Electronics > Beginners
Power source, internal resistance and Ohm's law?
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cur8xgo:

--- Quote from: Mr D on June 06, 2019, 07:39:15 pm ---Thanks, but can you clarify your analogy?

Is voltage like the weight that's going to fall one inch, or like the distance (1 inch) an arbitrary weight is going to fall?

But i like your analogy for another reason:

You asked: "how much work can this thing do if I drop it?"

If there is nothing for the weight to interact with, it'll fall forever. So it'll do an infinite amount of the least possible amount of work!?

Is that like, in an ideal circuit, the current becoming infinitely high when a voltage pushes current down a wire with no resistance?

--- End quote ---

The voltage is potential energy, so its the combination of the weight and the height.

Since super conductors are on the edge of physics I'm not really sure how much current flows when a voltage is applied. It would be interesting to look up how super conducting experiments have been run and what the current levels have been, and supplied how.

So instead of imagining zero resistance, which you will never, ever see, unless you are literally working with super conductors, lets just say a very small resistance, which is small enough to be "ideal" for all intents and purposes but still non-zero enough to allow understanding at an extreme. Lets say 100 micro ohms.

An entire circuit with no resistance is not necessarily "ideal" since it wouldn't really do anything unless you are messing with super conductors. However a section of a circuit between two components could be considered ideal since usually energy wasted in wire resistance is not the goal.

If the building doesn't interact with anything as it falls, no work is being done. So think of an object falling towards a planet with no atmosphere. No work is being done, as there are no interactions. But something is clearly happening. Potential energy is being converted to kinetic energy. 

I _think_ that in a super conductor, something similar is happening. No work is being done, but the current (and/or resultant magnetic field?) represents a kinetic energy of sorts?

Nerull:
The power rails in some of the high end workstation motherboards can supply hundreds of amps at 1.8V. Meanwhile, neon sign transformers supply tens of thousands of volts but can barely output a few milliamps. Voltage tells you nothing about how much power a supply can output.

Voltage is the change in energy of a unit charge moving through an electric field.

As an analogy to height, you could think of a change in kinetic energy per unit mass. There isn't a named unit for that to my knowledge, but it could be expressed as J/kg. A 1 kg unit dropped from some specific height will gain 1J of kinetic energy, and this height would be your 'height-volt'.

You could drop a 1 tonne weight from the same height. It will gain 1000 times more kinetic energy, but its 'height-volt' measurement would be equal because it is normalized to a unit mass.
rstofer:
Ohm's Law holds because it is likely impossible to achieve 0 Ohms of resistance.  Even 10-100 is not zero.  Division by zero is not a realistic problem.

There are many Ohm's Law tutorials on the Internet.
Brumby:
I really don't know why so many people have gone to such great lengths to give answers to questions that weren't asked.

I will address your points:

--- Quote from: Mr D on June 04, 2019, 10:26:06 pm ---So i guess this is the result of EveryCircuit not simulating the internal resistance of the battery?

--- End quote ---
Absolutely correct.


--- Quote ---Is this normal in every simulator (nonsensical answers at the extremes), or do more serious ones take this into effect?

--- End quote ---
I am no expert - but I would expect that if you used a native definition of a voltage source, the internal resistance would not be included, i.e. it would be considered to be zero.  If you wanted to simulate a real-world voltage source, you could create one by selecting an appropriate voltage source with an appropriate series resistor.
If there were any predefined such models, there would need to be a huge variety as not all 1V voltage sources will have the same ESR.  For a 1.5V example, take AAAA, AAA, AA, C and D cell batteries - each of those will have different ESR - and each of THOSE will differ between chemistry and brands.  Then you have power supplies, solar cells and whatever else can generate a potential - and all of those will have different ESR.
Setting up the exact values for a real world source will usually be quite straightforward - and it will save having to search through a potentially large range of predefined models in the hope of finding the right match.


--- Quote ---Or am i in fact talking nonsense?

--- End quote ---
Not at all.  It is a very fair question.

You now know that you have to be aware of the difference between the ideal world of the circuit simulator and the real world of electronics.

A simple example of this difference can be found in the simulation of an astable multivibrator.  Four resistors, two capacitors and 2 transistors in the classic structure will not start.  The reason is that if you use exactly the same value components in the two halves, then there will not be any way for the simulator to choose which side will turn on first.  You can get around this by making one value slightly different.  This introduces asymmetry and the simulator can then work out the starting condition where one side will turn on first.  After that, the running phase will settle down to what you are familiar with.

This problem never happens in a real multivibrator circuit, because the chances of getting four pairs of identical components is basically zero.
Mr D:
Thanks Brumby,

In my short time on this forum, i've noticed quite a lot of resistance from some people to the idea of using a simulator for learning anything about electronics.

Personally i think i've learnt quite a lot a short time from simulators.

But some people, instead of addressing the principal that i'm asking about, simply say in "in a real circuit, that's not how it works". So they expect me, as well as grasping the basic principal, to also grasp the subtleties of real-world component interaction at the same time!

Ok, so EveryCircuit is a bit of a toy, fair enough.

A question:

Are there other simulators, like Multisim, that, given enough information about what component you've chosen, will factor into the simulation the real world-ness that would make it more realistic?

And do they, if instructed to do so, even add in some fuzzy or randomness (into for example, resistor values) so that an astable multivibrator would simulate more accurately?

The reason i'd like to keep learning with simulators is that it's just quicker to throw components around on a screen, create multiple versions to compare, do a quick hour of learning without first covering the kitchen table with multimeters, breadboards etc.

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