I-V characteristics and power in circuits

Last time

Before we start: last time, in Stationary waves, strings and pipes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A one-way gate for current

Connect a battery the wrong way round to a radio, and some radios stop working to protect themselves. The part doing the protecting is a diode: current flows easily one way, and almost not at all the other way. To understand it, we look at its I–V characteristic.

Three characteristics

A fixed resistor at constant temperature: a straight line through the origin, in both quadrants. The resistance is constant. A filament lamp: a curve through the origin, symmetric, with the gradient falling as the p.d. rises. The resistance rises with temperature. A semiconductor diode: almost no current for reverse bias or small forward voltages, then a sharp rise above about point six volts. For all three, resistance at a point is V over I at that point.

Diode resistance

In reverse bias the diode has a very high resistance, so the current is almost zero. In forward bias below the threshold, the current is still near zero. The resistance is large. Above the threshold, the current climbs fast, and the resistance falls as V increases. At zero point six volts and seven point five milliamperes, the resistance is eighty ohms.