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