Suppose this parallel plate capacitor is connected to a 9 volt battery. A volt is a measure of a potential difference. Simply put, this is the potential energy of each charge – it measures how much energy a charge gains by moving through that potential. So this 9 volt battery will have a 9 volt potential change across the plates.
But what happens if you push one of the boards so that the distance between them decreases a little bit? Well, since the capacitor is still connected to the 9 volt battery, the potential still needs to be at 9 volts. However, if the electric field remains the same, a shorter distance will mean a lower potential. The only way to compensate for the reduced spacing is to increase the charge on the board. This extra charge will come from the battery, which looks like a current. On the other hand, if you move the plate further away, the charge will flow out of the capacitor and create a current.
In other words, moving the board back and forth creates a changing current. This is the basis of how condenser microphones work. When you have sound, it vibrates in the air. These oscillations then push a plate of the condenser microphone to produce a varying current. You can then record this current and save it for later, and you can send it to amplifiers and speakers for louder sound.
The nice thing about condenser microphones is that one of the condenser plates can be very thin and flexible. This means it can move quickly in response to higher frequency sounds, so you might not be surprised that many high-end microphones are of this type. Of course, a small downside is that these microphones require voltage to be applied, which means they need power. This could come from the small battery in the microphone, or more likely from the power supply of the audio receiver/amplifier.
Now let’s look at a slightly different type of microphone: the electret microphone, sometimes called an electret condenser microphone. What the hell is an electret? The name should remind you of something familiar: magnets. Although it is possible to generate a magnetic field with an electric current (such as with an electromagnet, As Wile E. Coyote shows), most people probably think of something like a permanent bar magnet. They are made of materials with tiny regions that also generate magnetic fields called magnetic domains. When these domains are aligned in the same direction, you get a magnet with north and south poles.
Instead of using permanent north and south poles to create a magnetic field, electrets use positive and negative charges to create an electric field. It’s a bit like socks coming out of the dryer with static and sticking to things. (Well, socks don’t stay charged, but electrets do.) While socks may simply have too much negative charge due to some extra electrons, or positive charge due to lack of electrons, electrets can actually be neutral. Even if an object has the same number of positive and negative charges, it can still generate an electric field if there is “charge separation”. Imagine a molecule that is slightly positive on one side and negative on the other. It’s still neutral, but creates an electric field.