17. "Does it matter which way round you connect a capacitor? I've seen some capacitors with a positive sign at one end and a negative sign at the other. What does that mean?"

Good questions. The answer to the first question ... "Does it matter which way round you connect a capacitor?" is yes and no! ... it depends on the type of capacitor. With capacitors with a "simple" dielectric like a sheet of paper or polythene between the plates, it doesn't matter since to charges they "look" the same no matter which plate is positive and/or negative.

Now for the second question ... So-called "electrolytic capacitors" - the ones with a "+" sign near one connecting wire and a "-" sign near the other wire - are different from regular capacitors. Electrolytic capacitors are often characterized by having a large capacitance but small operating potential difference (i.e., maximum voltage). In order to obtain a large capacitance, one needs to reduce the spacing between the plates to as small a value as possible. This is achieved in an electrolytic capacitor by having, initially, a solution called an electrolyte between the plates (which might be thin sheets of aluminum). An electrolyte is a conducting solution that has a special property that, if an electric current is passed through it from one plate to another, a substance from the electrolyte is produced at one of the plates. For example, if a sheet of paper soaked in aluminum borate (an electrolyte) is placed between two sheets of aluminum and a current is passed through the assembly in the direction shown,

oxygen is produced at the upper interface, which reacts with the electrolyte in that region to form a thin layer of aluminum oxide on the upper plate. Aluminum oxide is an insulator so it forms an insulating-dielectric layer. This layer can be extremely thin, i.e., much narrower than the thickness of the sheet of paper, so, since the electrolyte itself is a conductor, the device is a capacitor with a much higher capacitance than a simple paper capacitor because the thickness of the dielectric is so small.

There are three operational problems with electrolytic capacitors.

1. Firstly, since the aluminum oxide layer is very thin it has a small dielectric strength, i.e., the breakdown voltage (=E_max.d) is not very great, and so there is a relatively low limit on the potential difference that can be applied across it.

2. Secondly, the potential difference must be applied in the "proper" direction, i.e., in the same direction as when the film was first formed. If the potential difference is applied in the opposite direction, the film will "dissolve" and the capacitative effect will be destroyed. This limits the use of these types of capacitors to DC (direct current) applications; clearly, they cannot be used in AC (alternating current) circuits!

3. Thirdly, they are not very reliable and deteriorate with age so they are generally not used in high-grade applications.

Good questions ... thanks!

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