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# What Does Electricity Do? Help (page 2)

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By — McGraw-Hill Professional
Updated on Sep 5, 2011

## Resistors

Some substances, such as carbon, conduct electricity fairly well but not very well. The conductivity can be changed by adding impurities such as clay to a carbon paste. Electrical components made in this way are called resistors . They are important in electronic circuits because they allow for the control of current flow. The better a resistor conducts, the lower is its resistance; the worse it conducts, the higher is the resistance.

Electrical resistance is measured in ohms , sometimes symbolized by the uppercase Greek letter omega (Ω). In this book we’ll sometimes use the symbol Ω and sometimes spell out the word ohm or ohms , so that you’ll get used to both expressions. The higher the value in ohms, the greater is the resistance, and the more difficult it is for current to flow. In an electrical system, it is usually desirable to have as low a resistance, or ohmic value , as possible because resistance converts electrical energy into heat. This heat is called resistance loss and in most cases represents energy wasted. Thick wires and high voltages reduce the resistance loss in long-distance electrical lines. This is why gigantic towers, with dangerous voltages, are employed in large utility systems.

Practice problems of these concepts can be found at: Direct Current Practice Problem

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