# Homework Help: Very simple problem

1. Feb 1, 2005

### nemzy

A fixed inductance of 1.04 µH is used in series with a variable capacitor in the tuning section of a radio. What capacitance tunes the circuit to the signal from a station broadcasting at 5.30 MHz?

Hmm, i have no idea how i can relate capacitance with frequency and with indunctance? is there a formula that i am overlooking? I cant find it anywhere in my book

2. Feb 1, 2005

### da_willem

For a capacitor and inductance the relation between current and voltage are:

$$I=C\frac{dV}{dt}$$
$$V=L\frac{dI}{dt}$$

Now if you a apply an ac signal the impedance will depend on frequency. E.g with a sinosoidal signal with frequency $$\omega$$: $I=I_0 e^{j \omega t}$ (do you know this complex notation?) differenting and integrating yield for the impedances:

$$Z_C=\frac{1}{j \omega C}$$
$$Z_L=j\omega L$$

3. Feb 1, 2005

### Curious3141

Da Willem gave you two vital formulas for the reactances of the (pure) capacitance and (pure) inductance. Use those, in complex form to find an expression for the total impedance of a series combination of them.

Now, using that expression for the total impedance, can you find the value of $\omega$ for which the impedance is a minimum ? What is the value of that minimum impedance ? What frequency does this occur at ($\omega = 2\pi f$) ? What state is said to exist at this frequency (hint : r_s_n___e) ?

EDIT : Sorry, upon closer reading of the question, the required r_s_n__t frequency is given, they want you to find the value of C that causes that state at that given frequency. Still, work through the algebra above as I prescribed, it'll greatly aid understanding and it'll be satisfying to get it from first principles.

Last edited: Feb 1, 2005
4. Feb 1, 2005

### poolwin2001

w=1/(LC)^1/2
TOO lazy to use latex

5. Feb 1, 2005

### Curious3141

We try not to give away the answers until the poster has demonstrated serious effort in trying it out himself. I could've easily typed that out and been done with it. :uhh: