Science Matters (5 page)

Roller coasters are a microcosm of the universe. You were probably a bit too preoccupied to think about it the last time you
rode one, but as you climbed and zoomed down those hills, hitting all the fancy loops and bumps, you demonstrated the basic laws of how energy behaves. Everything you do or see requires energy, and that energy always follows two basic rules:

and

These two laws suggest good news and bad news. The first rule says that energy, the ability to perform useful work, comes in many different forms, and these forms are interchangeable. You can shift energy from one form to another the way you shift money between bank accounts. But just as transferring money neither adds to nor detracts from your wealth, changing energy from one form to another does not change the total amount of energy available. Energy cannot be created or destroyed, so the total energy of an isolated system stays the same. This powerful scientific statement is known as the first law of thermodynamics. It’s the good news.

The bad news about energy is that its conversion always leads from more concentrated (i.e., more useful) to less concentrated (less useful) forms. When you burn coal or gas, the laws of nature require that some of the high-grade energy in the fuel must be dispersed as heat in the atmosphere, where it cannot be recovered to perform useful work. This limitation on how we can use energy is called the second law of thermodynamics.

These two laws of thermodynamics govern all processes that involve heat and other forms of energy.

Thermodynamics relies on three concepts: work, energy, and power. Each of these words has an everyday meaning, but scientists use them in slightly different, specialized ways.

As far as physicists are concerned, work is done every time a force is used to move something. The amount of work done depends on how much force is used and how far the object moves (work equals force times distance). When you lift groceries, open a door, or throw a ball, you apply a force over a distance and move an object. You do work. The greater the force applied or the longer the distance moved, the more work is done. If you try to move something and fail (try to lift a car, for example, or push a wall) you haven’t done any work. You may have generated a force, but it was not applied over a distance. No matter how much effort you expend, if nothing moves, no work is done. Physicists can thus actually prove that bricklayers do more work than lawyers.

Energy is the ability to do work—the ability to exert a force.

Power is a measure of how quickly work is done: work done divided by the time it takes to do it. If you run up a flight of stairs instead of walking, you require more power, even though the total amount of work done is the same in both cases. In sports, the player who can generate power at the highest rates throws farther, hits harder, and runs faster.

Energy comes in many forms. It can be converted from one form into another, but all the forms have one thing in common: they involve a system that is capable of exerting a force.

A boulder perched at the edge of a cliff has the potential to do work. If it is allowed to fall, it will exert a force as it creates a depression in the earth below. It must therefore possess energy while it’s sitting quietly at the top. We call this potential energy, where the term “potential” signifies that the system could do work, but isn’t doing so at the moment. Think of the boulder as storing energy against future use.

If you dam a river, the water high in the reservoir has gravitational potential energy. We store this energy until we want to use it, at which time the water is allowed to fall and generate hydroelectric power. This is a common technique for generating electrical power, particularly in the northwestern United States.

There are many different kinds of potential energy. Stretch a rubber band or compress a spring and they are ready to snap back, exerting a force over a distance as they do so. We say the rubber band has elastic potential energy. Coal, gasoline, and other combustibles store chemical potential energy, which is released to do work when they are burned. Energy can also be stored in magnets, in batteries, in the surface tension of drumheads and soap films, and in many other systems in nature.

Anything that moves possesses energy. A fastball headed toward home plate, a rotating waterwheel, a speeding car, or a falling leaf can do work. You see this any time a moving object stops. A moving baseball exerts a force on the catcher’s mitt, compressing the material and leaving an indentation. It exerts a force over the distance of the compression. A speeding car that stops suddenly (by hitting a tree, for example) exerts a force that makes the tree
move. A rock slide or avalanche can obliterate an entire town. By virtue of the fact that it is moving, each of these objects can do work and therefore possesses kinetic energy.

Every atom in a material is in motion, either moving freely (as in a gas) or vibrating (as in a solid). The kinetic energy of all these atoms is related to the phenomenon we call heat. The more vigorously atoms move, the greater the heat energy in the material of which they are part. This way of explaining the nature of heat says, in effect, that heat is simply a special kind of kinetic energy—the kind associated with atoms in motion.

The recognition that heat is a form of energy was one of the great insights of nineteenth-century science, and is the foundation stone of thermodynamics. Although the notion seems simple to us (particularly when we picture heat as vigorously moving atoms), it is far from obvious that there is a connection between a pot of boiling water and a boulder perched on a hilltop.

When electrical current flows in a wire, electrons are moving. The kinetic energy of those electrons is one sort of electrical energy—one that lights your lamps, runs your stereo, and perhaps even cooks your food.

Sound is a special kind of kinetic energy caused by regular patterns of atomic movement. We hear sounds because our ear drums vibrate in response to moving air molecules.

Visible light carries with it energy that is related to electricity and magnetism. Other forms of radiation, such as radio waves and X-rays, carry the same kind of energy.

In the twentieth century, a new category was added to the roster of known types of potential energy—mass. The equivalence
of mass and energy is embodied in science (and in folklore) by Einstein’s famous equation:
E = mc
²
. The equation says that mass can be converted into other forms of energy (and vice versa).

One of the most important features of energy is that it easily shifts from one form to another. When you ride a bicycle, chemical energy in your cells changes into the motion of your legs and, ultimately, into kinetic energy as the bike moves. When you climb a hill, some of your energy is converted to gravitational potential, and when you coast down the other side of the hill, that potential energy is converted back into kinetic energy as you speed up.

No matter how often energy is converted from one form to another, there is one simple rule that always applies. The total energy—the sum of energy in all its forms—is the same after the conversion as it was before. Energy cannot be created or destroyed, only shifted from one type to another. We say that the total energy is “conserved,” and call this first law of thermodynamics the “conservation of energy.” Conservation of energy is one of the deepest and most widely applicable principles in the sciences, and operates everywhere from the largest supercluster of galaxies to the cells in your body to the smallest, most fundamental pieces of matter that we know about, the quarks. It is one of the great integrating principles of science.

The first law of thermodynamics is the joy of amusement park owners and the bane of dieters. It tells us that the higher the roller coaster climbs, the faster we’ll go. It also tells us that the chemical energy or calories we take in as food must either be
used to do work (exercise) or stored (as fat). The spate of new diet strategies that come out every year are a testimony to the power of the first law and to the futility of trying to avoid it.

An easy way to picture the operation of the first law is to think about that roller coaster ride. You start at the bottom and powerful motors gradually pull the coaster up a steep incline to its highest point. During this phase of the ride electrical energy is converted to gravitational potential energy. At the top, gravity takes over and the car starts down. The car exchanges its gravitational potential for kinetic energy as it descends, moving faster and faster until it gets to the bottom. At this point in the ride, all of the original gravitational potential energy has been transformed to kinetic energy, and the car is moving as fast as it can go. As it starts back up the hill, it slows until it almost comes to rest at the next summit. And so it goes, potential and kinetic energy passing back and forth as the car goes up and down.

In the absence of friction, this transfer between potential and kinetic energy would go on forever. At each location on the track the total energy of the car—the sum of potential and kinetic energies—is exactly the same. Energy is neither created nor destroyed. The process is like transferring $100 back and forth between two bank accounts.

In the real world, there are other shifts of energy—the heat energy of wheel friction, the sound energy of straining wood and steel, the vibrational energy of the wooden framework and ground. Energy transferred into these categories does not flow back into either potential or kinetic energy; it is lost to the environment. Thus, on each hill the amount of potential energy is a little less than it was on the preceding one (that’s why the second hill on a roller coaster is always lower than the first). The effect of friction is analogous to taking a dime out as a service charge every time you shift your $100 from one
account to another. The dime doesn’t disappear, of course—it goes to the banking company—but eventually the amount of money in your account is reduced to zero. In the same way, a roller coaster will eventually come to a halt, even if no brakes are applied. The energy hasn’t disappeared, but it has shifted from the roller coaster to other, less exciting accounts.

Living systems operate a lot like roller coasters. We take in chemical potential energy in the form of food, which is modified and stored in our cells. That chemical potential energy is constantly used to keep us warm, drive our muscles, and perform work. You can’t do anything without energy. Even thinking and sleeping consume it. So life is a constant struggle to gather and use energy.

Not only can energy be converted from one form to another, it can also be moved from one place to another. This is particularly true for heat energy. Heat flows from the burner on your stove into your food, for example, and from a drink into an ice cube. The transfer of heat plays a crucial role in many systems in nature, and is involved in effects as different as the movement of continents on Earth and the growth of plants. Heat moves from one place to another in three different ways: conduction, convection, and radiation. We experience all of these effects every day of our lives.

Put a metal spoon in a bowl of hot soup and the handle becomes hot in a matter of seconds—metal conducts the heat. If you had a very powerful microscope that allowed you to see inside the spoon, you would see its atoms near the soup moving
very fast because they had suffered collisions with fast-moving water molecules in the soup. Those fast-moving atoms in the metal then collide with slower-moving molecules farther up the spoon, which collide with atoms still farther up and so on. Eventually, the atoms at the very end of the handle are moving fast and we say the spoon is hot.
Conduction
thus relies on the transfer of heat energy through the motion and collisions of individual atoms.

Water boiling on a stove and hot air rising from the asphalt on a parking lot in summertime are both examples of heat transfer by
convection
. Think about the water in the pot. Heat from the stove causes the molecules of water in the bottom layer of the pan to move faster. Some heat travels up through the water by conduction, but the process is too slow and cumbersome to move all the energy that is pouring in. The water at the bottom heats up, expands, and rises, to be replaced by cold water from the upper regions of the pot. When this heated water gets to the top it cools off and sinks, to be replaced by newly heated water from the bottom. The cycle of heating and cooling goes on, creating what is known as a convection cell. Convection depends on heat being carried from one place to another by the bulk motion of warmed materials, rather than through collisions between individual atoms.

Hold your hand out toward a fire and you feel warmth, even though neither convection nor conduction is operating. Heat reaches you by
radiation
. In this case, infrared radiation (an invisible cousin of ordinary light) travels from the fire to your hand, carrying energy in the process. Every object in the universe gives off heat by radiation. Indeed, for something like a satellite or a star in the vacuum of space, radiation is the
only
way that heat can be given off.

The words “temperature” and “heat” are often used interchangeably, but scientists think of the two terms in quite distinct ways. Heat refers to the total amount of atomic kinetic and potential energy in a material. Two gallons of ice water hold twice as much heat energy as one gallon. Temperature, on the other hand, is a relative term. Two objects are at the same temperature if no heat flows between them. Put a pound of metal into a gallon of ice water and they will soon be at the same temperature—32° Fahrenheit. But the metal and ice water do not hold the same amount of heat energy, because it takes more energy to make the atoms of water vibrate.