17.3. Resistivity http://www.ck12.org
promise in producing devices that require much less energy since electrical resistance, as we will see in this chapter,
converts a good deal of electrical energy into unwanted heat. Of course, the practical use of superconductors is very
limited by the necessity to keep them at extremely low temperatures. However, some advances have been made in
recent years. Scientists have learned to manufacture materials that become superconducting at temperatures as high
as 160K(− 113 ◦C).
Resistors
Every electrical device (and every circuit) has some amount of resistance. There are, however, circumstances when
additional resistance is required in a circuit. Resistance can help protect delicate circuitry by limiting the current
through a particular segment (branch) of a circuit.
In order to add the required resistance to a circuit, electronic components are made calledresistors(adding resistance
is not their only purpose). Resistors have resistances that range from one ohm to millions of ohms. Figure17.9
shows some common resistors found in electronic equipment. The colorful bands on the resistors are a code used to
designate the amount of resistance of the resistor. For example, the fifth resistor from the left inFigure17.9 has a
sequence of colors: Green, Blue, Green, Silver: The first two colors represent 56 (see code) the third color represents
the multiplier Green=× 100 k. Therefore the resistor has 56× 100 kohms of resistance or 5. 6 × 106 Ω. The last
color represents the tolerance of the resistor. This is a measure of accuracy. Silver is 10%, which means that the
actual resistance of the resistor lies somewhere between ten percent above to ten percent below the stated value of
5. 6 × 106 Ω.
→ 5. 6 × 106 Ω±( 5. 6 × 105 Ω)→ 6 , 160 , 000 − 5. 040 , 000 → 6. 2 × 106 Ω∗ 5. 0 × 106 Ω
FIGURE 17.9
Common resistors.http://demonstrations.wolfram.com/4BandColorCodeForResistors/