A little bit of physics (Несколько текстов для зачёта), страница 6

Collecting and sorting used plastics is an expensive and time-consuming process. While about 35 percent of aluminum products, 40 percent of paper products, and 25 percent of glass products are recycled in the United States, only about 5 percent of plastics are currently recovered and recycled. Once plastic products are thrown away, they must be collected and then separated by plastic type. Most modern automated plastic sorting systems are not capable of differentiating between many different types of plastics. However, some advances are being made in these sorting systems to separate plastics by color, density, and chemical composition. For example, x-ray sensors can distinguish PET from PVC by sensing the presence of chlorine atoms in the polyvinyl chloride material.

If plastic types are not segregated, the recycled plastic cannot achieve high remolding performance, which results in decreased market value of the recycled plastic. Other factors can adversely affect the quality of recycled plastics. These factors include the possible degradation of the plastic during its original life cycle and the possible addition of foreign materials to the scrap recycled plastic during the recycling process. For health reasons, recycled plastics are rarely made into food containers. Instead, most recycled plastics are typically made into items such as carpet fibers, motor oil bottles, trash carts, soap packages, and textile fibers.

To promote the conservation and recycling of materials, the U.S. federal government passed the Resource Conservation and Recovery Act (RCRA) in 1976. In 1988 the Plastic Bottle Institute of the Society of the Plastics Industry established a system for identifying plastic containers by plastic type. The purpose of the "chasing arrows" symbol that appears on the bottom of many plastic containers is to promote plastics recycling. The chasing arrows enclose a number (such as a 1 indicating PET, a 2 indicating high density polyethylene (HDPE), and a 3 indicating PVC), which aids in the plastics sorting process.

By 1994, 40 states had legislative mandates for litter control and recycling. Today, a growing number of communities have collection centers for recyclable materials, and some larger municipalities have implemented curbside pickup for recyclable materials, including plastics, paper, metal, and glass.

Statistical Mechanics

Statistical Mechanics, in physics, field that seeks to predict the average properties of systems that consist of a very large number of particles. Statistical mechanics employs principles of statistics to predict and describe particle motion.

Statistical mechanics was developed in the 19th century, largely by British physicist James Clerk Maxwell, Austrian physicist Ludwig Boltzmann, and American mathematical physicist J. Willard Gibbs. These scientists believed that matter is composed of many tiny particles (atoms and molecules) in constant motion. These scientists knew that determining the motions of the particles by assuming each particle individually obeys Newtonian mechanics is unworkable, because any sample of matter contains an enormous number of particles. For example, a cubic foot of air contains about a trillion trillion (1 followed by 24 zeroes) particles. Rather than dealing with all of these microscopic particles individually, Maxwell, Boltzmann, and Gibbs developed statistical techniques to average the microscopic dynamics of individual particles and obtain their macroscopic (large-scale) thermodynamic features. Through their calculations they discovered that temperature is a measure of the average kinetic energy of microscopic particles. They also found that entropy is proportional to the logarithm of the number of ways a given macroscopic system can be microscopically arranged.

Statistical mechanics had to be extended in the 1920s to incorporate the new principles of quantum theory. The nature of particles is regarded differently in quantum theory than in classical physics, which is based on Newton's laws of motion. In particular, two classical particles are in principle distinguishable; just as two cue balls can be distinguished by placing an identifying mark on one, so in principle can classical particles. In contrast, two identical quantum particles are indistinguishable, even in principle, requiring new formulations of statistical mechanics. Furthermore, there are two quantum mechanical formulations of statistical mechanics corresponding to the two types of quantum particles—fermions and bosons. The formulation of statistical mechanics designed to describe the behavior of a group of classical particles is called Maxwell-Boltzmann (MB) statistics. The two formulations of statistical mechanics used to describe quantum particles are Fermi-Dirac (FD) statistics, which applies to fermions, and Bose-Einstein (BE) statistics, which applies to bosons.

Two formulations of quantum statistical mechanics are needed because fermions and bosons have significantly different properties. Fermions—particles that have odd half-integer spin—obey the Pauli exclusion principle, which states that two fermions cannot be in the same quantum mechanical state. Some examples of fermions are electrons, protons, and helium-3. On the other hand, bosons—particles that have integer spin—do not obey the Pauli exclusion principle. Some examples of bosons are photons and helium-4. While only one fermion at a time can be in a particular quantum mechanical state, it is possible for multiple bosons to be in a single state.

The phenomenon of superconductivity dramatically illustrates the differences between systems of quantum mechanical particles that obey Bose-Einstein statistics instead of Fermi-Dirac statistics. At room temperature, electrons, which have spin , are distributed among their possible energy states according to FD statistics. At very low temperatures, the electrons pair up to form spin-0 Cooper electron pairs, named after the American physicist Leon Cooper. Since these electron pairs have zero spin, they behave as bosons, and promptly condense into the same ground state. A large energy gap between this ground state and the first excited state ensures that any current is “frozen in.” This causes the current to flow through without resistance, which is one of the defining properties of superconducting materials.

Friction

INTRODUCTION

Friction, force that opposes the motion of an object when the object is in contact with another object or surface. Friction results from two surfaces rubbing against each other or moving relative to one another. It can hinder the motion of an object or prevent an object from moving at all. The strength of frictional force depends on the nature of the surfaces that are in contact and the force pushing them together. This force is usually related to the weight of the object or objects. In cases involving fluid friction, the force depends upon the shape and speed of an object as it moves through air, water, or other fluid.

Friction occurs to some degree in almost all situations involving physical objects. In many cases, such as in a running automobile engine, it hinders a process. For example, friction between the moving parts of an engine resists the engine’s motion and turns energy into heat, reducing the engine’s efficiency. Friction also makes it difficult to slide a heavy object, such as a refrigerator or bookcase, along the ground. In other cases, friction is helpful. Friction between people’s shoes and the ground allows people to walk by pushing off the ground without slipping. On a slick surface, such as ice, shoes slip and slide instead of gripping because of the lack of friction, making walking difficult. Friction allows car tires to grip and roll along the road without skidding. Friction between nails and beams prevents the nails from sliding out and keeps buildings standing.

When friction affects a moving object, it turns the object’s kinetic energy, or energy of motion, into heat. People welcome the heat caused by friction when rubbing their hands together to stay warm. Frictional heat is not so welcome when it damages machine parts, such as car brakes.

CAUSES OF FRICTION

Friction occurs in part because rough surfaces tend to catch on one another as they slide past each other. Even surfaces that are apparently smooth can be rough at the microscopic level. They have many ridges and grooves. The ridges of each surface can get stuck in the grooves of the other, effectively creating a type of mechanical bond, or glue, between the surfaces.

Two surfaces in contact also tend to attract one another at the molecular level, forming chemical bonds. These bonds can prevent an object from moving, even when it is pushed. If an object is in motion, these bonds form and release. Making and breaking the bonds takes energy away from the motion of the object.

Scientists do not yet fully understand the details of how friction works, but through experiments they have found a way to describe frictional forces in a wide variety of situations. The force of friction between an object and a surface is equal to a constant number times the force the object exerts directly on the surface. The constant number is called the coefficient of friction for the two materials and is abbreviated µ. The force the object exerts directly on the surface is called the normal force and is abbreviated N. Friction depends on this force because increasing the amount of force increases the amount of contact that the object has with the surface at the microscopic level. The force of friction between an object and a surface can be calculated from the following formula:

F = µ × N

In this equation, F is the force of friction, µ is the coefficient of friction between the object and the surface, and N is the normal force.

Scientists have measured the coefficient of friction for many combinations of materials. Coefficients of friction depend on whether the objects are initially moving or stationary and on the types of material involved. The coefficient of friction for rubber sliding on concrete is 0.8 (relatively high), while the coefficient for Teflon sliding on steel is 0.04 (relatively low).

The normal force is the force the object exerts perpendicular to the surface. In the case of a level surface, the normal force is equal to the weight of the object. If the surface is inclined, only a fraction of the object’s weight pushes directly into the surface, so the normal force is less than the object’s weight.

KINDS OF FRICTION

Different kinds of motion give rise to different types of friction between objects. Static friction occurs between stationary objects, while sliding friction occurs between objects as they slide against each other. Other types of friction include rolling friction and fluid friction. The coefficient of friction for two materials may differ depending on the type of friction involved.

Static friction prevents an object from moving against a surface. It is the force that keeps a book from sliding off a desk, even when the desk is slightly tilted, and that allows you to pick up an object without the object slipping through your fingers. In order to move something, you must first overcome the force of static friction between the object and the surface on which it is resting. This force depends on the coefficient of static friction (µ_s) between the object and the surface and the normal force (N) of the object.

A book sliding off a desk or brakes slowing down a wheel are both examples of sliding friction, also called kinetic friction. Sliding friction acts in the direction opposite the direction of motion. It prevents the book or wheel from moving as fast as it would without friction. When sliding friction is acting, another force must be present to keep an object moving. In the case of a book sliding off a desk, this force is gravity. The force of kinetic friction depends on the coefficient of kinetic friction between the object and the surface on which it is moving (µ_k) and the normal force (N) of the object. For any pair of objects, the coefficient of kinetic friction is usually less than the coefficient of static friction. This means that it takes more force to start a book sliding than it does to keep the book sliding.

Rolling friction hinders the motion of an object rolling along a surface. Rolling friction slows down a ball rolling on a basketball court or softball field, and it slows down the motion of a tire rolling along the ground. Another force must be present to keep an object rolling. For example, a pedaling bicyclist provides the force necessary to the keep a bike in motion. Rolling friction depends on the coefficient of rolling friction between the two materials (µ_r) and the normal force (N) of the object. The coefficient of rolling friction is usually about  that of sliding friction. Wheels and other round objects will roll along the ground much more easily than they will slide along it.

Objects moving through a fluid experience fluid friction, or drag. Drag acts between the object and the fluid and hinders the motion of the object. The force of drag depends upon the object’s shape, material, and speed, as well as the fluid’s viscosity. Viscosity is a measure of a fluid’s resistance to flow. It results from the friction that occurs between the fluid’s molecules, and it differs depending on the type of fluid. Drag slows down airplanes flying through the air and fish swimming through water. An airplane’s engines help it overcome drag and travel forward, while a fish uses its muscles to overcome drag and swim. Calculating the force of drag is much more complicated than calculating other types of friction.

EFFECTS OF FRICTION