Everything Science Grade 12

CHAPTER 14. ELECTRONICS 14.2

current is called the impedance (Z), and its unit is also the ohm. Impedances are
calculated using this formula:

Z =

�

X^2 + R^2 (14.4)

where X is the total reactance of the inductors and capacitors in the circuit, and R is
the total resistance of the resistors in the circuit.

It is easier to understandthis formula by thinkingof a right angled triangle. Resistances
are drawn horizontally,reactances are drawn vertically. The hypotenuseof the triangle
gives the impedance. This is shown in Figure 14.4.

Resistance R

Reactance X

Impedance Z

Figure 14.4: Visualisingthe relationship betweenreactance, resistance and impedance.

DEFINITION: Impedance

The maximum voltage divided by the maximumcurrent for any circuit.

The unit of impedance is the ohm.

It is important to remember that when resistorsand inductances (or capacitors) are in
a circuit, the current will not be in phase withthe voltage, so the impedance is not a
resistance. Similarly thecurrent won’t be exactly 90 ◦out of phase with the voltage so
the impedance isn’t a reactance either.

See simulation: VPpvr at http://www.everythingscience.co.za)