Page 48 - Teach Yourself Electricity and Electronics

28 Electrical units

Resistance has another property in an electric circuit. If there is a current flowing
through a resistive material, there will always be a potential difference across the resis-
tive object. This is shown in Fig. 2-4. The larger the current through the resistor, the
greater the EMF across the resistor. In general, this EMF is directly proportional to the
current through the resistor. This behavior of resistors is extremely useful in the design
of electronic circuits, as you will learn later in this book.

2-4 Whenever a resistance carries a current, there is a voltage across it.

Electrical circuits always have some resistance. There is no such thing as a perfect
conductor. When some metals are chilled to extremely low temperatures, they lose
practically all of their resistance, but they never become absolutely perfect, resistance-
free conductors. This phenomenon, about which you might have heard, is called
superconductivity. In recent years, special metals have been found that behave this
way even at fairly moderate temperatures. Researchers are trying to concoct sub-
stances that will superconduct even at room temperature. Superconductivity is an
active field in physics right now.
Just as there is no such thing as a perfectly resistance-free substance, there isn’t a
truly infinite resistance, either. Even air conducts to some extent, although the effect is
usually so small that it can be ignored. In some electronic applications, materials are
selected on the basis of how nearly infinite their resistance is. These materials make good
electric insulators, and good dielectrics for capacitors, devices that store electric charge.
In electronics, the resistance of a component often varies, depending on the condi-
tions under which it is operated. A transistor, for example, might have extremely high
resistance some of the time, and very low resistance at other times. This high/low fluc-
tuation can be made to take place thousands, millions or billions of times each second.
In this way, oscillators, amplifiers and digital electronic devices function in radio re-
ceivers and transmitters, telephone networks, digital computers and satellite links (to
name just a few applications).

Conductance and the siemens

The better a substance conducts, the less its resistance; the worse it conducts, the
higher its resistance. Electricians and electrical engineers sometimes prefer to speak