While a pinhole will project the image of a scene on a piece of film, it doesn't collect very much light. That's why a pinhole camera requires very long exposures. A better camera makes use of a converging lens. If you hold a magnifying glass several inches away from a white sheet of paper, you will see that it forms a real image of anything on the other side of it—particularly bright things such as light bulbs or well-lighted windows. A typical camera uses a converging lens that's not unlike a magnifying glass to form an image of this sort. You could use a magnifying glass to build a camera, but I'd suggest that you start with a camera and rebuild it yourself. Go to a company that processes film and see if they will give you any used disposable cameras. These cameras are of essentially no value to them and they either discard them or recycle them. If you ask around, you should find a photo shop that will give you a couple. You can then disassemble them. You'll find a very nice lens, a shutter system, a film advance mechanism, and so on. You can use a toothpick or small screwdriver to turn the exposure dial backward so that the camera behaves as though it still has film left. You can then "advance the (non-existent) film" by turning the film sensing gears in the back of the camera with your fingers until the shutter cocks. Finally, you can press the shutter release and watch the shutter open the lens to light. Disposable cameras are great because if you break something in your experimenting, you can just throw away your mistake.
As yet, there is no direct relationship between those two forces. Our best current understanding of gravitational forces is as disturbances in the structure of space itself while our best current understanding of electromagnetic forces involves the exchanges of particles known as virtual photons. However, physicists are trying to develop a quantum theory of gravity that would identify gravitational forces with the exchange of particles known as gravitons. How closely such a quantum theory of gravity would resemble the current quantum theory of electromagnetic forces (a theory called quantum electrodynamics) is uncertain. It's also uncertain whether those two quantum theories will be able to merge together into a single more complete theory. Only time will tell.
These two observations—that salt melts ice and that salt makes ice colder—are actually consistent with one another. When you add salt to ice, you make a relatively ordered mixture—pure crystalline ice and pure crystalline salt. This orderly arrangement is looked on unfavorably by nature; given a chance, nature tends to maximize randomness. There is a much more disorderly arrangement available—salt water—and nature tends toward disorderly arrangements. When you put the salt and ice together, nature's tendency toward randomness begins to drive the system to rearrange. The ice begins to melt so that the salt can dissolve in it. Although the melting of ice requires energy, the randomness this melting and dissolving produces makes this process take place. The energy needed to melt the ice is extracted from the remaining ice and that ice gets colder. When you're making ice cream, some of the energy needed to melt the ice also comes from the ice cream mix, so that it gets colder, too. If there is enough salt around, the ice will melt completely to form very cold salt water—the desired result with salt on a slippery sidewalk. The salt water remains liquid well below the normal freezing temperature of water because forming ice crystals would require the salt and water to separate from one another—an orderly and therefore unlikely event. In short, nature's trend toward disorder causes salt to melt ice, even though that melting lowers the temperatures of everything involved well below the freezing temperature of pure water.
The tide is caused primarily by the moon's gravity. Gravity is what keeps the moon and earth together as a pair—the moon and earth orbit one another because each is exerting an attractive force on the other. While they are effectively falling toward one another as the result of this gravitational attraction, their sideways motion keeps them from smashing together and they instead travel in elliptical paths around a common center of mass. But the moon's gravity is slightly stronger on the near side of the earth than it is on the far side of the earth. As a result, the water on the near side of the earth bulges outward toward the moon. The water on the far side of the earth also bulges outward because the earth itself is falling toward the moon slightly faster than that more distant water is. The distant water is being left behind as a bulge.
There are thus two separate tidal bulges in the earth's oceans: one on the side nearest the moon and one on the side farthest from the moon. But the earth rotates once a day, so these bulges move across the earth's surface. Since there are two bulges, a typical seashore passes through two bulges a day. At those times, the tide is high. During the times when the seashore is between bulges, the tide is low. Because the moon moves as the earth turns, high tides occur about 12 hours and 26 minutes apart, rather than every 12 hours. Since local water must flow to form the bulges as the earth rotates, there are cases where the tides are delayed as the water struggles to move through a channel. However, even in those cases, the high tides occur every 12 hours and 26 minutes. The sun's gravity also contributes to the tides, but its effects are smaller and serve mostly to vary the heights of high and low tide.
A relay is an electromagnetically operated switch. It contains a coil of wire that acts as an electromagnet. Since electric currents are magnetic, this coil of wire develops north and south magnetic poles whenever current passes through it. A metal core is often placed inside the coil of wire to enhance its magnetism. Adjacent to the coil of wire is a moveable piece of iron. While iron normally appears nonmagnetic when it's by itself, it becomes highly magnetic whenever it's exposed to a nearby magnetic pole. The iron piece becomes magnetic as current flows through the coil and the two are attracted toward one another. As the iron piece shifts toward the coil, it moves various electric contacts that are attached to it. These contacts close some circuits while opening others. The coil remains magnetic and continues to hold the iron piece near it until current stops flowing through the coil. When the current does stop, the coil loses its magnetism and so does the iron piece. A spring in the relay then pulls the two apart and the electric contacts return to their original positions.
A telephone uses an electric current to convey sound information from your home to that of a friend. When the two of you are talking on the telephone, the telephone company is sending a steady electric current through your telephones. The two telephones, yours and that of your friend, are sharing this steady current. But as you talk into your telephone's microphone, the current that your telephone draws from the telephone company fluctuates up and down. These fluctuations are directly related to the air pressure fluctuations that are the sound of your voice at the microphone.
Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend's telephone. Thus as you talk, the current through your friend's telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuations reproduce the sound of your voice. Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior.
I can't think of an easy way to make sound waves visible while they travel through air, but it's relatively easy to make sound waves visible as they travel through materials. If you choose a system in which the sound waves bounce back and forth many times through a material, you can sometimes see the sound waves as they move. For example, partially fill a crystal wine glass with water and then rub your wet finger gently around the rim of the glass. With some practice, you'll be able to get the wine glass to emit a pure tone as your finger alternately sticks and slips its way around the glass rim. As this tone appears—the vibration of the crystal glass itself—the water will begin to exhibit beautiful ripple patterns. You should be able to see these ripples by looking at a bright light reflected from the water's surface. The ripples are sound waves that are travel through the water, back and forth, as the glass vibrates.
Another system that makes the movement of waves visible is a stiff, thin aluminum plate that's supported rigidly and horizontally at only one point. If you sprinkle fine sand lightly over the surface of this plate and then bow its edge with a violin bow, it will begin vibrating with a clear tone. As it vibrates, the sand will drift into places where there is very little surface motion—the nodes of the vibrating surface. Once again, sound waves are traveling back and forth across this surface and the up-down motions squeeze the sand into certain parts of the plate. In this case, the surface's vibrations and the sound waves in that surface are the same thing—in example of the fact that vibrations and sound waves are intimately related and are in many respects exactly the same thing.
Except during an eclipse, one half of the moon's surface is bathed in sunlight while the other half is in shadow. The phases of moon occur because we can only see half the moon at any moment and the fractions of lighted and shadowed moon that we see vary with about a four-week cycle—the lunar month. For example, when the moon is almost on the opposite side of the earth from the sun, we see only the lighted side of the moon and the moon appears full. When the moon is on the same side of the earth as the sun, we see only the shadowed side of the moon and it appears almost non-existent—a new moon. Each lunar month, our vantage point gradually evolves so that we see the new moon become a growing crescent moon, a half moon, a gibbous moon, and a full moon, a gibbous moon, a half moon, a shrinking crescent moon, and finally a new moon again. You can see this effect by illuminating a soccer ball with a bright flashlight and then walking around the soccer ball. You'll see the phases of the soccer ball.
The frequency characteristics of a transformer are determined principally by the materials in the transformer's core. Power flows from the primary circuit to the secondary circuit by way of the magnetization of the transformer's core. With each half-cycle of the alternating current in the primary circuit, the transformer's core must magnetize and demagnetize. A transformer core's ability to magnetize and demagnetize properly depends on the frequency of the alternating current in the transformer's coils. If that frequency is too low, the core may saturate—reach its maximum possible magnetization—during the half-cycle. In that case, the core will not be able to transfer the requisite amount of energy to the secondary coil and the power transferred between the two coils will be inadequate. That's why low frequency transformers often contain huge iron cores—cores that avoid saturation by spreading out the magnetization and stored energy over large volumes of iron.
On the other hand, if the frequency of current in the primary is too high, the core may be unable to magnetize and demagnetize fast enough to keep up with it and the power transfer will again be inadequate. The core may also become hot due to friction-like losses in the core material. That's why high frequency transformers use special core materials such as ferrite powders or even air. Although air (or really empty space) can't store large amounts of energy in small volumes when it magnetizes, it can respond extremely quickly. Air-core transformers operate well at extremely high frequencies.
A fan motor is an induction motor, with an aluminum rotor that spins inside a framework of stationary electromagnets. Aluminum is not a magnetic metal and it only becomes magnetic when an electric current flows through it. In the fan, currents are induced in the aluminum rotor by the action of the electromagnets. Each of these electromagnets carries an alternating current that it receives from the power line and its magnetic poles fluctuate back and forth as the direction of current through it fluctuates back and forth. These electromagnets are arranged and operated so that their magnetic poles seem to rotate around the aluminum rotor. These moving/changing magnetic poles induce currents in the aluminum rotor, making that rotor magnetic, and the rotor is dragged along with the rotating magnetic poles around it. After a few moments of starting, the spinning rotor almost keeps up with the rotating magnetic poles. The different speed settings of the fan correspond to different arrangements of the electromagnets, making the poles rotate around the aluminum rotor at different rates.