As a current flow through a series circuit containing a resistor, a voltage will develop across the end terminals of that component. The voltage will be in phase with the current. The value of the resistor is found by applying AC Ohm's Law where R = V ÷ I. TEST YOUR UNDERSTANDING: In this lesson on AC Voltage & Current we introduce reactance (AC resistance) and phase relationships. Work though each topic to understand the overall concept. Note we are applying AC Ohm's Law which is the same as that for DC circuits except it uses capacitor or inductor reactance at a known frequency rather than pure resistance. For the moment we are not concerned with frequency and only consider the reactance of the capacitor and inductor as a fixed value resistance, although it will not remain so if you change the frequency. Work through the first three topics to familiarise yourself with the basic principles. The third topic shows the same voltage will apply across the resistor, capacitor and inductor when the same current flows through the series circuit. The voltages developed across the inductor will lead the current by 90°and that for the capacitor lag by the same angle. Applying the Parallelogram in the last topic make Va and Vb the same, say 100V and Va phase +90° and Vb -90°. What happens to the resultant voltage? Play around and explore how the resultant is dependent upon the phase angles either cancelling or aiding.
