Physics: The Story of How Things Work für 12-Jährige

Physics begins with a promise: that ordinary things can be put in order and tested again. Dew on grass, echoes in hallways, thermometers and motors, ships that float and balloons that rise, magnets that pull nails and lightning that crackles across the sky. This is not just curious stuff on the side of life—it is the very stuff science is made of. Ordinary knowledge is a mix of experience and rumor; scientific knowledge arranges it so that effects are tied to causes and ideas are linked to measurements. In this work, we use some simple words: a hypothesis is a suggested explanation, a theory is a well-tested framework that ties many findings together, and a law is a rule that always holds, often so clear that it can be calculated. To speak together, we use common measures, especially the metric system with its steady units.
Matter fills space. Energy is the ability to make something happen. The molecular idea ties all this together: all substances are made of incredibly small molecules that never stop moving. How tightly they lie and how strongly they attract each other decide whether something is solid, liquid, or gas. Heat them up, and the molecules move faster. In solids, they hold their neighbors and resist changes in shape. In liquids, they slide past each other. In gases, they roam widely and fill spaces. Some changes—melting ice, boiling water, dissolving sugar—preserve identity and are called physical. Others, like burning magnesium or scorching sugar, make something new and are chemical. Physics mostly cares about physical changes and the measurable relationships they obey.

Proof of molecular motion is everywhere. A smell spreads across a room even if no one blows. If you let out vapors of ammonia and hydrochloric acid a short distance apart, they rush toward each other and form a visible cloud about midway. Gases also run at different speeds through porous cups: hydrogen outruns air, swells a container, and can make a small jet; when the supply is cut, hydrogen slips out faster than air can get in, creating suction. The pressure a gas exerts is not because it is tightly packed—steam shrinks to a fraction of its volume when it condenses—but because molecules keep hitting the walls without stopping. Heat makes the hits harder; pressure rises or the gas expands.
Look closely at very small particles suspended in a liquid, and they dance jerkily. That is Brownian motion: uneven little hits from molecules hitting them from all sides. Liquids also spread dissolved substances, they evaporate into drier air and cool surfaces because the fastest molecules escape first. Water can go through thin membranes and be pulled toward stronger solutions. That is called osmosis. Liquids and gases flow, but not equally easily. Some are thicker than others, and even gases have a little 'thickness' that slows the flow, just as syrup flows more slowly than water.

Molecules are drawn to each other, both inside a substance and between different substances. When similar molecules hold together, it is called cohesion. When different surfaces attract each other, it is called adhesion. A thin skin on top of a liquid—surface tension—can do strange things: soap bubbles become tight and shiny, a thin steel needle can rest on the water's surface, wet brush hairs bunch into a point, and a tiny insect can actually be carried along on a stretched liquid skin. When two liquids with different surface tension meet, one can spread suddenly or pull together on the other.
In narrow spaces, adhesion sometimes wins over cohesion. Then water creeps up inside thin tubes, called capillary action. When the wall does not get wet, like mercury in glass, the liquid is pulled down. Wicks, towels, blotters, and sponges work that way. Waxed cloth lets water vapor through but not liquid water. Soil and forest floors save and share moisture through such tiny forces, and dry-land farming lives on them.
Dissolving and crystallization show that molecules 'know' each other. Over time and right conditions, substances build crystals with impressive order. Porous solids take up gas, and liquids dissolve gases more or less depending on temperature and pressure. That makes soda fizzy and determines how much oxygen fish get in warm summer water.

The holding together of solids shows in strength and springy properties. If you pull a thread within reasonable limits, it becomes the same length again. If you press down a spring and let go, it bounces back. Within a certain limit, elasticity is simple to describe: double the force gives double the stretch. If you go beyond the limit, the shape does not come back completely. Elasticity exists in push and pull, in bending and twisting, in shape and in volume. Outside forces disturb the balance between attraction and heat motion. If you remove the forces in time, the original shape rises again.
Other properties vary greatly because the internal structure varies: gold can be beaten as thin as leaf and still hold together; spring steel endures sudden blows; glass resists changing shape for a long time and then suddenly gives way with a snap. Such differences—toughness, brittleness, hardness—are not just words. They decide what we can build and how we must treat materials.

Where liquids and gases rest or flow, they press against everything around. Pressure increases with depth and depends on density. If you hold a card against the end of a tube and lower it into water, the water is kept out by air pressure until the forces inside and outside the card balance. Measure it, and do it again, a rule appears: at the same level in a liquid, pressure equals the weight of the column of liquid above that level. The total force on a surface depends on the area, the average depth, and how heavy the liquid is per unit volume.
Pressure in a liquid is the same in all directions at the same point. If you close the liquid in and add extra pressure at one spot, this pressure goes unchanged through the whole liquid. This insight was clarified by a thinker who saw what we now see everywhere: a small cylinder pushing on a liquid can lift heavy loads in a wide cylinder. Hydraulic presses and jacks do just that. We trade movement length for force, and the area ratio tells how much gain. It is like using a long arm to lift something heavy.
Water systems enjoy this—and suffer from it. An artesian well can flow by itself when water in a sloping layer lies higher than the hole we drill far away. Large steel pipes in a city get their flow needs smoothed out by high water towers. Small air cushions at taps dampen the shocks when we shut off quickly.

Liquids also carry things. An old law says that a body immersed in a liquid experiences an upward push equal to the weight of the liquid it displaces. A floating object pushes down exactly as much as the water pushes up, and how deep it sinks tells how heavy a load it carries. If you weigh a stone in air and then in water, the difference in weight equals the weight of the displaced water, and so you get the volume. That is how we find density and specific gravity. Floaters sink deeper in light liquids and shallower in heavy ones. They are called hydrometers and sit as tiny spies on density in everything from milk to acid.
Gases share a lot with liquids. They flow, they carry, but they also let themselves be squeezed easily. Air has weight. A column of mercury in a glass tube stands about three-quarters of a meter high, held up by air pressure. If we remove the air around, the column falls. If we carry the instrument up a mountain, it falls with height. That is how it was shown once and for all that air presses.
Another rule says that for a fixed amount of gas at the same temperature, volume will decrease when pressure increases, and vice versa. That gives us a simple tool to understand everything from real clouds to play with syringes.

From these properties grow clever tools. An air pump pulls air out of a container through valves. A water jet in a tube can create suction and empty air from a device; it is called an aspirator. Pumps that push gas into bottles or tires use the same game. Lift pumps get well water by letting air pressure push water into a cylinder when the piston makes the air inside a little thinner. But because air pressure can only lift water about ten meters, deeper wells need the cylinder down at the water level.
Force pumps push water out through valves. Air chambers at the outlet smooth the flow so it doesn't just come in jerks. A siphon, once filled, carries liquid over a rim as long as the outlet is lower than the surface we start from. A little glass diver in a water-filled bottle shows how pressure goes everywhere equally, and that gas can be squeezed more than liquid. Squeeze the bottle, the diver sinks. Let go, it rises.
Water hammer, which we usually avoid in pipes, can still be used in a smart machine: a hydraulic ram lifts part of a falling water stream upward with no other power than the fall itself. Balloons rise because they displace more air than they weigh. Parachutes slow falls by giving a large surface to air. Compressed air stops trains, measures gas in houses, and drives pumps that throw liquid upward by spinning blades.

A force is a push or a pull. We can measure it by how much it stretches something elastic, or by how much it changes motion. A spring scale is reliable because, within the elastic limit, equal extra push gives equal extra stretch. A force has size and direction and acts at a point. We can draw it as an arrow and add arrows together. Equal forces in the same line become bigger together. Two forces at an angle can be replaced by one—the resultant—which points along the diagonal between them. The force that would hold everything still points exactly opposite to the resultant and is the same size.
We also have to distinguish mass and weight. Mass tells how much stuff something has and how hard it is to change its speed. Weight is the Earth's pull on the thing, and changes a little with where we are. Motion comes in several kinds: sliding along, spinning, swinging. Speed is how fast and which way, acceleration is how quickly speed changes. A falling object accelerates. A ball thrown up is slowed by the same downward acceleration.
The amount of motion, momentum, is mass times speed. Changes in it measure what a force has done.

Three clear statements help us understand almost everything that moves. First: A body stays at rest or continues straight at the same speed if no net force acts. That is called inertia. When something swings in a circle, a force must pull inward toward the center, otherwise it would fly straight out. Cut the string on a sling, the stone flies off tangentially.
Next: The change in momentum is in the direction of the force and is proportional to how big the force is. Two forces act independently and add together in their effect. Finally: Action and reaction are equal in size and opposite in direction. When we push on the floor, the floor pushes on us. A scaffold pulls on the wall, and the wall pulls back. All this means we often have to split a force into parts along directions we care about. A sled pulled by a slanted rope gets a part that pulls forward and a part that lifts a little.
On an inclined plane, we can split the weight into a part that tries to pull the sled down and a part that presses it against the surface. For rotation, we use torque: force times the perpendicular distance to the axis. When clockwise torques equal counterclockwise ones, everything is balanced. Two equal, opposite, parallel forces form a couple that just tries to twist.

Things pull on each other. We call it gravity, and the picture becomes tidy. Between two lumps, the pull increases when they get heavier, and decreases when they get farther apart, faster than with distance alone. Near Earth, everything is pulled toward the center; weight measures the pull. Big spheres behave for things outside as if all mass were gathered at the center. Weight varies a little with latitude and height, because Earth bulges at the equator and because rotation makes weight seem a little less there.
Imagine a perfectly smooth hole through Earth, from pole to pole. A body dropped in would feel less pull the closer it got to the center, where it would be zero, then turn and be pulled toward the other side. Without air and friction, it would swing back and forth in the same time no matter how far it went.
To gather all the little pulls on a body into one, we use the center of gravity. A vertical line through it is the direction of gravity. A body stands steady if that line falls within the footprint. Lower the center of gravity and make the footprint wider, and we increase steadiness. That is why we bend our knees when carrying something heavy.

Drop things, and they fall the same when air hardly slows them. Acceleration near Earth's surface is about the same everywhere. Starting from rest, speed increases steadily, and in equal time it falls more and more. By letting things roll down an inclined plane, we can measure this steady acceleration slower and clearer.
A pendulum—a small mass at the end of a string—swings back and forth. For small swings, the time for a full swing depends only on the length of the string and gravity, not on how heavy the bob is or how little it swings. A longer string gives a slower swing. Old clocks tell time by giving little pushes in the right rhythm for each tick. Clockmakers invented ways to keep the 'effective length' almost constant even though metal expands with heat, so the clock doesn't run fast in summer and slow in winter.

Work happens when a force makes something move in its own direction. It is the product of force and distance in that direction. Energy is the ability to do such work. If we lift a stone in gravity, we store gravitational energy that can be released when it falls. If we compress a spring, we store elastic energy. When something is moving, it has kinetic energy.
A heavy block lifted high can do a lot on the way down. In free fall, gravitational energy becomes kinetic energy, and everything we hit gets pressed, bent, or heated. Kinetic energy grows with the square of speed, so a little more speed means much more energy. Power is how fast work is done. One horsepower is how much work a strong horse was thought to do in a minute. One watt is one joule per second; a thousand watts is a kilowatt.
Energy changes form easily, but the total stays the same. A pendulum calmly exchanges height and speed, high energy in position becomes high energy in motion, and back again. Friction slowly drains it into heat. Coal stores old sunshine as chemical energy; when it burns, heat becomes motion in engines. Falling water drives turbines, a generator turns motion into electricity, a motor turns electricity into motion again.

No machine creates energy from nothing. The simple law for machines is accounting: Work out equals work in, minus small losses. In a perfect machine, input force times input distance equals load force times load distance. If we gain in force, we lose in distance. A lever shows the trade-off openly: we are balanced when the product of force and arm on each side is equal. A wheel and axle is like a lever with many arms around; the ratio of the radii gives the ideal multiplication. Pulley systems can only change direction, but can also multiply force by how many ropes hold.
Real machines lose to friction. Efficiency is useful work out divided by work in. An inclined plane trades long distance for gentle force; a wedge—a double inclined plane—forces things apart; a screw is an inclined plane around a cylinder and can give great force for a small turn of a handle. Friction resists when surfaces slide, but rolling is easier. We lubricate, polish, and use ball bearings to reduce losses. At the same time, we need friction: without it, feet slip and wheels spin without grip. We live on the balance.

Long ago, water power was taken with big wheels. A wheel that catches water from below uses the flow's speed; an overshot wheel higher up lets the weight of the water give the turning force. In steep terrain, we can send a sharp jet of water against cup-shaped blades on a wheel that almost reverses the jet and takes out much of its speed—a Pelton wheel. Modern turbines lie submerged and guide water smoothly through curved blades. Water changes direction and gives up nearly all its energy.
Electric power stations put turbine and generator together. How much power a site can give depends on how heavy the water per second that comes, and how many meters it falls. How much we actually get for lights and motors depends on the efficiency of machines and the lines that carry the electricity far. So we often raise the voltage high to reduce losses in wires, and lower it again where people live and use it.

Heat is energy wandering from something warmer to something colder. It can come from the sun, from burning, from electricity flowing through resistance, from friction or impact, and from squeezing gas. When we heat a substance, it can expand, become warmer, change state, change chemistry, and sometimes change electrical properties.
Heat and temperature are not the same. Temperature tells how 'hot' or 'cold' something is; heat is energy. Two equal cups of boiling water have the same temperature, but if one cup has more water, it has more total heat energy. There is a small unit that tells how much heat is needed to raise temperature: one calorie heats one gram of water by one degree. Another unit heats one pound by one degree and is called Btu. We use two common temperature scales: one with zero at ice and a hundred at boil, and another that sets ice at thirty-two and boil at two hundred and twelve. Further down lies a bottom where gases lose almost all heat motion. It is called absolute zero.
Gases expand steadily with temperature when pressure is kept constant; if both conditions are handled carefully, a fixed rule binds pressure, volume, and temperature together for a given amount of gas. Liquids expand less; mercury is steady and so good for thermometers. Water is strange: it is densest a little above freezing, and expands when heated from that point—but also when cooled below. That makes ice lighter than water and leaves its mark on winter, water, and climate. Solids expand the least, but not equally. So we leave gaps between rails, let bridges rest on rollers, use two metals that expand differently in thermometers and switches, and carefully choose metals that can stick to glass without cracking when lamps are made.

Heat moves in three ways. When it goes from molecule to molecule in a substance, it is called conduction. Metals conduct well. Porous substances like wool trap air and conduct poorly, and insulate. Air and water conduct poorly. A thermos bottle uses a vacuum and shiny walls to stop both conduction and radiation.
Radiation is heat that travels in straight lines in empty space and moves at the speed of light. It only warms when absorbed. Polished shiny surfaces reflect a lot and stay cooler under radiant heaters; blackened surfaces absorb more and become warmer. If we put a small fan wheel in a glass bulb under a lamp, it spins because gas near black surfaces gets warmer and bounces back harder.
Convection is heat transport in liquids and gases that move. Warm becomes lighter and rises, cold becomes heavier and sinks. The chimney pulls smoke because warm air rises. Rooms get air to circulate around a fireplace or radiator. By day, land warms faster than sea; that creates wind that changes direction from day to night. Heating a house is always a balance between giving heat and ventilating. A fireplace heats mostly by radiation and ventilates well, but sends much heat up the chimney. A stove and boiler add conduction and convection. Hot water systems circulate by density differences. Steam heats quickly and tolerates pauses. Air systems blow clean, tempered outside air over pipes and into rooms; small humidifiers add back moisture.
A small strip of two metals that bend differently with temperature can control dampers and valves via air or electricity. That is the thermostat, which keeps a room about as we like it.

Water requires a lot of heat to change temperature. That is called high specific heat and makes climate milder, and gives heating systems steady warmth but slow change. When we melt ice, a lot of heat is needed without the temperature rising. That heat is called heat of fusion and comes back when water freezes. If we sprinkle salt on ice, we lower the melting point, make cold saltwater, and can freeze ice cream without a freezer.
Crystals melt at a fixed temperature. Irregular glasses soften over a range. Water expands when it freezes, ice floats and splits rocks in winter. Pressure can melt ice a little below zero; a tight wire can cut through an ice block by melting underneath and letting the water freeze again above. When liquid evaporates, it takes the fastest molecules and cools what is left. Less pressure lowers the boiling point and helps evaporation.
Boiling happens when the vapor pressure inside tiny bubbles becomes equal to the pressure outside. At sea level, water boils at about a hundred degrees; in the mountains at lower temperature; in pressure cookers that withstand pressure, at higher. If we turn liquid to vapor and back on purpose, we can purify and separate substances. And by letting a gas be compressed, cooled by giving off heat, then expand and vaporize a liquid again, we can create cold and freeze food economically.

Work and heat are two sides of the same coin. While drilling cannon barrels, people noticed it got hot and stayed hot without stopping, and an idea came: Heat is made from motion that is stopped. Later, a patient man measured what work is needed to heat water a certain amount. He found a fixed ratio. That means when we rub, hit, or compress—it becomes heat, and when we let heat flow through an engine, we can get work out.
The steam engine is the grandfather of many machines. Steam under pressure pushes a piston. Valves open and close in rhythm, a connecting rod turns push into rotation, and a heavy flywheel keeps steady speed. A clever inventor found it pays to cool the exhaust steam somewhere other than in the cylinder itself. After that, engines became more efficient, and factories and ships could grow.
In an internal combustion engine, a mixture ignites inside the cylinder. Four strokes make a cycle: intake, compression, power, exhaust. A flywheel smooths it out. Such engines are small, fast, and easy to start. Large steam turbines let steam brush blades arranged in rows and spin shafts quietly and fast with few vibrations.

Magnets attracted iron long before anyone could explain it, and a magnetic needle pointed north and south. If we rub or coil electric current around iron, it becomes magnetic. Like poles repel, unlike attract. Magnetism gathers mostly at the ends. If a magnet gets too hot, it loses much of its strength. Hardened steel holds magnetism better, while soft iron takes magnetism easily but releases it quickly when the field disappears. If you break a magnet in two, you get two whole magnets each with its own north and south end. That suggests magnetism lives in millions of little 'molecular magnets' that can align the same way.
If we draw a magnetic field, we see curved lines that go from the north end into space and in toward the south end, and close the circle inside the magnet. Iron pulls the lines into itself. Small filings on paper over a magnet draw the pattern. The Earth itself is a big magnet. A compass points roughly to a magnetic pole. The difference between true north and magnetic north is called variation and varies from place to place. The needle also dips downward in northern regions and upward in southern. Large iron structures become magnetic from Earth's field; vertical poles in our region usually get a south end at the top.

If we rub certain sticks, they attract light things. That is static electricity from friction. It turns out there are two kinds of charge, which attract and repel in pairs. A small instrument with two thin leaves can show if something is charged; the leaves spread when they get the same charge. When we charge by rubbing, the two surfaces we rubbed become equally charged with opposite signs. Touching can move charge. If we hold a charged object near a conductor without touching, it pushes electrons around in the conductor and changes the charge distribution there. That is called induction.
Today we say that tiny particles, electrons, carry negative charge. Positive charge sits in more stable atomic nuclei. Metals let electrons move easily and are called conductors. Glass and wax hold them in place and are called insulators. Charge collects on the outer surfaces of a conductor, and piles up extra at sharp points. There it can leak into the air with a quiet hiss and a faint light. Lightning is a huge discharge between cloud and ground or between clouds. It heats the air so suddenly that we hear a bang, and it can injure and kill. Lightning rods lead the charge safely into the ground.
Voltage difference is like a height jump for charge. Capacitance is the ability something has to hold charge at a given voltage. If we put another conductor nearby without touching, the capacitance increases. Two plates close together can hold a lot of opposite charge. That is how a capacitor stores charge. An old glass with metal inside and outside was the first. If we discharge it across a coil, it oscillates back and forth a few times. Such oscillations send out invisible waves.

There are machines that create static electricity by touching and using induction. A simple plate can give a disk charge again and again. Clever devices with rotating disks and small combs gather many little charges into a steady stream of sparks.
If we want steady current over time, we use a chemical cell—a battery. Zinc and copper in an acid solution create a voltage between them. If we connect them with a wire, current flows. Small bubbles on the electrodes make the current weaker after a while; we can prevent that with other substances that 'eat' the bubbles before they hinder. Impure zinc breaks down locally; a tiny thin mercury coating can protect it. For short and occasional jobs, a saltwater cell with manganese dioxide is handy. Compact versions without liquid became 'dry batteries.' Where we need steady current for a long time, other compositions are used that prefer to deposit metal rather than make gas.
Where current flows, a magnetic field is born. Around a straight wire, the lines are circles. If we point our thumb in the direction of current, our fingers curl the same way as the field. A coil behaves like a bar magnet. Soft iron in the middle makes the magnet much stronger. We can work at a distance with many turns of wire. More turns or more current gives more pull. That is how the telegraph ticks: a key far away opens and closes the circuit so an electromagnet pulls a small arm. The signal from a weak current can start another magnet that closes a strong current locally; that is called a relay. A doorbell gets its own current to interrupt itself and strike the bell again and again. And a single powerful spark across a coil can make a small steel needle become magnetic for a long time.

To use electricity well, we want to measure it. A magnetic needle in a field from a current swings out and stays. A light coil hung between strong poles swings out at an angle that nicely relates to the current. That is the basis for ammeters and voltmeters. A coulomb is a packet of charge. An ampere is one coulomb per second. How big an ampere is can be determined by how much silver it deposits on a plate in a given time in a liquid bath. Resistance stands in the way of current, depends on length, cross-section, material, and temperature, and is measured in ohms. Voltage is measured in volts. A simple law binds them: current equals voltage divided by resistance. If we connect resistors in series, the resistances add. If we connect them side by side, they share the current in a way that makes the total resistance smaller.
Battery cells can be put in series to get higher voltage, or in parallel to get lower internal resistance and more current. We can measure unknown resistances by seeing how current and voltage relate, or use a bridge where a well-chosen adjustment helps us find a point where the needle is still.
Current can move matter. In a solution with metal ions, positive ions go toward a negative plate and deposit as metal. Negative ions go to the positive and either gas off or bond. How much moves is neatly proportional to how much current and how long. We can use that for plating, refining metals, but also as a danger: pipes in the ground can be corroded if 'stray' currents go in and out of them.
Some batteries can be recharged. Storage batteries store electrical energy as chemical and give it back later. Those with lead and acid are heavy, have low internal resistance, deliver a lot of current, and give about two volts per cell. Others, with different chemicals, trade weight, cost, and longevity in other ways.

Electric heat is common: current in a resistance becomes heat. How much depends on current, resistance, and time. We protect wires with thin wires that melt first when current is too high. Lamps shine because a thin wire becomes white-hot in a vacuum. Tungsten can take high temperature and gives more light per watt than carbon. An arc between two carbon rods burns brightly. In direct-current arcs, the positive rod wears faster, and we have to move them. In enclosed chambers, the carbon lasts longer, but needs special power supplies.

When magnetism in a core changes, a voltage appears in a coil around it. Push a bar magnet into a coil, the meter needle jumps. Pull it out, the needle jumps the other way. Leave the magnet still, little happens. Rotate a coil in a steady field, we constantly change how much field passes through the coil and create alternating voltage. The direction is such that it opposes the change that created it. Energy is conserved here too.
Small generators with strong magnets can give alternating voltage. Larger machines let the magnetic field be created by current in their own coils. That way both voltage and current are larger. How we connect the coils decides how the voltage holds up when we connect a load. Alternating voltage is taken out through smooth rings, while rectifying the voltage inside the armature happens with a commutator that flips the connection exactly when the current reverses inside, so we get direct current out. Many coils and segments make everything flow smoothly.
A motor is the same machine run backward: current in a field creates forces that make the coil turn. If we reverse the direction of the field or the current, the motor also reverses. A coil with few turns and a large secondary coil can make brief high voltages when we interrupt the current quickly in the first. A capacitor across the break dampens the spark and sharpens the break. A transformer is the idea lifted to elegance: a primary winding sends alternating current in a closed iron core that is laminated to prevent heat eddies. The alternating magnetism induces voltage in a secondary winding proportional to how many turns it has. That way we step voltages up and down with small losses. Some tubes and arcs conduct easily one way and block the other, and could make steady current from alternating. Large mercury arcs were such a solution.

A telephone carries speech by turning sound into small variations in current and turning them back into motion far away. A thin iron disc over a coil near a small magnet is pulled closer and farther when current through the coil changes, creating sound. At the other end, small carbon grains are under pressure. Sound changes compress and release the pressure, changing resistance. A small current through them is thus modulated in step with the sound. A coil can lift these variations to send them far. Small switches and bells control connection and signaling, and with a shared source for the microphone, entire cities can talk together.

Sound is order in motion. It needs matter to travel through. A string, a tuning fork, vocal cords—anything that vibrates rhythmically makes compressions and rarefactions in air. In water and iron, sound goes faster than in air. Air carries it a couple of hundred meters per second, a little faster when it is warm. In water and iron, it goes many times that fast.
Waves can be transverse or longitudinal. In water, we see the crest go along while the water goes up and down. In air, molecules go back and forth. One full swing is a compression followed by a rarefaction. How big the displacement is, we sense as loudness. How often the swings come, we sense as pitch. The distance between two crests times how often crests come is the speed. Sound reflects. The angle out equals the angle in. A shout toward a cliff wall comes back only if the round trip is enough longer that the ear does not blend start and end. Indoors, reflections help us hear better. Old ear trumpets and soundboards collect and send sound waves.
Music is rhythmic and periodic motion; noise is irregular. Tone has loudness, pitch, and quality. Loudness grows with displacement, and with larger vibrating surfaces through soundboards, and falls off with the square of distance. Thin air carries sound less well. Pitch depends on frequency. Beautiful intervals follow simple number ratios: double, three-half, four-thirds. To be able to play in all keys, these ratios are slightly compromised in what is called equal temperament. Resonance amplifies motion when the driver and natural frequency match. Two identical forks awaken each other. A tube of air under a fork can sound loudly when its quarter-wave matches the fork's frequency, with a small adjustment for the end. Two nearly equal tones can make soft-loud-soft patterns, beats, at the rhythm of the frequency difference.

Instruments roughly divide into membranes and plates (drums), air (flutes, organ, cornets), and strings (piano, violin). Strings have simple laws: double the length gives half the pitch, more tension raises pitch by the square root, thicker string lowers it. Real strings sing not only their fundamental tone but have many overtones in integer ratios. The mix of these gives timbre. Sand patterns on plates can draw beautiful shapes at the places that do not move. Small flames and needles on wax can make sound visible and preservable, so it can be analyzed and played again. Air in tubes resembles strings, with open tubes holding half a wave and closed tubes a quarter. Blow harder, and higher vibration patterns appear. Valves and holes change effective length. The ear itself is a filter; small, finely tuned parts in a cochlea-shaped part each respond to its own tone and send a signal on to the brain.

Light is also a kind of motion, but it does not need matter. It comes from something that shines, and we see it when it is reflected or let through something. Usually light travels straight. A small opening draws an upside-down picture on the opposite wall. The strength of light decreases with the square of distance. Mirrors follow one rule: angle in equals angle out. Rough surfaces scatter in many directions; smooth ones gather and send neatly.
A flat mirror creates an image that appears to stand behind the mirror as far as the object stands in front. Large glass plates and clever lighting once fooled theater audiences with 'ghosts.' Curved mirrors can gather or scatter. Inward-curving ones collect light from far away to a point midway between the mirror's center and surface. If we place an object inside that point, we get an upside-down or upright image depending on where it stands. Outward-curving mirrors always create upright, small images that appear to lie behind.
When light goes from one substance to another, it bends. Straight on, it goes nearly straight through, but at an angle it bends toward the normal in a denser substance, and away from the normal when leaving. How much is given by the refractive index. Water about one and a third, ordinary glass about one and a half, diamond much more. In a plate, the ray is just shifted sideways a little. In a prism, the ray bends toward the base and is refracted again on the other side. If we put two prisms against each other, they point like a convex lens and gather light. Beyond a certain angle out of a dense substance, light does not escape into the thinner one; it is totally internally reflected. That way prisms can turn and reflect without silver. Layers of air with different density and temperature can bend light so we see sky in the desert as a 'lake,' or ships mirrored in the sky. Those are mirages that fool the brain but obey simple physics.

Lenses make images sharp. A convex lens gathers parallel light to a focal point. A concave lens spreads it. Small holes give blurry spots because many rays from the same point do not meet again. The lens brings them together at a point. If we place an object closer than the focal length, we get an upright, magnified image. Place it farther away, we get an upside-down, real image that can be captured on a screen.
The eye is a lens and a screen. The cornea and lens focus a real, upside-down image on the retina. At rest, it is set for seeing far. To see near, small muscles tighten and the lens becomes thicker. Most people cannot see sharply closer than about twenty to thirty centimeters. A magnifying glass lets us hold the object closer without straining. A nearsighted eye focuses distant light too early and needs concave correction. A farsighted eye focuses too late and needs convex. Uneven curvature gives distorted images, corrected with cylindrical lenses.

Instruments use the same simple rules. A camera focuses real images on a light-sensitive surface; chemistry makes them permanent. A projector sends an enlarged, upside-down image toward a screen. A microscope has a short focal length in front and creates a small but real image, which an eyepiece magnifies. A telescope for distant objects puts a long focal length in front and lets an eyepiece magnify a small, sharp image. A short version, used in pocket binoculars, creates upright images by combining a convex front and a concave rear. Binoculars with prisms use total internal reflection to fold the light path, make the tubes shorter, and turn the image right-side up.
A prism shows that white light is a mixture. A narrow slit through a prism lays out the colors nicely. Another prism placed correctly can gather them back into white. Short-wavelength light is bent more than long-wavelength; violet separates more than red. That is why a simple lens has colored edges: different colors focus at different places. If we combine a convex crown glass lens with a concave flint glass lens, we can cancel the color spread while keeping the gathering. That is called achromatic and is necessary for good images.
The colors we see depend on what the surface does. A black object absorbs almost all. A white object sends out again almost all it got. A red cloth sends out mostly red and absorbs the rest. A green bottle lets mostly green through and stops more of the others. If we mix light, two colors can become white if they are proper opposites. If we mix paint, we take away and often end up with gray and brown. A rainbow is a prism in nature: sunlight bends into drops, reflects once inside, and bends out again. The rays go at specific angles to the eye—violet lowest, red highest. If we look at the sun through a narrow slit and prism, dark lines appear where gases in the sun's outer layer absorbed specific wavelengths. The same gas, hot and thin, emits bright lines exactly there. That is how we can read the 'chemistry' of stars from afar.

We see color as a mix of three basic sensitivities. Some lack one of them and confuse certain colors. Printing with three printing plates—each made with light that passes through specific filters—can imitate many colors by printing with the right inks. That is magic built on measurement.

What is light really? Early on, some imagined tiny balls flying in straight lines. Another thinker imagined waves in something invisible that filled everything. Two problems troubled the wave idea: what is this 'something,' and why does light seem to travel straight? Much later, puzzle pieces came together. Light bends around the shadow of very small things, but only a little—because the wavelength is so small. And electric sparks seem to send out waves in a field that contains both electricity and magnetism, a field that also carries light and radio.
When two light rays meet, it happens that they strengthen each other—or cancel out. If we press two glass plates almost together, leaving just a thin wedge of air between them, we see colored stripes in reflected light. Reflections from the top and bottom surfaces meet and either help each other or hinder, depending on the thickness. With monochromatic light, it becomes bright and dark in rows. With white light, one color disappears at a certain thickness, and we see a hint of the opposite color there—and it changes outward.

Sound in air is longitudinal. If we try to stop it with grids set crosswise, it still goes through. Light, on the other hand, can be weakened and even stopped by making it vibrate in only one direction. Two crystals can show this. If we let light through both when they have the same orientation, it goes fine. If we rotate one ninety degrees, it becomes dark. Light has transverse vibrations. That supports the wave idea against the old particle picture.
Invisible radiations widen the picture. If we close a capacitor with a small circuit, it oscillates not just once but many times, quickly. A similar circuit nearby, tuned to the same frequency, receives energy and 'sparks' also. When it was shown openly that such oscillations send out waves that travel at the speed of light, doubt vanished. A small tube with metal filings was sensitive: when a wave came, the filings cohered and conducted. That made wireless telegraphy possible. Spark transmitters charged and discharged capacitors many times per second, and antennas tuned just right sent out. Receivers tuned to the same frequency and picked up.
If we put current through air in a nearly empty glass tube, the glass glowed where the rays hit. They came from the negative electrode and were bent by magnets. That showed they were electrons, small carriers of negative charge. And where they stopped abruptly in glass or metal, another radiation appeared that went straight ahead, blackened photographic plates, went through soft tissue and made some crystals glow, but barely bent or reflected. It was called X-rays. Today we know it is light at very high frequency—waves made by rapid deceleration of electrons.

Some minerals fogged photographic plates by themselves, without light and without spark. They sent something out all the time. From a dark ore, a tiny part was separated that did the same, but much, much stronger. This part could send out three kinds of things. The first was heavy, positive bits that were stopped by thin paper. The second was very light and fast, negative bits. The third was rays that went through a lot and resembled X-rays in strength. A simple screen with a substance that flickers when something hits showed tiny flashes when one of the heavy ones hit. All this pointed toward atoms not being indivisible balls, but small systems with parts and strong bonds that sometimes break and give off energy much larger than ordinary burning.
The voice eventually became wireless, not just dots and dashes. A telephone with wire turns sound into changes in current through the carbon in the transmitter. Large, clean oscillations at high frequency can sit as a 'carrier,' and we can vary their size in time with the sound. A small tube with three parts made this possible. A tiny glowing wire releases electrons, a plate attracts them, and the current can be controlled by a fine wire in between. Tiny variations in this fine wire's voltage cause large variations in the current to the plate. Connected with an oscillating circuit and an antenna, the tube produces steady, clean waves. Sound can ride as a backpack on these, and be taken off again in the receiver. The same tube could both detect and amplify. The audible energy in the earphone came from the tube and power supply, not from the wave in the air itself.

Alternating current does more than just reverse every half cycle. When the field around a wire grows and shrinks, it breeds voltage in nearby wires. The transformer lives on that. But it also means alternating current meets two kinds of resistance: one that actually uses energy as heat, and one that does not eat energy but pushes the current behind or ahead of the voltage. That from coils comes from a property called inductance. That from capacitors comes because they take and give charge in step with the oscillation and let alternating current through more easily than direct current. Together this resistance is called impedance. The law we learned still applies, but now with this combined resistance instead of the old one.
Losses in power networks grow with the square of current. So we raise the voltage high when we send far, so that current can be smaller for the same power, and lower it before we use it. When voltage and current do not oscillate in step, there is a difference between how much 'apparent' power we calculate (voltage times current) and how much real power we use in work and heat. The ratio tells how well we use the network.
Alternating current can be delivered on one phase, like to houses, or three phases, three similar oscillations shifted by one-third of a period. The three create a rotating field in a stationary winding system, which turns a rotor. Generators for such often have the field rotating with direct current and the windings that give current fixed. Motors come in several types. A series motor, which runs the same way whatever the direction, works for tools and small appliances. An induction motor without brushes lives on the field that rotates and induces current in a 'squirrel-cage' rotor; it runs almost as fast as the field, just a little behind. A synchronous motor locks onto the field's oscillation and runs at an exact speed determined by how fast the network oscillates and how many poles it has.

When we look back through all these topics, the practice is the same. We look for causes in nature, not in whims. We measure, we use the same measures, we attach words to concrete actions and numbers. Matter and energy change form, but the sum stays. Molecules dance unseen and reveal themselves in pressure and diffusion, evaporation and crystal shine. Forces combine, balance, bend motion, and borrow and repay in equal amounts. Liquids carry, press and rush, and clear laws give pumps and ships, barometers and brakes. Heat sneaks and flows and radiates, hides in phase change, trades with work on a fixed course, and governs climate and comfort.
Electricity and magnetism weave together, from sparks in a room to lightning over land, from Earth's steady field to networks of machines. Together, as electromagnetism, they carry messages through wire and empty air, light streets and rooms, spin motors and turbines, and let voices and music cross oceans without a thread. Sound and light are different in whether they need a medium and in their direction of vibration, but both are waves that reflect and refract, that overlap and reveal distant sources by fixed laws. And finally the gaze that began with dew and echoes reaches into the atom, where energies far beyond fire and flame show in quiet glimmers. From first to last page, the subject is not a list but a chain where each link strengthens the others, so that familiar things light up the general, the general explains the familiar, and the facts of the world become a pattern that can both be understood and used.