What means capacitance? How do we understand it?

Although we know that the capacitor can be charged and discharged, the voltage across the capacitor cannot be changed suddenly, and the capacitor can pass high frequency and block low frequency. However, many people have not straightened out their thinking and feeling that the capacitor is unpredictable. Some simple principles should explain capacitors.

What means capacitance
What means capacitance?

The principle of “charging and discharging”

The first understand the principle of “charging and discharging” of capacitors, and the rest is all-natural. Science and engineering knowledge are like this, the foundation is not solid, and everything that follows is playing the piano to the cow. What is the principle of capacitor charging and discharging? Very simple, the silk rubs against the glass rod, and the glass rod becomes electrified, right? Why is the glass rod charged? Because the electrons on it transfer to the silk, microscopically, there is a significant loss of negatively charged electrons, and there is a “deficit,” so it has a positive charge. It is called “static electricity.”
Further, we put the charged glass rod in contact with the electroscope. Is the aluminum foil open? Why open it? Because part of the electrons on the electroscope transfer to the glass rod. Neutralizing the “positive charge,” it carries and makes the electroscope itself charged. It is all about the migration of electrons, right? Of course, these are talking about static electricity. We know that static electricity can be “discharged” through conductive objects. Autumn and winter are the seasons of the high incidence of static electricity. If the weather is dry, There have probably suffered from static electricity many times.
Connect the glass rod to the electroscope with a wire. Moreover, the electrons transfer along the wire. The same as if There were to touch the electroscope directly with the glass rod. In this process, “current” is generated in the wire. It is just that this current lasts for a short time, and it flashes a spark, and it has gone. Why is it gone? Because there is no “potential difference” between the glass rod and the electroscope. Half of it, now everyone is positively charged, and the potential is equal, so naturally, no current can continue to exist. If it can figure out a way to make the potential difference always exists.

Can the current always exist?

That is right. For example, through the galvanic cell reaction, we can always maintain a potential difference between the positive and negative electrodes of the galvanic cell; then, when connecting them with wires, we get a “steady current.” Similarly, what happens if we put two pieces of aluminum foil close together and connect them to the positive and negative electrodes of the primary battery with wires? The aluminum foil connects to the positive electrode charge, connected to the negative electrode charge. It can find a 6.3V 1000uF aluminum electrolytic capacitor and connect it to the positive and negative poles of the dry battery (pay attention to the positive and negative poles) – now, remove the capacitor and short its two pins with tweezers. Will it snap? One spark at the same time? This is the charging and discharging of the capacitor. The capacitor I mentioned earlier is an “aluminum electrolytic capacitor,” and this capacitor has a positive and negative pole! —Ah, It was already dizzy enough! Stop pulling weird Stuff in it! We guess that it will be whining like that. However, science and engineering are very “weird”: once you have mastered the fundamentals, the more things you pull in, the more it can help you understand the relevant knowledge in depth. ——The fun of studying science and engineering is precisely here. Let us go through it all over again. Still two pieces of aluminum foil, we touch them with a positively charged glass rod and a negatively charged rubber rod, respectively, so that they are respectively positively and negatively charged. Very good. You will notice that the two pieces of aluminum foil are attracted to each other. You know it is the electrostatic attraction – it is the electric field at the end of the day.
Now, we slowly separate the two aluminum foils. We need to resist electrostatic force; the farther we take it, the more work we do. At the same time, the potential difference between the two aluminum foils is also increasing. However, the number of charges on the two aluminum foils did not change during the entire process. What is this indicating? It means that when the two aluminum foils are far apart, charging them with the same amount of electric charge will require higher electrical work. Otherwise, the law of conservation of energy will violate.
Similarly, when two pieces of aluminum foil are relatively close, it is easier to charge them with the same amount of electric charge due to the attraction between different electric charges – this is a “filling knife” based on the principle of the electric field. A physical phenomenon can often explain from many different directions without necessarily insisting on energy conservation. Must be self-consistent internally). We call the “ease of charging” the “capacity.” From my previous discussion, with a bit of knowledge of electric field/potential energy/work. It is easy to deduce that the capacitance is proportional to the plate area and inversely proportional to the distance between the plates – the permittivity is too high, so we temporarily do not care. Well, you see, we have mastered the fundamentals, and we do not even need to ask for the capacitor formula. We can guess by ourselves. To make the capacitor bigger, one is that the area of ​​the plates should be larger (so it needs to be rolled up with a large piece of aluminum foil), and the other is that the distance between the plates should be below. An electrolytic capacitor uses a non-conductive aluminum oxide layer on the surface of the aluminum plate as a spacer and another electrode that acts as an electrolyte to form a capacitor. This oxide layer is skinny so that the electrolytic capacitor can achieve a large capacitance with a small volume. The downside is that it can only accept charging in one direction. Once reversed, the aluminum oxide layer will be eroded and destroyed, and the capacitor will break – here again with the knowledge of electrochemistry.
Physics/Chemistry is just that. The more you think about it, the more you realize that it seems that all knowledge ultimately boils down to a few simple truths. In the end, you do not seem to have learned anything new. You have a deeper and deeper understanding of Newton’s three laws of conservation of energy and so on. This profound, in turn, can help you decipher and even independently invent/discover new Stuff – it is like the formula for capacitance. The more you learn, the less you understand; but the more you can deal with more and more complex scenarios. Once you have this feeling and verify it in the new knowledge, you know that you have probably learned it.

Is the mysterious veil of capacitors gone?

Now, is the mysterious veil of capacitors gone? Once understood to this extent, the problem is not a problem at all. The principle of capacitor charging and discharging is understood. Then “the voltage across the capacitor cannot be abruptly changed” can naturally be deduced at once: the voltage across the capacitor determine by the amount of charge on the capacitor plates and the distance between the plates. The charge needs a There is no “teleport” in the charging and discharging distance, and the voltage across the capacitor cannot change suddenly.

Why don’t capacitors allow DC to pass through?

Similarly, why don’t capacitors allow DC to pass through? It is straightforward, it breaks in the middle, and the two pieces of aluminum foil are not touching. When it first connects to the power supply, there will be a current to charge it, but once the voltage between the two aluminum foils charges to the power supply voltage, there will naturally be no current in the circuit. Therefore, capacitors cannot let DC through. Why do capacitors allow alternating current to pass through? Because the AC circuit is equivalent to a power supply whose positive and negative electrodes are constantly changing. When the capacitor is first connected, it will also charge it like a DC circuit. However, after the capacitor charges, its output voltage is still changing (hence the capacitor current is 90° out of the AC circuit voltage), even though the voltage direction is changing. Therefore, when its voltage is lower than the voltage of the two poles of the capacitor, the capacitor begins to discharge; when its voltage polarity reverse, it charges the capacitor from the other direction. Because the voltage of the AC circuit is not fixed for a moment, the connection The capacitors in it naturally switch between charge and discharge all the time.
Moreover, this continuous charging and discharging process make the circuit always current. For example, a direct current is like a circular racetrack. In the external circuit, electrons start from the negative pole and run along the circuit to the positive pole. Inside the power supply, the electrons are moved from the positive pole to the negative pole by chemical energy/electromagnetic field, and it does not stop. So there is current. (To be precise, it is not necessary to move the electrons to the negative electrode; for example, the inside of the battery generates excess electrons at the negative electrode through a chemical reaction. Thereby maintaining the potential difference between the positive and negative electrodes – when the battery is not connected to the circuit, ” The excess electrons” also prevent the adverse electrode chemical reaction from continuing. Therefore, the new zinc plate of the battery negative electrode does not consume quickly. Until you connect the positive and negative electrodes, the “excess electrons” of the negative electrode are sent. Go to the positive pole, quickly drain.) Moreover, the capacitor blocks this process (the circuit is open), and when the electrons charge it up, the road is blocked.

The conclusion

The alternating current is like two cities AB. The car drives from A to B in the morning. Moreover, the car drives from B to A in the afternoon. The road between the two cities is broken, but two substantial parking lots have been built near the breakpoint, so the car can still start from A to B in the morning, but it will stop in the middle parking lot. In the afternoon, the parking lot in the middle parking lot car goes back to A. Some cars come from B simultaneously and stop on the breakpoint close to B. You see, although cars cannot drive from A to B, the flow of traffic from A to B and from B to A can exist.

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