RESISTANCE

  

Resistance is the opposition that a circuit or substance creates against the flow of electri-cal current. Referring to our water system example again, you can compare resistance to the diameter of the pipe. The narrower the pipe, the less water  that can flow through the pipe.

The only way  to get more water to flow through a narrow pipe would be to increase the water pressure. In the same way, a very thin wire offers more resistance to the flow of electricity than does a thick wire. The only way to increase the flow of electricity in a thin wire would be to apply more voltage to it.

Every electrical circuit contains at least some resistance. This is because no substance is a perfect conductor. For example, silver metal is an excellent conductor, but it’s not perfect — it will always put up some resistance to the flow of electricity through it.

The magnitude of electrical resistance in a circuit is measured in ohms. One ohm is equal to the resistance of a circuit in which one volt of EMF is applied to produce one ampere of  current. The abbreviation for the ohm is the Greek symbol omega.

CONDUCTORS, RESISTORS, AND INSULATORS

One of the first scientists to observe  the flow of electrical current  through various materials was Georg Ohm. In the early 1800s, Ohm performed experiments with electric current. He found that when current was applied to different materials, some materials carried the current readily and others didn’t. He found that different materials put up different degrees of resistance to the flow of electricity.

For example, he noticed that silver and copper readily permitted the passage of almost all of the applied electricity; therefore, he called these metals conductors. Some materials carried almost no electricity, even when a very high voltage was applied. Ohm called these materials insulators. Examples of insulators are glass, mica, porcelain, paper, plastic, and rubber. Other materials passed some (but not all) of the current, so these were called resistors. Carbon is an example of a resistor.

Using your knowledge about the structure of atoms, you can now explain the results of Ohm’s experiments. You learned that some atoms are constructed with free electrons in  their outermost orbits. A material in which electrons can be moved easily from one atom  to another by an outside force is a good conductor of electricity.

In comparison, other materials are made of atoms in which the electrons are very tightly  bound to their orbits.  In these atoms, it’s very difficult to remove electrons from their orbits, so the material is a poor conductor of electricity. If the electrons in an atom can’t  be moved from their orbits at all, the material is an insulator. If at least some of the electrons can be moved, the material is a resistor.

FIXED RESISTORS

We’ve already discussed how some materials resist  electrical flow. Now, we’ll examine how resistive materials can be used to make resistors. Resistors are devices that resist or oppose the flow of electrons through a circuit. Resistors are commonly used to protect circuits and control the flow of electricity through them. However, a resistor doesn’t completely prevent the flow of  electricity. This principle has many practical applications in industry.

The most common type of resistor is the carbon resistor, also called a fixed resistor.  A carbon resistor has a body made of resistive carbon surrounded by a composite sealing material. A stiff wire lead protrudes out of each end of the resistor. A series of colored bands mark the resistor. The color bands identify the resistor’s value as measured in ohms.

Most carbon resistors tend to have a very high resistance to electrical current. While carbon resistor may have a value as small as one-tenth of an ohm, values in the hundreds,  thousands, or even millions of ohms are more common. Carbon resistors are used in electronic circuits to control the flow of current, or to lower the voltage delivered to an electronic device (such as an integrated circuit).

Another type of resistor is the wirewound resistor. A wirewound resistor consists of a porcelain or ceramic tube that’s wrapped with a length of wire. Nichrome, a  type of wire that’s a poor conductor, is often used.

Wirewound resistors generally have lower resistance values than carbon resistors. Typical wirewound resistors have resistance values of tens, hundreds, or thousands of  ohms. The resistance value is largely determined by the length and thickness of the wire  used in the resistor, and how well the wire conducts electricity. These resistors are often used in the high-power circuits found in heaters and DC motor controls.