# Low pass filter

1. Aug 19, 2007

### jesuslovesu

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

Suppose you are developing a high-fidelity soundsystem. If you wish to deliver low-freq signals to a woofer, what device would you place in series with it?
A) Resistor
B) Capacitor
C) Inductor

2. Relevant equations

3. The attempt at a solution
According to my book, the answer is an inductor; however, I would have thought that it would be a resistor if because that's how a low pass filter is made. My book references the equation I0 = V0/(wL) when giving the answer. I'm not quite sure why it would be an inductor? Couldn't it be a resistor just as easily?

2. Aug 19, 2007

### learningphysics

The answer is an inductor... the inductor acts like a short at low frequencies, and acts as an open circuit at high frequencies...

If you wanted a highpass filter, you'd use a capacitor, because it acts the opposite way (short at high frequencies, open circuit at low frequencies)

A resistor acts the same independent of frequency, so that wouldn't work.

3. Aug 19, 2007

### learningphysics

What does the above equation say about the current at low frequencies and high frequencies (ie when w is high or low)?

4. Aug 19, 2007

### jesuslovesu

I can see how the frequency would effect the current with an inductor in the circuit

But what's the purpose of a low pass filter (with a resistor in series) then?
Like in this picture
http://en.wikipedia.org/wiki/Image:Low_pass_filter.svg

(or am I to assume that there is not a capacitor in parallel?)

Last edited: Aug 19, 2007
5. Aug 19, 2007

### learningphysics

Yes, definitely having resistors in conjunction with capacitors and inductors will give you much better control over the circuit...

One major reason to use resistors is to make the filter's behavior not affect the rest of the circuit... for example in the low pass filter picture you posted, if R is high enough then connecting this filter to the circuit that's producing V_in, won't affect that circuit's behavior very much (loading effects)... however a low resistance can change the initial circuit's behavior... then we have to analyze the entire circuit together... so that would be a poorly designed filter because V_in would change significantly... and it would change differently for different circuits that you connect it to.... you don't want the filter's behavior to change that Vin...

The resistors help the filter work as a black box... There are many other uses for the resistors also, but this is the first that came to mind for me.

But without inductors or capacitors, there will be no filtering of frequencies.

6. Aug 19, 2007

### Gokul43201

Staff Emeritus
Yes, (in addition to everything said by lp) there is nothing in the question that requires you to assume there is.