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What is capacitance?


What! The flash broke again? Let me see! But I just changed batteries! Must be something else not working.

The trouble turns out to be this little component. It looks a bit like...a battery! And just like a battery, this is a can full of electricity! It’s a little can, that can store and release electrical charge. But it’s not a battery.

It’s a capacitor. And a capacitor works differently than a battery does. One thing that a capacitor is really good at, is to discharge a whole lot of electricity at the same time. And that happens to be exactly what Philip’s flash needs to work. Let’s open the capacitor, to see what can be done.

Hmm...two long plates of metal: conductors… ...separated by a thin sheet of insulating paper... Connect a battery to the capacitor, and look what happens! The plate on the right is connected to the battery’s positive pole. Electrons, with their negative charges, are attracted from the right plate into the battery, leaving the plate positively charged. To the left, the opposite thing happens.

The negative pole of the battery repels electrons away from it and into the plate. Opposite charges, gather in the plates, attracted towards each other. But they can’t make the jump - and meet - because of the insulating paper - the dielectric. As more and more electrons are pulled away from the right plate and into the left plate, it becomes increasingly hard for the battery to move more electrons. Eventually, the battery doesn’t have enough voltage to continue, and the flow of electrons will stop.

Now, the capacitor is as fully charged as this battery can make it. The voltage across the capacitor, is the same as the voltage across the battery. If there had been any difference in voltage between the two, there would still be some electrons moving, to even out that difference. The amount of charge stored on the left is exactly the same as on the right, but with different signs: one is positive and the other is negative. At this point, you can disconnect the battery.

The capacitor will still store the charge. Instead of the battery, let’s connect the flash unit from Philip’s camera. There! The capacitor discharges really quickly - electrons rush from the left side, through the flash, to cancel out the positive charges on the right side. Wow!

Thanks! Got it so far? How much charge fits in one capacitor? Ah, that depends on how hard you push. Check this out: We charge up the capacitor again, but with another battery.

This time with a higher voltage. When we double the voltage, we double the amount of charge. Twice as many electrons are pushing, and want to jump from one plate to the other, inside the capacitor. For each volt that we increase the voltage, the amount of charge increases. How much the charge increases for each volt, is something you can calculate.

Take the amount of charge: “Q” divide it by how many volts: “V” - the voltage, write it as Q divided by V. And this division - charge per volt - is the answer to your question, “How much charge a capacitor can take?” Calculate this division and you get the capacitance of each capacitor. Capacitance describes a capacitor’s ability to store charge. Capacitance is measured by the unit Farad. The capacitance each capacitor has depends on the size of its conducting plates, how close together they are, and how good an insulator the dielectric is.

So...capacitance is not influenced by the voltage of the battery! We connected a battery with a higher voltage to the capacitor, but its capacitance stayed the same. You can say that again! I don’t think I can! Capacitance tells us: how much charge can the capacitor store, for each volt supplied from the battery.

Philip! Your capacity for taking selfies is measuring how much patience I have!