# Homework Help: Do capacitors block unwanted ac frequencies in the wires?

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1. Jul 16, 2015

### lonely_nucleus

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I have been studying capacitors and inductors recently and I have seen this video.

I have a few questions about capacitors and inductors.
Capacitors:
1.how do capacitors discharge?
2. What happens when you apply direct current through a capacitor?
3.Do capacitors only allow certain frequencies to have a high enough voltage in the wires?
4.what are some applications to capacitors besides filters?

Inductors:
1.What happens when you apply direct current through an inductor?
2.Do inductors only allow certain frequencies of AC to have a high enough voltage in the wires?, Do they have a high impedance to certain frequencies?
3.How does applied voltage lagging inductor current affect applications of the inductor?
4. What are some common applications to the inductor?

2. Jul 16, 2015

### Staff: Mentor

I suggest that you spend some time studying inductor and capacitor on Wikipedia first. Then come back here and ask again about the pays you don't understand.

3. Jul 16, 2015

### lonely_nucleus

I did.

4. Jul 16, 2015

### davenn

so what have you found out ?

your 4 capacitor questions are pretty standard and should have been easily answered with a bit of web searching
so start with them and list some things you have found out about each of your Q's . Write it on a post and we can see
if you are on the right track and correct you where needed

Dave

5. Jul 17, 2015

### William White

parts 4 of each question....use of inductor or capacitor.

Such devices are used for power factor correction; which is worth investigating

6. Jul 17, 2015

### Staff: Mentor

OK, I want you to consider the four variations of a simple circuit above. A voltage source Vin supplies the circuit. The output Vout is measured. The two resistances R are equal.

Now introduce the idea of impedance Z in AC circuits. Z can be any combination of resistive (R), capacitive (1/wC), and inductive (wL). No matter what combination, Ohm's law V=I*Z applies. w is proportional to the frequency. For low frequencies (almost DC) 1/wC is very large and wL is very small. For high frequencies, 1/wC is very small and wL is very large.

Now, can you tell what happens in each of the four circuits for two cases? First case, Vin is low frequency. Second case, Vin is high frequency. Don't worry about numbers, just think about Vout. How big will Vout be compared to Vin?

7. Jul 17, 2015

### lonely_nucleus

well your circuits are low pass and high pass filters because impedance of the components change at different frequencies.

What happens when you connect a dc battery to a capacitor? will the capacitor charge and then discharge after the capacitor voltage=battery voltage.?

8. Jul 17, 2015

### lonely_nucleus

this is not HW questions. These are questions that will give me a more solid understanding of capacitors and inductors when answered.

9. Jul 17, 2015

### Staff: Mentor

Good.

An ideal capacitor connected to a battery will charge to battery voltage. When disconnected, it holds the charge. Real life capacitors have leakage which will discharge it slowly.

10. Jul 17, 2015

### lonely_nucleus

Right.

This is a strange question: When capacitors are connected in series they have low impedance at only certain frequencies. Many frequencies yield high impedance. Can a high impedance AC frequency charge a capacitor?

Do capacitors filter frequencies in wires parallel to those capacitors?

11. Jul 18, 2015

### Staff: Mentor

In each of the 4 variations, you include a component connected from Vin to ground (drawn very close to the left side). Should we pay any heed to this component?

12. Jul 18, 2015

### Staff: Mentor

There is no ground in those drawings. Arrows just show where Vin and Vout are measured. They are not part of the circuit.

13. Jul 18, 2015

### Staff: Mentor

There is nothing of significance in my use of "ground", I could instead refer to your schematic's lower-most conductor as the common rail. The question concerning the component I identified still holds.

14. Jul 18, 2015

### Staff: Mentor

In each case, there is an R or C or L connected across the input. Is that what you mean? Sure they are significant. The L Shortsout the circuit at DC and the C shorts it out for high frequencies.

As I said in #12, the arrows showing how Vin is measures are not components of the circuit.

15. Jul 18, 2015

### Staff: Mentor

A DC source would not like you putting a short across its terminals. For similar reasons, a high frequency source would be expected to react badly if you in effect directly "short" its terminals with a large capacitor as you propose.

16. Jul 18, 2015

### Staff: Mentor

I'm not proposing anything practical. I was trying to explain principles using ideal components. No real life involved.

But I never said DC or infinite frequency, I merely said low or high frequencies.

17. Jul 18, 2015

### Staff: Mentor

Talking ideal components, adding R or L or C directly across Vin will have no effect on Vout in your circuits.

18. Jul 18, 2015

### Staff: Mentor

Ah yes, you're right. Good point.

19. Jul 20, 2015

### donpacino

it doesnt need to be an ideal source... it is simply the input to that circuit block