Need help understanding capacitors

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In summary, the electrons in a capacitor are not actually travelling from the negative to the positive plate. However, there is an attraction between the two plates which causes a magnetic field. This field indicates the presence of an equal amount of current between the two plates.
  • #1
remedemic
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So I'm reading that in a capacitor, an electric field is being manifested between two plates. There are no electrons actually traveling from the negative to the positive plate, correct? There is only an attraction?
 
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  • #2
Also, I understand that electrons are being gathered on the bottom plate (connected to the negative terminal), but could someone explain what is occurring to the top plate connected to the positive terminal?
 
  • #3
The simple view is the electrons depart from the top plate, pass through the battery, and arrive at the bottom plate. Thus the battery itself neither gains nor loses electrons. Its function is to provide the force that moves the electrons. The battery is analagous to a water pump - it makes the water move, but is neither a source, nor a sink, for water.
 
  • #4
remedemic said:
There are no electrons actually traveling from the negative to the positive plate, correct?
Yes, that's correct. Not through the dielectric medium anyway.

But there's something strange going on here, because despite the lack of flow of charge (i.e. conduction current) between the plates, the magnetic field (as per Ampere's Law) that you typically associate with an electrical current is still present between the two plates when it charges! In fact, the magnetic field between the plates indicate that the current between the plates is equal in magnitude as that of the conduction current in the connecting wires. It turns out there are two kinds of current, and the the latter, ghost-like, type between the plates is called displacement current and does not involve transportation of charge.

I hope that doesn't muddy the water for you, but you seem to be moving through the gears pretty quick so hopefully it will appeal to your curiosity more than it will confuse you.

EDIT: Dear OP, a wave of self-doubt washed over me after I posted this and re-read your OP--for some unknown reason, bringing up displacement current felt like a good idea, but that feeling is now completely gone. Sorry for any confusion.
 
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  • #5
At this stage it is probably preferable to stick with Sylvia's simpler concept.

[STRIKE]Her(?)[/STRIKE] Sylvia's mental model will get you a long way.

Think of the dielectric as an array of polar molecules. Water is a good example - it is odd shaped with + at one end and - at other... that makes it 'polar'..

120px-H2O_molecule_scheme_of_dipole.png


In absence of an electric field they'll be oriented randomly.
In presence of an electric field they'll twist around and align with the field.
That twisting into a new orientation takes energy , and that's how energy is stored in the dielectric. They'd like to snap back to random orientation.
Interesting - it's almost a mechanical phenomenon.


And that's why a capacitor can store energy.

Observe that pure water is a good dielectric , its ε about 80.
 
  • #6
Sylvia Else said:
The simple view is the electrons depart from the top plate, pass through the battery, and arrive at the bottom plate. Thus the battery itself neither gains nor loses electrons. Its function is to provide the force that moves the electrons. The battery is analagous to a water pump - it makes the water move, but is neither a source, nor a sink, for water.

The "simple" view of electrons actually 'travelling' a significant distance during an experiment is very risky. By the time your 'average capacitor' has charged up, the net movement of electrons in a circuit is a tiny fraction of a mm. Best just to talk in terms of 'charge' and, at a stroke, you have avoided the problem which many people have of electron flow being in the opposite direction to conventional current and the worry of how far the electrons may actually get. In a circuit, electrons are faceless - like the links in a bicycle chain.
 
  • #7
sophiecentaur said:
Best just to talk in terms of 'charge' and, at a stroke, you have avoided the problem which many people have of electron flow being in the opposite direction to conventional current and the worry of how far the electrons may actually get.

seconded .

Electrons in wires move imperceptibly slow*,
whatever charge is it moves comparable to speed of light irrespective of its sign.

You'll be in agreement with virtually all textbooks and professors if you speak of current in terms of moving positive charge. So your education will go easier.

Where I worked, engineers and technicians kept up a running good natured rivalry between "Engineer's Current" and "Real Current'...
My guys were well aware of the fact electron drift is slow.
But many of them still preferred to think in terms of negative charge when walking a circuit to write Kirchoff's law.. That was quite natural in vacuum tube days.

So go with the accepted terminology.
In most engineering circles you'll be looked at askance if you speak of electron flow.

There are some circles where folks were trained in negative current, and very competently.
It's good to be able to swap back and forth for it'll gain you respect both places.

*(they move fast in beam devices like CRT or electron microscope, but we are discussing circuits.)

old jim
 

What is a capacitor and how does it work?

A capacitor is an electronic component that stores electrical energy in the form of an electric field. It consists of two conductive plates separated by an insulating material, known as a dielectric. When a voltage is applied to the capacitor, it charges the plates, and the dielectric material prevents the charges from flowing across. This stored energy can then be released when needed.

What are the different types of capacitors?

There are various types of capacitors, including ceramic, electrolytic, film, and variable capacitors. Each type has different properties and uses. For example, ceramic capacitors are small and inexpensive, while electrolytic capacitors have a larger capacitance and are commonly used in power supply circuits.

How do I choose the right capacitor for my circuit?

The right capacitor for a circuit depends on the specific requirements of the circuit. Factors such as capacitance, voltage rating, and temperature stability should be considered. It is also essential to select a capacitor with the correct physical size and shape to fit into the circuit design.

What are the units of measurement for capacitance?

The unit of measurement for capacitance is the Farad (F), named after the physicist Michael Faraday. However, due to the small size of most capacitors, subunits such as microfarads (μF), nanofarads (nF), and picofarads (pF) are commonly used.

How can I test a capacitor to see if it is working correctly?

There are a few methods for testing a capacitor, including using a multimeter, an oscilloscope, or an LCR meter. These devices can measure the capacitance, voltage rating, and other properties of a capacitor to determine if it is functioning correctly. Another way to test a capacitor is to discharge it using a resistor and then measure the time it takes to recharge. A functioning capacitor should recharge quickly.

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