By Robert M. Hazen, Ph.D., George Mason University
The first law of thermodynamics is wonderful because energy can be changed from one form to another multiple times, and the total amount of energy is constant. However, the second law of thermodynamics places severe restrictions on how energy can be utilized. In fact, the second law of thermodynamics states that heat has a universal tendency to dissipate.
The First Versus the Second Law of Thermodynamics
The first law states that the total amount of energy is constant, but it says nothing about the way that energy can shift from one form to another. In everyday life, there are many limitations to these energy transfers. For example, a hot bowl of soup becomes cooler spontaneously, but a cold bowl of soup never becomes hot spontaneously.
Gasoline burns in the car to produce heat and exhaust, but the other way around never happens. And yet once again, that wouldn’t violate the first law of thermodynamics, because the total amount of energy in its different forms would remain constant. For example, objects fall downward in a gravitational field and release heat when they hit the bottom, and again the other way would never happen, which means objects don’t fall up.
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The Difference between Heat and Temperature
The most intuitive statement of the second law comes courtesy of British physicist William Thompson, Lord Kelvin. Kelvin said that heat tends to diffuse and move out evenly. This statement incorporates two related but different concepts—heat and temperature—that have very specific scientific meanings.
Heat is a quantity of energy and a measurable unit. The amount of heat an object holds can be measured in joules or in calories. But temperature, on the other hand, is a relative term. Two objects are at the same temperature if no heat flows spontaneously from one of them to the other. Two objects are at different temperatures if the heat does spontaneously flow from the warmer to the cooler object.
Temperature is measured with a thermometer. And every temperature scale, every thermometer, requires two reproducible reference points, such as the freezing and the boiling points of water. Also, there is a need for some physical property that changes, for example, the volume of mercury at a particular temperature. Therefore, the thermometer has some mercury at the bottom, and as the mercury gets cooler/warmer, its volume changes and moves up and down. And that gives a record of the temperature.
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The Concept of Heat Capacity
The difference between heat and temperature leads to another important property, and that’s heat capacity. Every material has the capacity to store heat energy, but some materials are much better at this than others. Think about placing a pound of copper and a pound of water on identical burners. Which one heats up more quickly? Common experience states that the metal gets hotter much faster, and this is because metals are unable to store very much heat energy.
But water is an extremely efficient energy storage, hence, water has a very high heat capacity. Heat capacity, in fact, is the amount of energy that a substance can hold, and since water is so efficient, this has significant implications for the climate. A useful measure of heat capacity is called ‘specific heat’, a scientific term.
How Heat Moves in the Second Law of Thermodynamics
There are three different ways to move heat. Conduction, convection, and radiation. Conduction is the transfer of heat from atom to atom in a solid object. Imagine a spoon in hot water, the handle becomes hot very quickly. That’s the process of conduction, the heat conducts from one end of the spoon to the other. Conduction is a rather slow process because temperature is maintained at a constant level.
The second method of moving heat is called convection, the transfer of heat through the motion of a whole body of a fluid, either a gas or a liquid. This includes boiling water, ocean currents, or the wind. Those are good examples of heat movement by convection.
There’s a third way heat moves, radiation. That’s the transfer of heat in the form of light energy or in heat energy, moving 186,000 miles per second through a vacuum. The Sun radiates energy, and that’s a way for it to transfer its heat energy, which is 93 million miles away, through the vacuum of space.
Learn more about the nature of energy.
Different Methods to Reduce Heat Transfer
One of the key technologies that scientists look for is ways to eliminate or reduce heat transfer. Humans, and nature as well, have devised many different ways of reducing the transfer of heat. Animals living in cold climates, for example, use insulation of various sources.
They develop fur, or they have feathers. The fur or feathers cuts down the efficiency of heat transfer by convection. If the little pockets of air are trapped, the cycling of air can’t proceed. Hence, the way heat is lost from an object is restricted to conduction, a much slower process. And that’s how insulating materials work, in clothing, in animals, and so forth.
Common Questions about an Introduction to the Second Law of Thermodynamics
Heat capacity is the amount of energy that a substance can hold. A useful measure of heat capacity is called ‘specific heat’.
According to the second law of thermodynamics, there are three heat motions: Conduction (transferring heat from atom to atom in a solid object), convection (heat transfer through the motion of a whole body of a fluid, either a gas or a liquid), and radiation (the transfer of heat in the form of light energy or in heat energy).
The second law of thermodynamics incorporates two related but different concepts—heat and temperature. Heat is a quantity of energy and a measurable unit and can be measured in joules or calories. But temperature, on the other hand, is a relative term. Temperature is measured with a thermometer.
