### Coils and Direct Current

IN DC CIRCUITS, RESISTANCE CAN BE EXPRESSED AS A NUMBER RANGING FROM ZERO (REPRESENTING a perfect conductor) to extremely large values. Physicists call resistance a scalar quantity, because it can be expressed on a one-dimensional scale, as shown in above figure.

Suppose you have some wire that conducts electricity very well. If you wind a length of the wire into a coil and connect it to a source of dc (following figure), the wire draws a large current. It doesn’t matter.

An inductor connected across a source of dc
whether the wire is a single-turn loop, or whether it’s lying haphazardly on the floor, or whether it’s wrapped around a stick. The current amperes is equal to the applied voltage in volts divided by the wire resistance in ohms. It’s that simple.

You can make an electromagnet, as you’ve already seen, by passing dc through a coil wound around an iron rod. Electromagnets are known for the high current they draw from batteries or power supplies. The coil of an electromagnet heats up as energy is dissipated in the resistance of the wire. If the voltage of the battery or power supply increases, the wire in the coil gets hotter. Ultimately, if the supply can deliver enough current, the wire will melt.