First Law of Thermodynamics

The fi rst law of thermodynamics states that the change in an object’s internal energy equals the heat fl owing into or out of the object, minus the work the object does on its surroundings. An object loses energy by doing work or because heat fl ows out of it, and it gains energy if heat fl ows into it or work is done on the object.

Internal energy is the random motion of the object’s molecules (which as mentioned in the fi rst chapter are always moving). All moving objects have energy, called kinetic energy, but people cannot see the motion associated with internal energy since atoms and molecules are so small. Yet this energy exists, even in an object such as a glass of water that is sitting on a table; the water appears to be still since its molecules have no ordered motion—they are not all moving to the left or to the right—but the molecules are moving randomly in different directions, and this motion is energy. As the previous chapters discussed, heat fl owing into the object increases this motion, and heat fl owing out decreases it.

Work is the result of a force acting over a distance, such as lifting a weight or pushing a cart. Doing work requires a source of energy, which comes from the object doing the work. In physics, the law of energy conservation states that energy is neither created nor destroyed but can be transformed into one form or another. Lifting a weight is work, but the energy required by the process is “stored” in the potential of the lifted weight—when it falls, the weight can do work, as in dams that use falling water to produce electricity. But pushing a cart across a level street is also work, yet the cart does not seem to gain any potential energy— the cart cannot get back on its own or do work in the process.

The fi rst law of thermodynamics accounts for all energy transformations, which include the disordered motion created by friction. In the example of the pushed cart, the tires and the street became a little warmer because they were rubbed together. Energy is conserved, but orderly motion, such as the movement of a cart across a street, can be transformed into disordered motion of the object’s molecules—internal energy.

Heat can do work, and heat engines are the subject of chapter 4. But there are special circumstances involved in using random internal motion to do work, and the fi rst law of thermodynamics is not the whole story. The second law, discussed below, addresses this issue.

58 Time and Thermodynamics

Convection currents are often used for cooling purposes. As one of the mechanisms of heat transfer, these currents are well suited to do so. The circulating air or water draws off heat, and then deposits it elsewhere.

A car radiator works by the circulation of a liquid through pipes and hoses in and around the engine. The liquid is usually water, with antifreeze compounds added so that the liquid does not freeze during the winter. Water, having a large heat capacity, does an excellent job of absorbing heat from the engine. But that heat must be deposited somewhere before the water can come around to the engine again and absorb some more. This is the job of the radiator, which exposes the water to air rushing by (due to the car’s motion or, when it is not moving, a fan). This is another convection current, generated by movement of the car through the air. The water is cooled by this “breeze” and returns to the engine to pick up some more heat.

The radiator has a name that sounds like the term radiation, which is another mechanism of heat transfer. Although some radiation is always involved in warm objects, the primary job of the car’s radiator is to use convection currents to cool down the engine.

Elephants have radiators, in the same sense that cars do. In an elephant, the radiators are their ears. They work because warm blood flows through vessels in an elephant’s ears, which are large and thin. The convection current cools down the blood, which then passes through the body again and picks up some more heat. But in cold weather an elephant would prefer to hide its ears, or at least keep them close to its body, in order to conserve heat and maintain body temperature.

Large power plants, which produce much of the country’s electricity, also have problems with overheating. Considering the huge electric currents that power plants must generate, this is not sur- prising. Many power companies use convection currents in the same way as automobile radiators, by circulating cold water.

But there is a problem with this method when it is used by power companies. Power plants, unlike cars, are never traveling down the highway at a high rate of speed. How does the circulating water get rid of its heat? The heat has to go somewhere—the first

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This elephant is keeping its ears close to its body to conserve warmth on a chilly day in December. (Elizabeth Kirkland)

law of thermodynamics insists upon it. Energy does not simply vanish.

The solution to this problem has unfortunate consequences. Since the water used by power companies does not get rid of its heat, the power company gets rid of the warm water and draws some more cold water from the same source. The source is usually a stream, river, or lake. The circulation of water is therefore not contained within the power plant but consists of the whole water source—the power plant draws off cold water and returns warm water. This process has been described as thermal pollution; in the same way that released chemicals create chemical pollution, released heat creates thermal pollution. Rivers and lakes that experience thermal pollution can suffer adverse effects, especially to their wildlife. The laws of physics are fairly strict about this, and the heat produced by power plants has to go somewhere. But this does not mean that delicate natural environments should be destroyed, and physicists and engineers are working on ways to cool down power generators with as little environmental disrup- tion as possible.

The other two heat transfer mechanisms (conduction and radiation) can also control temperature, but they are not as widely

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used as convection. Engineers design certain parts of a spaceship to lose excess heat by radiation; this is necessary in the vacuum of space because there are no convection currents. Soldering electronic components involves the application of high temperature, but heat can ruin the component. To prevent this, soldering tech- nicians attach a heat sink to the electronic component. The heat sink is a large piece of metal that conducts away a lot of the heat generated by soldering.

First Law of Thermodynamics