the mechanics of their microscopic behavior.