Force and motion

FORCES ARE PUSHES OR PULLS that change the motion of objects. To make a stationary object
move, or a moving object stop, a force is needed. A force is also required to change the speed or direction of an object. This change in speed or direction is known as acceleration. Acceleration depends on the size (magnitude) of the force, and on the mass of the object. The effects of forces were first summarized by Isaac Newton in his three laws of motion. The international unit of force, named after him, is the newton (N), which is approximately equal to the weight of one apple. Gravity—the force of attraction between any two masses—can be measured using a newton meter (spring balance). Forces are put to useful effect in machines. A simple machine, such as a wheel and axle, is a device that changes the size or direction of an applied force. It allows an applied force (the effort)
to produce another force (the load). A lever uses a bar that turns on a fulcrum to exert force. In all simple machines, there is a relationship between force and distance. A small force (in a compound pulley, for instance) moves through a large distance to lift a heavy object a small distance. This is called the Law of Simple Machines.

Light

LIGHT IS A FORM OF ENERGY. It is a type of electromagnetic radiation, like Xrays or radio waves. All electromagnetic radiation is produced by electric charges: it is caused by the effects of oscillating electric and magnetic fields as they travel through space. Electromagnetic radiation is considered to have both wave and particle properties. It can be thought of as a wave of electricity and magnetism. In that case, the difference between the various forms of radiation is their wavelength. Radiation can also be said to consist of particles, or packets of energy, called photons. The difference
between light and X-rays, for instance, is the amount of energy that each photon carries. The complete range of radiation is referred to as the electromagnetic spectrum, extending from low energy, long wavelength radio waves to high energy, short wavelength gamma rays. Light is the only part of the electromagnetic spectrum that is visible. White light from the Sun is made up of all the visible wavelengths of radiation, which can be seen when it is separated by using a prism. Light, like all forms of electromagnetic radiation, can be reflected (bounced back) and refracted (bent). Different parts of the electromagnetic spectrum are produced in different ways. Sometimes visible light— and infrared radiation—is generated by the vibrating particles of warm or hot objects. The emission of light in this way is called incandescence. Light can also be produced by fluorescence, a phenomenon in which electrons gain and lose energy within atoms.

Electricity and magnetism

ELECTRICAL EFFECTS result from an imbalance of electric charge. There are two types of electric charge, named positive (carried by protons) and negative (carried by electrons). If charges are opposite (unlike), they attract one another, while like charges repel. Forces of attraction and repulsion (electrostatic forces) exist between any two charged particles. Matter is normally uncharged, but if
electrons are gained, an object will gain an overall negative charge; if they are removed, it becomes positive. Objects with an overall negative or positive charge are said to have an imbalance of charge, and exert the same forces as individual negative and positive charges. On this larger scale, the forces will always act to regain the balance of charge. This causes static electricity. Lightning, for example, is produced by clouds discharging a huge excess of negative electrons. If charges are “free”—in a wire or material that allows electrons to pass through it—the forces cause a flow of charge called an electric current. Some substances exhibit the strange phenomenon of magnetism—which also produces attractive and repulsive forces. Magnetic substances consist of small regions called domains. Normally unmagnetized, they can be magnetized by being placed in a magnetic field.Magnetism and electricity are inextricably linked, a fact put to use in motors and generators.

Energy

ANYTHING THAT HAPPENS—from a pin-drop to an explosion—requires energy. Energy is the capacity for “doing work” (making something happen). Various forms ofenergy exist, including light, heat, sound, electrical, chemical, nuclear, kinetic, and potential energies. The Law of Conservation of Energy states that the total amount of energy in the universe is fixed—energy cannot be created or destroyed. It means that energy can only change from one form to another (energy transfer). For example, potential energy is energy that is “stored,” and can be used in the future. An object gains
potential energy when it is lifted; as the object is released, potential energy changes into the energy of motion (kinetic energy). During transference, some of the energy converts into heat. A combined heat and power station can put some of the otherwise “waste” heat to useful effect in local schools and
housing. Most of the Earth’s energy is provided by the Sun, in the form of electromagnetic radiation. Some of this energy transfers to plant and animal life, and ultimately to fossil fuels, where it is stored in chemical form. Our bodies obtain energy from the food we eat, while energy needed for other tasks, such as heating and transport, can be obtained by burning fossil fuels—or by harnessing natural forces like wind or moving water—to generate electricity. Another source is nuclear power, where energy is released by reactions in the nucleus of an atom. All energy is measured by the international
unit, the joule (J). As a guide, one joule is about equal to the amount of energy needed to lift an apple one meter.

Chemical reactions

A CHEMICAL REACTION TAKES PLACE whenever bonds between atoms are broken or made. In each case, atoms or groups of atoms rearrange, making new substances (products) from the original ones (reactants). Reactions happen naturally, or can be made to happen; they may take years, or only an instant. Some of the main types are shown here. A reaction usually involves a change in energy. In a burning reaction, for example, the making of new bonds between atoms releases energy as heat and light. This type of reaction, in which heat is given off, is an exothermic reaction. Many reactions, like burning, are irreversible, but some can take place in either direction, and are said to be reversible. Reactions can be used to form solids from solutions: in a double decomposition reaction, two compounds in solution break down and re-form into two new substances, often creating a precipitate (insoluble solid); in displacement, an element (e.g., copper) displaces another element (e.g., silver)
from a solution. The rate (speed) of a reaction is determined by many different factors, such as temperature, and the size and shape of the reactants. To describe and keep track of reactions, internationally recognized chemical symbols and equations are used. Reactions are also used in the laboratory to identify matter. An experiment with candle wax, for example, demonstrates that it contains carbon and hydrogen.

The periodic table

AN ELEMENT is a substance that consists of atoms of one type only. The 92 elements that occur naturally, and the 17 elements created artificially, are often arranged into a chart called the periodic table. Each element is defined by its atomic number—the number of protons in the nucleus of each of its atoms (it is also the number of electrons present). Atomic number increases along each row
(period) and down each column (group). The shape of the table is determined by the way in which electrons arrange themselves around the nucleus: the positioning of elements in order of increasing atomic number brings together atoms with a similar pattern of orbiting electrons (orbitals). These appear in blocks. Electrons occupy shells of a certain energy. Periods are ordered according to the filling of successive shells with electrons, while groups reflect the number of electrons in the outer shell (valency electrons). These outer electrons are important—they decide the chemical properties of the atom. Elements that appear in the same group have similar properties because they have the same number of electrons in their outer shell. Elements in Group 0 have “filled shells,” where the outer
shell holds its maximum number of electrons, and are stable. Atoms of Group I elements have just one electron in their outer shell. This makes them unstable—and ready to react with other substances.

Atoms and molecules

ATOMS ARE THE smallest individual parts of an element . They are tiny, with diameters in the order of one ten-thousand-millionth of a meter (10-10 m). Two or more atoms join together (bond) to form a molecule of a substance known as a compound. For example, when atoms of the elements hydrogen and fluorine join together, they form a molecule of the compound hydrogen fluoride. So molecules are the smallest individual parts of a compound. Atoms themselves are not indivisible they possess an internal structure. At their center is a dense nucleus, consisting of protons, which have a positive electric charge (see p. 316), and neutrons, which are uncharged. Around the nucleus are the negatively charged electrons. It is the electrons that give a substance most of its physical and chemical properties. They do not follow definite paths around the nucleus. Instead, electrons are said to be found within certain regions, called orbitals. These are arranged around the nucleus in “shells,” each containing electrons of a particular energy. For example, the first shell (1) can hold up to two electrons, in a so-called s-orbital (1s). The second shell (2) can hold up to eight electrons, in s-orbitals (2s) and p-orbitals (2p). If an atom loses an electron, it becomes a positive ion (cation). If an electron is gained, an atom becomes a negative ion (anion). Ions of opposite charges will attract and join together, in a type of bonding known as ionic bonding. In covalent bonding, the atoms bond by sharing their electrons in what become molecular orbitals.

The variety of matter

MATTER IS ANYTHING THAT HAS A MASS. It includes everything from natural substances, such as minerals or living organisms, to synthetic materials. Matter can exist in three distinct states—
solid, liquid, and gas. A solid is rigid and retains its shape. A liquid is fluid, has a definite volume, and will take the shape of its container. A gas (also fluid) fills a space, so its volume will be the same as the volume of its container. Most substances can exist as a solid, a liquid, or a gas: the state is determined by temperature. At very high temperatures, matter becomes plasma, often considered to be a fourth state of matter. All matter is composed of microscopic particles, such as atoms and molecules. The arrangement and interactions of these particles give a substance its physical and chemical properties, by which matter can be identified. There is a huge variety of matter because particles can arrange themselves in countless ways, in one substance or by mixing with others.Natural glass, for example, seems to be a solid but is, in fact, a supercool liquid: the atoms are not locked into a pattern and can flow. Pure substances known as elements combine to form compounds or mixtures.
Mixtures called colloids are made up of larger particles of matter suspended in a solid, liquid, or gas, while a solution is one substance dissolved in another.