Everything Science Grade 12

14.2 CHAPTER 14. ELECTRONICS

Example 1: The impedance of a coil

QUESTION

Calculate the maximumcurrent in a coil in a South African motor which has

a resistance of 5 Ω and an inductance of 3mH. The maximum voltage across

the coil is 6 V. You can assume that the resistance and inductance are inseries.

SOLUTION

Step 1 : Calculate the reactance of the coil

XL= 2πfL = 2π× 50 × 0 ,003 = 0,942 Ω

Step 2 : Calculate the impedance of the coil

Z =

√

X^2 + R^2 =

�

0 , 9422 + 5^2 = 5,09 Ω

Step 3 : Calculate the maximumcurrent

Imax= Vmax/Z = 6/ 5 ,09 = 1, 18 A.

Example 2: An RC circuit

QUESTION

Part of a radio containsa 30 Ω resistor in series with a 3 μF capacitor. What is

its impedance at a frequency of 1 kHz?

SOLUTION

Step 1 : Calculate the reactance of the capacitor

XC=

1

2 πfC

=

1

2 π× 103 × 3 × 10 −^6

= 53,05 Ω

Step 2 : Calculate the impedance