The Basics of Resistance

The Basics of Resistance

Introduction

In the Charge And Charge Motion page, we note that current is the result of transferring electrons from atom to atom in a material. In almost all cases, there is some opposition to the motion of electrons in a material. This results in some loss of energy when transferring charges through a material—in general, this energy loss results from conversion of electrical energy to heat 1.

Resistance characterizes the loss of energy associated with passing an electrical current through a conductive element. Units of resistance are volts per ampere, or Ohms, and are represented by the greek symbol Ω. A high resistance corresponds to a large energy loss associated with current passage through a material, while low resistance corresponds to small energy loss associated with current passage through a material. As extreme examples, perfect conductors have zero resistance and perfect insulators have infinite resistance.

Figure 1. Resistor circuit symbol.

Resistors are circuit elements whose characteristics are dominated by this energy loss. The circuit symbol for a resistor is shown in Fig. 1. Resistors are probably the simplest and most commonly used circuit elements. Common uses for resistors are to limit currents to acceptable levels or achieve desired voltage levels in a circuit. Currents may need to be limited for a couple of reasons:

  1. To protect components. Most components can only withstand certain current levels; attempting to exceed these levels may cause the component to fail. Resistors can ensure that currents stay within acceptable levels.
  2. To reduce the current demand from other portions of the circuit. Generally, a complicated circuit can be considered to consist of a number of sub-circuits. Normally, the current which can be delivered by any given sub-circuit is limited—attempting to draw more than the allowable current causes the overall circuit to malfunction. Resistors can be used to limit the current drawn from any given sub-circuit to acceptable levels.

Important Points

  • Resistance describes the energy required to move a specified current through a device. Its units are volts per ampere, or ohms. The symbol for resistance is Ω.
  • Resistors are circuit elements which provide a specified resistance value. These components are commonly used to limit current, or provide a desired voltage level. They are the most commonly used electrical circuit elements.


1Yes, there are such things as superconductors which experience no losses when transferring electrons. Those materials are rare enough so that we won't really worry about them here.

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