Everything Science Grade 12

14.2 CHAPTER 14. ELECTRONICS

maximum voltage to maximum current is called the reactance — thistime inductive
reactance (XL). The value of the reactance depends on its inductance (L).

XL=

Vmax

Imax

= 2πfL (14.2)

Time

Current

Voltage

Figure 14.3: The current peaks (has its maximum) one quarter of a wave after the
voltage when an inductor is connected to an alternating voltage.

DEFINITION: Reactance

The ratio of the maximum voltage to the maximum current when a

capacitor or inductor isconnected to an alternating voltage. The unit

of reactance is the ohm.

While inductive and capacitive reactances are similar, in one sense theyare opposites.
For an inductor, the current peaks 90 ◦after the voltage. For a capacitor the current
peaks 90 ◦ahead of the voltage. When we work out the total reactance for an inductor
and a capacitor in series, we use the formula

Xtotal= XL− XC (14.3)

to take this into account. This formula can alsobe used when there is more than one
inductor or more than one capacitor in the circuit. The total reactanceis the sum of
all of the inductive reactances minus the sumof all the capacitive reactances. The
magnitude (number) in the final result gives the ratio of maximum voltage to maximum
current in the circuit asa whole. The sign of the final result tells you its phase. If it is
positive, the current peaks 90 ◦after the voltage, if it isnegative, the current peaks 90 ◦
before the voltage.

If a series circuit contains resistors as well, then the situation is more complicated. The
maximum current is stillproportional to the maximum voltage, but the phase difference
between them won’t be 90 ◦. The ratio between themaximum voltage and maximum