# LC Circuits with variable frequency

1. Mar 31, 2009

### greenday1260

1. The problem statement, all variables and given/known data

A variable capacitor with a range from 10 pF to 420 pF is used with a coil to form a variable-frequency LC circuit to tune the input to a radio.

(a) What ratio of maximum frequency to minimum frequency that can be obtained with such a capacitor?
ANS:__________

(b) If this circuit is to obtain frequencies from 0.54 MHz to 1.60 MHz, the ratio computed in (a) is too large. By adding a capacitor in parallel to the variable capacitor, this range can be adjusted. What capacitance should be added to obtain the desired frequency range?
ANS:___________pF

(c) What inductance should the coil have to obtain the desired frequency range?
ANS:___________H

2. Relevant equations

$$\omega$$= $$\frac{1}{\sqrt{LC}}$$

3. The attempt at a solution
no idea pls help

2. Apr 1, 2009

### George Jones

Staff Emeritus
For (a), assume that the value of $L$ is unknown, but fixed (constant).

What value for the variable capacitance gives you the biggest frequency?

What value for the variable capacitance gives you the smallest frequency?