7

Flow of electricity through metals

For a given metal wire, the electric current is measured for different voltages, and the result is displayed in a current–voltage diagram as in Figure 1. Such a diagram is called a characteristic diagram for a resistive element, the measured function is a resistive characteristic curve (it is characteristic of the charge transport in the particular wire and of the dimensions of the wire).

When the voltage is increased, the charge current increases as well. If the changes are not too great, the characteristic curves for metals are straight lines. We say that we have a linear characteristic (Figure 1). For different wires the curves are straight lines having different slopes. The slope is higher for shorter, thicker wires, and it higher for some materials than for others.

If voltage and current increase more strongly for a given wire—as might be the case for the filament of a light bulb—the current increases more slowly with increasing voltage (diagram in Figure 2).

Interpretation

The slope of the straight characteristic lines in the diagram of Figure 1 indicates how easily charge flows through the given wire. The measure of how easily charge flows is called the electric conductance, again in analogy to hydraulics. The inverse of the conductance is the resistance.

The linearity of the characteristic curves suggests a (structural) similarity between the transport mechanism for charge and that for fluids in laminar flow. Charge transport through metals is said to satisfy Ohm’s relation. The nonlinearity of the characteristic in the diagram of Figure 2 might suggest non-ohmic behavior. However, that is not the case. Charge flows through materials of the same type for both diagrams (Figures 1 and 2). Therefore, we should expect the transport mechanisms to be the same. The difference is caused by heat produced in the resistive element causing a strong change of temperature, leading to an increase of the resistive property of the metal (the higher the temperature, the higher the resistance).