Circuits themselves are as old as electricity. A circuit is literally a complete loop through which electric current can flow. For example, a flashlight contains a circuit whenever it's turned on—the current flows from the battery's positive terminal, through the switch (which is on), through the filament of the light bulb (which glows), and back to the battery's negative terminal. The battery then gives the current some more energy and sends it around this "circuit" again and again.
But electronic "circuits" are much more modern. Here the word circuit is equivalent to "device," "board," or "chip." Such electronic devices date to somewhere around the beginning of the twentieth century. As radio developed, with tube amplifiers and other electronic components, so did these circuits. Modern electronic systems place many of the components involved in an electronic device on a single sheet of plastic or fiberglass and many of the components on that board may exist on the surface of one or more tiny silicon wafers. These single wafer circuits, called integrated circuits, were invented in 1959 by Texas Instruments and became commercial products at Fairchild Semiconductors in 1965.
When the atoms that make up a metal assemble together, some of their electrons become delocalized—they stop associating with specific atoms and can move throughout the overall metal. Most importantly, these mobile electrons can respond to the presence of electric fields and electric forces by accelerating and traveling through the metal. When you turn on a flashlight, you are creating a system in which positive charges on one terminal of the battery and negative charges on the other terminal can begin to push electrons through the flashlight's wires. The mobile electrons in those wires are negatively charged and they accelerate toward the positive terminal of the battery. New electrons from the negative terminal of the battery replace the departing electrons and soon a steady flow of electrons through the flashlight is established.
The answer is somewhat different for older electromechanical meters than for modern electronic meters. I'll start with the electromechanical ones and then briefly describe the electronic ones. An electromechanical meter has a coil of wire that pivots in a nearly friction-free bearing and has a needle attached to it. This coil also has a spring attached to it and that spring tends to restore the coil and needle to their zero orientation. Because the spring opposes any rotation of the coil and needle, the orientation of the needle depends on any other torque (twist) experienced by the coil of wire—the more torque the spring-loaded coil experiences, the farther the coil and needle will turn away from the zero orientation. The needle's angle of deflection is proportional to the extra torque on the coil.
The extra torque exerted on the spring-load coil comes from magnetic forces. There is a permanent magnet surrounding the coil, so that when current flows through the coil it experiences a torque. Because a current-carrying coil is magnetic, the coil's magnetic poles and the permanent magnet's magnetic poles exert forces on one another and the coil experiences a torque. This magnetic torque is exactly proportional to the current flowing through the coil. Because the torque on the coil is proportional to the current and the needle's angle of deflection is proportional to this torque, the needle's angle of deflection is exactly proportional to the current in the wire.
To use such a meter as a current meter (an ammeter), you must allow the current flowing through your circuit to pass through the meter. You must open the circuit and insert this ammeter in series with the rest of the circuit. That way, the current flowing through the circuit will also flow through the meter and its needle will move to indicate how much current is flowing.
To use such a meter as a voltage meter (a voltmeter), some current is divert from the circuit to the meter through an electric resistor and then returned to the circuit. The amount of current that follows this bypass and flows through the electric resistor is proportional to the voltage difference across that resistor (a natural phenomenon described by Ohm's law). The voltmeter system thus diverts from the circuit an amount of current that is exactly proportional to the voltage difference between the place at which current enters the voltmeter and where it returns to the circuit. The needle's movement thus reflects this voltage difference.
In an electronic voltmeter, sensitive electronic components directly measure the voltage difference between two wires. Virtually no current flows between those two wires, so that the meter simply makes a measurement of the charge differences on the two wires. An electron ammeter uses an electronic voltmeter to measure the tiny voltage difference across a wire that is carrying the current. Since the wire also obeys Ohm's law, this voltage difference is proportional to the current passing through the wire.
A light switch controls the flow of electricity through a circuit—a complete, unbroken loop through which electric charges can move. When the light switch is on, these electric charges can move in an endless loop. This loop starts with a trip to the power company—actually to the power transformer near your home—where the charges pick up electric energy. They then flow through wires to the light switch, then to the light bulb where they deliver their electric energy, and finally back to the power company to obtain more energy. The same charges complete this loop over and over again. The loop is called a circuit.
But when you turn off the light switch, you open or break the circuit. One of the wires connecting the power company to the light bulb suddenly has a gap in it and the current of electric charges can no longer flow. The switch itself actually contains two separated wires and a mechanical device that connects them only when the switch is in its on position. The precise structure of the mechanical switching device differs from switch to switch, but the behavior is always the same: the switch disconnects the two wires—and thus breaks the circuit—whenever you turn the switch off.
As current flows through a battery, from its negative terminal to its positive terminal, the battery does work on that current. It must pull positive charges away from the negatively charged negative terminal and push them toward the positively charged positive terminal. An alkaline battery needs 1.5 joules of energy to transfer each coulomb of positive charge in this manner. This transfer operation consumes the stored chemical potential energy inside the battery and eventually causes the battery to go dead. Just because you don't see anything moving in the wires or in the battery doesn't mean that something substantial isn't occurring inside the battery—it undergoes electrochemical reactions whenever current is flowing through it.
Yes. While the other batteries in the string will pump positive charge from their negative terminals to their positive terminals, the reversed battery will extract energy from the positive charge as it flows from that battery's positive terminal toward its negative terminal. The charge will lose energy and the battery will gain energy. Some of the battery's additional energy will go into recharging the battery—converting its used chemicals back into their original forms. But some types of batteries are better at recharging than other. Those that aren't meant to be recharged may turn most of this energy into thermal energy and thus waste it.
Those different alkaline battery sizes are chemically equivalent, which is why they all produce the same voltage rises for currents passing through them from their negative terminals to their positive terminals. The same chemical reactions allow each of these batteries to pump the charges, giving each coulomb of positive charge about 1.5 joules of energy—a voltage rise of 1.5 joules-per-coulomb or 1.5 volts. Where these batteries differ is in how many charges they can pump each second—their maximum currents—and in how many charges they can pump before running out of chemical potential energy—their total stored energy. The bigger cells (C and D) can handle far more current than the smaller cells (AAA and AA) and they also contain more stored energy.
No. An electric fence needs at least one real wire. When you put a large electric charge on this wire, anyone who touches it and the ground at the same time will serve as the path through which that charge will flow into the ground. They will receive a shock. But without either the charged wire or the ground, they won't carry any electric current and they won't receive a shock.
An electric circuit is racetrack for electric charges. It must be a complete loop—a "circuit"—so that the charges don't pile up somewhere along the track. The simplest circuit has a source of energy for the electric charges (e.g., a battery) and a device that takes energy away from the electric charges (e.g., a light bulb). When the charges are in motion through the circuit, they are an electric current. By convention, current points in the direction of positive charge flow, so you can imagine a stream of positive charges circling this circuit over and over again, with current pointing always in the direction that those positive charges are moving. As the current passes through the battery, entering it at the battery's negative terminal and leaving it at its positive terminal, the charges pick up energy. The battery is converting some of its stored chemical potential energy into electric energy and giving that energy steadily to the current flowing through it. The battery is "pumping" the charges from its negative terminal to its positive terminal. The current continues around the circuit and then passes through the light bulb. In the light bulb, the charges give up most of their energies to the filament and the filament becomes white hot. The current continues out of the bulb and returns to the negative terminal of the battery to pick up more energy. This simple circuit is present in a flashlight. The same charges complete this circuit millions of times each second, shuttling energy from the battery to the bulb.
The classic technique is to insert two dissimilar metal strips into the potato in order to build a simple battery. You can then run an electronic clock with the power provided by that battery. But the energy in that battery is coming from chemical reactions of the metals and not really from the potato. If you really want to use a potato as the power source for a clock, you should dry the potato out and burn it. You can use the heat of the fire to run a steam engine or to generate electricity.
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