Science Matters by Robert M. Hazen Read Free Book Online Page A

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 oneaccount 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.
HEAT

Moving Heat
    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 movingvery 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