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Question:

Which of the following combination should be selected for better turning of an LCR circuit used for communication?

R=25Ω,L=1.5H,C=45µF

R=25Ω,L=2.5H,C=45µF

R=15Ω,L=3.5H,C=30µF

R=25Ω,L=1.5H,C=35µF

Solution:

For tuning an LCR circuit, its selectivity should be high. High selectivity is observed in circuits with high-quality factor. Q=√(L/R)√(1/C) Putting value in above equation:
For option (a), Q=7.3
For option (b), Q=8.28
For option (c), Q=9.43
For option (d), Q=13.67
As Q=√(3.5/15)√(1/(30x10⁻⁶))=13.67
Hence, option (d) is the best combination.