Question

Which of the following graph represents the impedance Z of the L-C-R series resonant combination?

(A)	(B)
(C)	(D)

Options

•  A

• B

• C

• D

Answer 1

B

Explanation:

The impedance of the series L-C-R circuit is given by,

Z = `sqrt(R^2 + (X_L - X_C)^2)`

= `sqrt(R^2 + (2pifL  - 1/(2pifC))^2)`

The value of the impedance (Z) first decreases with the increase of frequency (f) of the AC source and becomes minimum at the resonance frequency (f = f₀) because at the resonance frequency, X_L = X_C, thus Z_min = R.

Z begins to rise as we raise the AC source's frequency more, f(f > f₀). Thus, option b displays the right graph.