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Thermodynamics is the Connection Between Heat and Work

By Ron Kurtus (revised 9 November 2013)

Thermodynamics is the study of the connection between heat and work and the conversion of one into the other.

This study is important because many machines and modern devices change heat into work (such as an automobile engine) or turn work into heat (or cooling, as in a refrigerator). There are two laws of thermodynamics that explain the connection between work and heat. But first, it must be shown how mechanical energy can be equivalent to heat energy.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Mechanical equivalent of heat

Experiments showed that the amount of heat created is proportional to the work done. This relationship is called the mechanical equivalent of heat and can be expressed by the equation:

W = JH

where

Note: A calorie is the amount of heat required to raise the temperature of one gram of water 1°C. It is not to be confused with a Calorie (capital "C") used in dieting.

Application

Using this equation, you could calculate the amount of heat generated from the work required to stop a moving car.

The way you do this is to calculate the kinetic energy (KE) of a car from its mass and velocity in joules (KE = ½ mv²). Since the work required to stop a moving car equals its kinetic energy, the total amount of heat generated in the brakes and tires to stop the car would be:

H = (KE)/4.18.

Laws of thermodynamics

There are two major laws concerning thermodynamics.

First Law of Thermodynamics

The First Law of Thermodynamics is the law of Conservation of Energy. It states that energy cannot be created or destroyed. Instead, it is converted from one form to another, such as from mechanical work to heat, from heat to light, from chemical to heat or such.

One example of that is how the kinetic energy of a moving car is converted into heat energy at the brakes and tire surfaces.

Another example is when chemical energy is released in burning and is converted into light and heat energy.

Second Law of Thermodynamics

The Second Law of Thermodynamics has several variations, which will be explained below.

Some heat is wasted in conversion

One version of the Second Law of Thermodynamics states that some heat is wasted when converting heat into mechanical energy.

In other words, in a car engine, not all of the heat created from the exploding gasoline is used in turning the engine or moving the car. Some of the heat simply heats the engine. The percentage of heat turned to work is called the thermal efficiency of the engine.

Heat flows from high to low

The Second Law of Thermodynamics also states that heat normally flows from high temperature to low temperature. For example, when you heat the end of a metal rod, the heat will gradually travel to the cool end and heat it up.

Heat sink

Another example of this part of the Second Law of Thermodynamics concerns what is called a heat sink, which is an object that absorbs heat from another. Usually, a large mass that absorbs heat from an object of smaller mass.

The effect is seen in waterbeds. The reason waterbeds use heaters to warm the water is because otherwise the heat from your body (at 98.6° F) will flow to the cooler water (at room temperature of 72° F). Since there is so much water in a waterbed, it would take much energy from your body to heat the water to body temperature. Thus, you can feel chilled from the loss of body heat.

Entropy

Another variation on the Second Law of Thermodynamics states that the energy available for work in the universe is continually decreasing.

This is also stated as, "The entropy of the universe is continually increasing." Entropy is the measure of the disorder of a system. In other words, in any closed system, objects are getting more and more mixed. Mixtures do not "unmix" by themselves.

Applications of energy conversion

Following are two applications of thermodynamics or the conversion of energy.

Internal combustion engines

An application of the conversion of energy is the type of engine used in a car, an internal combustion engine. The way this engine works is that gasoline and air is mixed and exploded in a cylinder. That explosion is the internal combustion, changing chemical energy to heat energy.

Since gases want to expand when they are heated, they exert pressure on the piston in the cylinder, causing it to move and turn a shaft. Thus, the heat energy is converted into mechanical energy.

Refrigerator

Another application of energy conversion is the refrigerator. Electrical energy is converted into mechanical energy in an electric motor. This motor operates a pump, which expands the gas, causing it to become cold. This is converting mechanical energy into heat (cold) energy.

As you recall, when a gas in a cylinder is compressed it heats up. The pump in the refrigerator compresses a special gas, condensing it into a liquid at a higher temperature. The liquid is held in a tube called a condenser. In most refrigerators, a fan forces air across the condenser, transferring the heat to the surrounding air.

If a gas in a cylinder expands, it cools off. The liquid refrigerant is then expanded through a restriction device into an evaporator inside the refrigerator where it becomes a gas again. This expansion absorbs heat from inside the refrigerator, thus cooling the contents of the refrigerator. Another fan spreads the cold air through the refrigerator by convection.

Summary

The First Law of Thermodynamics is the law of conservation of energy. The Second Law of Thermodynamics also states that heat normally flows from high temperature to low temperature. The refrigerator is an application of these heat laws.


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