Voltage and Current

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kpfowler21
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Voltage and Current

Post by kpfowler21 » Wed Jun 10, 2020 1:19 pm

Good afternoon,

I was recently assigned as a technical accession trade school for the US Navy. i teach Basic electronics, Electron Theory, Ohms Law, etc... The students will be required to maintain heavy mobile equipment such as tractors, mobile power plants, mobile crane

I have been doing a lot of research on this but there are some things i just don't understand and i would appreciate some different ways of comprehending this information.

I am having a hard time explaining what voltage is to students, maybe it is because i don't fully understand it myself. I know it is considered a difference of potentials and an excess of electrons at one point compared to a deficiency at another. I know that it acts as a pressure system on current.

What i don't fully understand is, if current is the flow of electrical charges from negative to positive(we do not teach conventional current), how does voltage go from positive to negative? How can voltage/emf act as an electrical "pressure" on current if they go opposite ways? Why is current the same at all points in a series circuit if you have multiple loads providing resistance?

When it comes to electromagnetic induction, i know i need a conductor rotating in a magnetic field, but what does a change in the number of lines in magnetic force going through the conductor mean? What is actually happening with to the electrons in the conductor at that point? And what is the change in number of lines doing?

Additionally, i am pretty confused on ground. why do i need a ground? is ground where excess electrons are produced?

Any help is much appreciated

Thank you

Kyle

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haklesup
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Re: Voltage and Current

Post by haklesup » Thu Jun 11, 2020 12:24 pm

I have a lot of fun analogizing electronics to plumbing, A great deal of electron flow and storage can be visualized as water flowing through various sized pipes and held in containers. For example Voltage is the potential of the ability to cause flow, so it is like the level of water in a bucket and that bucket may be like a capacitor. The current flows like a river proportional to the channel width (the resistance to flow) and as long as the pipe is smooth, the water flows at the same speed along it, the pressure may vary along that pipe and again, that's like a voltage drip.

Too much water in the bucket causes it to overflow, its like blowing an electrolytic cap, a short circuit or electrical leakage is like a leak in a pipe and it in turn causes its own current flow and pressure (voltage) differentials as a result.

it only goes so far since flowing water does not create an electric or magnetic field but it can be used in a range of network analysis or conceptual comparisons.

Ground is simply the reference that all other voltage measurements will be made with respect to. It is arbitrarily assigned 0V because any given ground could have a potential difference to some other ground. Not all "Ground" lead to earth. Often we call it VSS when dealing with digital chips. Any circuit will generally have a Return path to the lower voltage side of the supply, that is also another way to describe ground. In the plumbing analogy, earth ground would be sea level and circuit ground would be the bottom of an empty bucket.

In the AC world, we call ground the Neutral because the green conductor is called the "Safety Ground" to avoid confusion but nevertheless, the neutral would be the ground reference for measurements in an AC system.

Electromagnetism is harder to analogize to mechanical systems but often the math is very similar to mechanical springs and dashpots. There may be some rotating machinery analogies but I haven't nailed it down. Maybe this, in a piston motor, 4 or 6 pistons take turns pushing the crankshaft, in an AC motor, multiple poles made up of overlapping coils of wire take turn pushing on the rotor, depending on the type of motor, how those fields rotate or are switched in or out makes the motor types. Fortunately EM experiments are generally easy to demonstrate on the bench using high school science class level apparatus. The right hand rule is useful as it shows when fields add or subtract due to current flowing through wires.

I've sometimes wondered if I could build a Class A or AB audio amplifier using only plumbing. A tank would be the voltage source, water flowing from it would be the current that powers the circuit, a pump would be needed to make it a "circuit" and that would be the external power supply. Changes in pipe diameter make resistors, open containers make capacitors, and some sort of water pressure activated valve could make the function of a transistor where a small difference in pressure can cause a large change in current. etc.

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Joseph
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Re: Voltage and Current

Post by Joseph » Thu Jun 18, 2020 6:19 am

how does voltage go from positive to negative?
A thought about that question occurred to me. Recall that protons have positive charge.

dyarker
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Re: Voltage and Current

Post by dyarker » Thu Jun 18, 2020 10:06 am

Electron flow, outside the source - electrons flow from the negative terminal through the external circuit to the positive terminal of the source.

Inside the source the electrons are forced toward the negative terminal. The extra electrons at the negative terminal is what made it negative in the first place. Atoms missing electrons at positive terminal make it positive (protons without balancing electrons). In batteries (cells) a chemical reaction is the "force" that moves the electrons to the element/compound at the negative. In a generator it is the moving magnetic field forcing electrons toward the negative terminal.

In short, electrons flow from negative to positive outside the source, and from positive to negative inside the source.

If you draw little arrows on the circuit you'll see they point in a loop. (Even though the word description may sound like electrons go in reverse direction inside the source.)

Added - In photovoltaic cells the sun lights' energy forces electrons from molecules in cell. The only way back to a now positive molecule is through the external circuit. Again negative to positive outside the source (photo cell), and positive to negative inside.

Cheers,
Dale Y

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haklesup
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Re: Voltage and Current

Post by haklesup » Thu Jun 18, 2020 10:33 am

Going back to the plumbing analogy, Consider a system with 2 buckets of water connected by a tube. We know if we raise one, water will flow from it to the other bucket until they have the same level. If water level determines head pressure in the tube, it is like voltage, it causes water Current to flow until both buckets have the same voltage, I mean level. Obviously if one moves the buckets so the other one is higher, we of course see the current flow the other way, As long as the lower bucket has a lower water level and water current is still flowing then it is like having a negative voltage. Where is ground here? It is either the common level where current stops flowing or it is the spot you are standing where that water will spill if it leaks from the bucket. Depends on your POV for the circuit analysis.

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Weaver84
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Re: Voltage and Current

Post by Weaver84 » Fri Aug 28, 2020 11:18 pm

This kind of textbook has already said very clearly, just need to carefully understand the definition.

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