Electrical Resistance

Also affecting the current flow in a circuit is electrical resistance, measured in ohms. All materials have resistance—even good conductors. There are four factors that determine the level of electrical resistance:

  1. Type of material: This refers to how many free electrons a material has.
  2. Length of the conductor: As length increases, so does resistance.
  3. Diameter of the conductor: The larger the conductor, the greater the amount of current it can carry.
  4. Temperature of the conductor: The higher the temperature, the harder it is for free electrons to pass through and the higher the electrical resistance.
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FIGURE 36-4
A. Measuring resistance with an ohmmeter. B. Measuring voltage drop of the same resistance.

While all materials have some resistance to current flow based on the number of free electrons the material has, a resistor is a component designed to extract energy from the current flow by forcing it through a restriction in the circuit. A typical resistor has a set resistance, usually marked or coded on its surface. Electrical resistance is somewhat like the electrical equivalent of friction in the mechanical world: It is the degree to which a material opposes, or resists, the passage of an electrical current. Good conductors have low resistance, while insulators have high resistance. Electrical energy lost through resistance is converted into heat.

Resistance is measured in ohms. Under most conditions, except temperature change, the resistance of an object is a constant and does not depend on the amount of the voltage or the amount of current passing through it. At the same time, Ohm’s law tells us that if we increase current flow through a resistance, the voltage used by that resistance will increase. So in that sense, the resistance did not change, but the voltage drop did Figure 36-4. When diagnosing circuits, we usually use a voltmeter to look for excessive voltage drops that reduce the available voltage at the load. An ohmmeter would not normally work in this case, since it generally cannot read less than 0.1 ohms. This amount of resistance in a 10-amp circuit would cause a 1.0-volt drop, which is way beyond the allowable maximum voltage drop for a wire, which will be discussed further in the Meter Usage and Circuit Diagnosis chapter. The relationships between current, voltage, and resistance are calculated using Ohm’s law. This concept will be discussed in detail later in this chapter.