# Capacitors. What are they for?

1. Mar 14, 2013

### Jaymo3141

I'm taking electricity and magnetism at school and i we are learning about capacitors. I get how they work and what they do. However, i don't see how they're useful. What is their purpose other than storing energy? why not just have the battery supply the energy?

2. Mar 14, 2013

### Staff: Mentor

Capacitors can charge and discharge really fast and hold precise amounts of energy, with little loss or degredation.

3. Mar 14, 2013

### Jaymo3141

so how is that useful? I'm really trying to give myself some context to all this new knowledge. I can calculate the capacitance and charge and do all the math but none it really means anything because I have no idea what these devices are really used for.

4. Mar 14, 2013

### davenn

a few of other uses for capacitors

1) DC isolation between stages in a circuit
2) RF bypass
3) when put in parallel with an inductor, you have a tuned circuit ( also known as a tank circuit)

Dave

5. Mar 14, 2013

### Naty1

6. Mar 14, 2013

### Staff: Mentor

Camera flash, soft start of a motor.

7. Mar 14, 2013

### Drakkith

Staff Emeritus
Have you read up on Basic Electronics? If not, I highly recommend picking up a book on the subject, as it explains all the basic concepts of circuits and their common components, including capacitors. They are, quite simply, an integral part of practically all modern electronic devices.

8. Mar 14, 2013

### Floid

Well the other big use for them is filtering. If you look at the impedance of a capacitor, it decreases as frequency increases. So if you want to remove high frequency components from a signal then you can connect a capacitor to ground. The low frequency components see the capacitor as high impedance so they will skip the capacitor and keep going through the circuit (assuming the rest of the circuit is lower impedance than the capacitor). But the high frequency components see a low impedance path to ground, so they take the capacitor which keeps them from traveling through the rest of the circuit.

Why is this useful? Well this very thing is present on every digital circuit card. You use capacitors to remove high frequency components from your DC power supplies so that your digital IC work correctly.

For supplying power you use a capacitor over a battery mainly because a capacitor can supply power much quicker than a battery. One example that was already given was flash. Another example is your home air conditioning unit, there is a large capacitor that is used to jump start the compressor.

9. Mar 16, 2013

### mheslep

The air tank analogy for a capacitor might be helpful. Like the air tank, the capacitor is little harm by charging and discharging. However, like the pump, batteries are degraded because it has moving parts of kind (ions).

10. Mar 18, 2013

### carlgrace

The DRAM in your computer is using capacitors to store the bits. That's pretty useful.

A digital camera uses the capacitance of a reverse biased photodiode to sense light. That's pretty useful.

An analog-to-digital converter (in a audio circuit, or a cell phone basestation, or a million other places) uses capacitors to store charge and manipulate it to generate a digital value that is proportional to the input voltage. That's pretty useful.

A radio uses a capacitor to filter out unwanted frequencies to receive a specific channel. That's pretty useful.

An electrical power system uses capacitors to filter out oscillations that could cause equipment to malfunction. That's pretty useful.

A computer uses capacitors to tune it's clock frequency. That's pretty useful.

I could go on...

11. Mar 18, 2013

### Crazymechanic

strange nobody mentioned the age old use of filter capacitors after bridge rectifiers to smooth out the left over "ac parasitic".
so yeah just in case I added.

12. Mar 18, 2013

### carlgrace

An electrical power system uses capacitors to filter out oscillations that could cause equipment to malfunction. That's pretty useful.

13. Mar 18, 2013

### psparky

Power factor correction between the electric company and all the big factories would be one of their largest uses financially. Coincidentally, these are generally the largest capacitors.

Motors are inductive by nature. This makes the current lag the voltage causing more power consumption for the same output at the shaft of the motor. Capacitors can and do correct this by "swinging" the current closer in phase with the voltage causing less power consumption. This can all be worked out mathematically by the use of the following. Also describes their behavior in almost any situation.

Impedance of a inductor: JωL
J= 1<90
ω= 2∏f (essentially the frequency of the circuit)
L= size of inductor (Henries)

So it's inductance clearly varies with the frequency of the circuit. Note, when talking DC, ω=0 and it acts like a short when powered by a battery once charged. It will accept any frequency and then act according to it's mathematical model described.

Impedance of a capacitor: 1/(JωC)

Capacitor's impedance also clearly changes with frequency. Again, when talking DC, ω=0 and capacitor acts like and open circuit.....infinite impedance.

Note that the "J" above shifts the current out of phase with the voltage at 90 degrees.....either a current vector pointing straight up....or a current vector pointing straight down.

Last edited: Mar 18, 2013
14. Mar 21, 2013

### harborsparrow

A capacitor is necessary for creating a oscillating signal, which is the basis of everything. If you want a sine wave, you'll need a capacitor to force the signal to go back and forth.

15. Mar 21, 2013