Most of us have taken a photograph indoors, using the flash lamp built in to the camera. When “Flash” mode is selected there’s usually a short delay before the camera is ready. That delay is caused by the electronics in the camera charging up a device called a capacitor. The job of the capacitor is to store electrical energy and then release it all at once in a sudden burst. It’s this pulse of energy that enables the lamp to throw out the very bright flash of light.
Capacitors are found in many other electronic devices besides cameras. Televisions, radios, microwave ovens and even garage door openers all take advantage of the capacitor’s ability to store electrical energy, but how do they work?
A capacitor consists of two parallel surfaces with insulating material between them. If one surface is connected to the positive terminal of a battery and the other to the negative terminal, electrical current will try to flow. This might seem counterintuitive, given that the insulating material forms a break in the circuit, so let’s examine what happens.
The parallel surfaces are the key. With the battery acting as a kind of pump, negative charge builds up on one surface. Now because opposite charges attract, a corresponding positive charge builds up on the other plate. What this means is that current will flow in our circuit, despite the break caused by the insulating material, but here’s where things get really interesting: each surface can only accept a certain quantity of charge, after which it becomes full.
(It might help to visualize this by thinking of cars entering a parking lot. At first there are lots of empty spaces and cars can flow in, but as the spaces fill the flow of cars slows until, when every space is taken, it stops.)
So once the surfaces are full of charge the current stops flowing. The battery is still pumping though, so the charge that built up on them can’t escape. However, when the battery is removed the charge can flow back off the surfaces, in the opposite direction to which it came from.
This is what happens in a camera when “Flash” mode is selected. Electrical charge builds up in a capacitor and stays there until the camera button is pressed. That releases a flood of electricity which rushes through the lamp and gives a brief and very intense burst of light. The same method is used in other devices like electric motors that need a boost of energy to get them started.
The other use of capacitors is to smooth out changes in electrical flow around a circuit. When used this way a capacitor acts as a reservoir, storing up charge when there’s lots of it and releasing it back into the circuit when the flow drops. This smoothing capability is particularly useful in audio circuits because it helps minimize unwanted noise.
Capacitors can be found in practically every household appliance. For example, you’ll see quite a few on the motherboard of a desktop computer. Most of these look like small cylinders about ¼” in diameter and standing up from the board, although others are shaped like little brown oysters. This might prompt you to wonder where the parallel surfaces discussed previously have gone. The answer is that they’ve been rolled up into a cylinder so as to make them fit into the computer. If it weren’t for this trick our computers would still be as large as filing cabinets!
Lastly, a word of warning is in order if you do decide to go hunting capacitors. Some of these, especially those used to kick-start electric motors, can store enough charge to injure or even kill someone on the receiving end. When dealing with such equipment it’s not enough to just disconnect it from the electrical supply because the capacitor will still be charged. This is why it’s best to leave working on such equipment to a professional electrician.
For further reading:
http://www.antonine-education.co.uk/electronics_as/electronics_module_1/topic_9/physics_of_capacitors.htm
http://www.britannica.com/EBchecked/topic/182915/electricity/71559/Principle-of-the-capacitor
http://physics.info/capacitors/
