You know electric charge. It sparked your finger when you walked across a room on a dry day. It makes your clothes stick together and make snappy noises when you take ‘em out of the dryer. It also gives rise to lightning.
It’s called “electric” because that was the old Greek word for amber. Amber was one of the few materials the Greeks had that would hold an electric charge. By rubbing a smooth piece of amber with a cloth, they could make it attract pieces of paper.
Electric charge comes in two varieties, which we call positive and negative.
If the same charge is put on two objects, they physically repel one another. If opposite charges are put on two objects, they attract one another.
When that electric spark occurs between two objects, we say electric current has flowed from one object to the other. The reason for thinking this is that after the spark has gone between them, the two objects don’t attract one another any more. So somehow the charge has balanced out.
So long as an electric charge isn’t flowing, it’s called a static charge. In the clothes dryer, we just call it “static”, for short.
Some materials “insulate” electric charge, in the following sense. An object made of such a material can hold a negative charge on one part and a positive charge on another part at the same time. Another negatively charged object will be repelled from the negatively charged part, and attracted to the positively charged part. Such materials are called insulators. Examples are: amber, plastics, dry wood, glass, and rubber.
Some materials conduct electric charge. A charge put on one part of an object made of such a material immediately flows all over the object. You can’t make one part of the object negatively charged and another part positively charged. Such materials are called conductors. Metals are the most important conductors. Salt water is also a conductor.
Nothing is 100 percent, though. Different metals conduct electricity to different degrees, and charge slowly leaks through most insulators.
If a material is used in a way that makes use of both properties, that it conducts a charge but slows its flow, it is called a resistive material. Graphite is an example of a resistive material.
Until atomic theories took hold, electricity was thought of as a kind of fluid. Usually, an excess of the fluid was thought to cause a negative charge and lack of the fluid was thought to cause a positive charge.
Nowadays, electric flow is thought of as the movement of individual particles called electrons, each of which caries a certain negative charge.
There are also particles in matter that have positive electric charge, namely protons. Protons don’t typically move much in electric circuits, though, since they’re stuck inside the nuclei of atoms. When we speak of electric current in a circuit, we’re thinking of a flow of electrons.
In the case of conductive fluids, such as salt water, atoms are much freer to move around than in electric circuits, and in that case we think of positively-charged atoms (or ions) actually taking part in the flow of electric current.
The analogy with fluid flows are very helpful in understanding many electric phenomena.
For example, electric charge can flow through a wire, just as water flows through a pipe. Pressure makes water flow through a pipe, and likewise a kind of pressure makes charge flow through a wire.
Just as you can measure the total flow of water per second in a pipe, you can measure the total flow of charge through a wire.
Just as a constriction in a pipe slows the flow of water through the pipe, a section of resistive material in a wire slows the flow of charge through the wire.
Just as it takes power to pump water up hill, it takes power to charge a battery. Just as power is expended in pushing water through a constriction in a pipe, power is expended in pushing electricity through resistive material.
The electric quantity analogous to pressure in a fluid is measured in volts, after an 18th century Italian scientist named Volta.
The total flow of charge through a wire, amounts to how many electrons cross a certain point in the wire every second. The unit of measurement of this quantity is called the ampere, after an 18th century French scientist. For short, we call them amps. An amount of electric flow is accordingly called amperage.
The unit of measurement of the total charge on an object is the coulomb, after an 18th century French scientist. When we speak of the capacity of an object to hold a charge, the unit commonly used it the farad, after an 18th century English scientist named Faraday. Either one is an amount of electric charge, though.
The unit of electric power is the watt, after an 18th century Scottish scientist.
The unit of resistivity to electric flow is the ohm, after an 18th century German scientist.
Guess which century most of the theories about electricity were formulated in! Guess which continent they were formulated on!
An electric circuit consists of components connected by metal wires. The components alter the way that electric charge flows through the wires.
The familiar battery can provide voltage in a circuit. It
contains layers of chemicals that cause electrons to move in a
certain direction, from its negative pole to its positive pole.
It is marked with a rating of how much voltage there is across
the two poles, and a (
−) for the negative pole and a
+) for the positive pole.
If a metal wire is used to connect the two poles of a battery, electrons will flow from the negative end to the positive end. (The wire will also get hot because it isn’t a perfect conductor, and the battery will quickly exhaust all its power.)
A resistor is an electric component whose function is to slow the flow of current in the circuit. One common kind is a little cylinder of graphite with metal wires coming out of either end. These are painted with colored stripes that indicate the resistivity, in ohms.
Another kind of resistor is a filament, which is a coil of metal wire that can withstand high temperature but resists electric current to some degree. When a current is passed through a filament, it heats up because of this resistance. Filaments are commonly used in light bulbs and heaters. They are marked with the voltage that should be put through them, and the power, in watts, that they will then give off as light and heat.
An electric charge can be stored and then quickly released by a component called a capacitor. A common type of capacitor consists of two pieces of metal foil (or plates) with an insulator such as wax paper between them.
If you put an electric charge on one plate of a capacitor, it will stay there because it can’t cross the insulator to the other plate. If you then put a wire between the two plates, the charge will flow through the wire to balance the charges on the two plates—the capacitor is then said to be discharged.
Capacitors typically look like a cylinder or blob with two wires coming out one end, and are marked to indicate their capacitance, the maximum charge that can be stored on them, in micro-farads, or “µF”.