FIG. 5-1 A rubber rod stroked with fur becomes negatively charged. When it is touched against a plastic ball, some of the negative charge flows to the ball. The plastic ball then flies away because like charges repel each other.

FIG. 5-2 A glass rod stroked with silk becomes positively charged. When one plastic ball is touched with a negatively charged rubber rod and another plastic ball is touched with a positively charged glass rod, the two balls fly together because unlike charges attract each other.

FIG. 5-3 When a rubber rod is stroked against a piece of fur, charges that were originally mixed together evenly become separated so that the rod becomes negatively charged and the fur becomes positively charged.

FIG. 5-4 An atom consists of a central nucleus of protons and neutrons with electrons moving around it some distance away. Shown is a simplified model (not to scale) of the most common type of carbon atom, which has six protons, six neutrons, and six electrons. Two of the electrons are relatively near the nucleus, the others are farther away.

FIG. 5-5 Electric charge is not continuous but occurs in multiples of ±e = ± 1.6 x 10^-19 coulomb.

FIG. 5-6 The forces between electric charges. When a rubber rod that has been stroked with fur is brought near a negatively charged plastic ball, the force on the ball is greater when the rod is held close to it and also greater when the rod has been vigorously stroked.

FIG. 5-7 (a) The force between two charges varies inversely as the square of their separation; increasing the distance reduces the force. (b) The force is proportional to the product of the charges.

FIG. 5-8 A charged object attracts an uncharged one by first causing a separation of charge in the latter.

FIG. 5-10 A gas such as air becomes ionized when x-rays disrupt its molecules. A molecule losing an electron becomes a positive ion; a molecule gaining an electron becomes a negative ion. Ultraviolet light, radiation from radioactive substances, sparks, and flames also cause ionization to occur.

FIG. 5-11 The ampere is the unit of electric current. The flow of charge in a circuit is like the flow of water in a pipe except that a return wire is needed in order to have a complete conducting path. An electric current is always assumed to go from the + terminal of a battery or generator to its — terminal through the external circuit.

FIG. 5-12 The flow of electric charge in a wire is analogous to the flow of water in a pipe. Thus having the water fall through a greater height at (b) than at (a) yields a greater flow of water, which corresponds to using two batteries to obtain a higher potential difference and thereby a greater current. .

FIG. 5-13 A 12-V storage battery consists of six 2-V cells connected in series.

FIG. 5-14 (a) A short, wide pipe yields a large flow of water, which corresponds to using a short, thick wire that offers less resistance to the flow of charge. (b) A long, narrow pipe yields a small flow of water, which corresponds to using a long, thin wire that offers more resistance to the flow of charge.

FIG. 5-15 (a) Symbols for a battery and a resistance. (b) A current of 3 A flows in a circuit whose resistance is 4 ohms when a potential difference of 12 V is applied. The current direction is from the + terminal of the battery to the - terminal.

FIG. 5-16 Series and parallel connections. Household electric outlets are always connected in parallel so each one has the same applied voltage of 120 V.

FIG. 5-17 (a) Like magnetic poles repel each other; unlike magnetic poles attract. (b) Cutting a magnet in half produces two other magnets. There is no such thing as a single free magnetic pole.

FIG. 5-18 The iron atoms in an unmagnetized iron bar are randomly oriented, whereas in a magnetized bar they are aligned with their north poles pointing in the same direction. The ability of iron atoms to remain aligned in this way is responsible for the magnetic properties of iron.

FIG. 5-19 Patterns formed by iron filings sprinkled on a card held over three bar magnets. The filings align themselves in the direction of the magnetic field. It is convenient to think of the pattern in terms of "field lines," but such lines do not actually exist since the field is a continuous property of the region of space it occupies.

FIG. 5-20 Oersted's experiment showed that a magnetic field surrounds every electric current. The field direction above the wire is opposite to that below the wire.

FIG. 5-21 Magnetic field lines around a wire carrying an electric current. The direction of the lines may be found by placing the thumb of the right hand in the direction of the current; the curled fingers then point in the direction of the field lines. In the right-hand diagram the current flows into the paper.

FIG. 5-23 The magnetic field of a loop of electric current is the same as that of a bar magnet.

FIG. 5-24 An electromagnet consists of a coil with an iron core, which considerably enhances the magnetic field produced.

FIG. 5-25 A magnetic field exerts a sidewise push on an electric current. In this arrangement, the wire moves to the side in a direction perpendicular to both the magnetic field and the current. A handy way to figure out the direction of the force is to open your right hand so that the fingers are together and the thumb sticks out. When your thumb is in the direction of the current and your fingers are in the direction of the magnetic field, your palm faces in the direction of the force. To remember this rule, think of your thumb in terms of hitchhiking and so as the current, of your parallel fingers as magnetic field lines, and of your palm as pushing on something. The same rule holds for the force on a moving positive charge. The force on a moving negative charge is in the opposite direction.

FIG. 5-26 The electron beam of a television picture tube is directed by magnetic fields to cover the screen in a pattern of horizontal lines.

FIG. 5-27 Equal and opposite forces are exerted by parallel currents on each other. The forces are attractive when the currents are in the same direction, repulsive when they are in opposite directions.

FIG. 5-28 A simple direct-current electric motor. The commutator reverses the current in the loop periodically so that the loop always rotates in the same direction.

FIG. 5-29 Electromagnetic induction. The direction of the induced current is perpendicular both to the magnetic lines of force and to the direction in which the wire is moving. No current is induced when the wire is at rest.

FIG. 5-30 An alternating-current generator. As the loop rotates, current is induced in it first in one direction (ABCD) and then in the other (DCBA). No current flows at those times when the loop is moving parallel to the magnetic field.

FIG. 5-31 How a 60-Hz (60 cycles/s) alternating current varies with time. The frequency of the current is the number of cycles that occur per second. Here each complete cycle takes 1/60 s. If a current in one direction in a circuit is considered +, a current in the opposite direction is considered -.

FIG. 5-32 A simple transformer. Momentary currents are detected by the meter when the current in coil A is started or stopped.

FIG. 5-33 Actual transformers usually have iron cores. The winding with the greater number of turns has the higher voltage across it and carries the lower current. The power in both windings is the same.

FIG. 5-34 A moving-coil microphone. When sound waves reach the diaphragm, it vibrates accordingly. The motion of the coil through the magnetic field of the magnet induces an alternating current in the coil that corresponds to the original sound. A loudspeaker is similar in construction except that an alternating current in its coil causes the diaphragm to vibrate and thereby produce sound waves.

FIG. 5-35 A tape recorder. The polarity and degree of magnetization of the magnetizable coating on the tape correspond to the pressure variations of the original sound wave.