| Electronics > Beginners |
| Some noob questions |
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| Ice-Tea:
--- Quote from: Mr D on July 17, 2018, 08:15:06 am ---So what is it about electricity that is so fundamentally less intuitive, that makes it so hard to understand, one needs to resort to an analogy of water? --- End quote --- Have you seen water? I would assume so. Have you actually seen electricity? I would think not. So, there ;) |
| PA4TIM:
It is possible to explain it on a fundamental level but that makes it very complex unless you are an ace in Physics. Now we only talk about DC voltage and current. The water model represents the functional model. Water as current is more or less usable to explain the effect. In real life electrons are travelling from minus to plus, besides that they travel very slow while the energy is moving very fast. Then there are electric and magnetic fields, static or dynamic. Water won't induce a water flow in a pipe next to it. A current flow can do that. Water will flow in the pipe, DC current does, AC, too but AC can also move on the surface of the wire and to make it even more complex, move outside the wire and that brings us to transmission lines and things like wave behavior that leads us to quantum mechanics. There is no way to understand that straight away and the amount of things you need to know is huge. There are only a hand full of people on this forum who really know the deepest fundamentals. I had the same problem when I started. I started with radio and every time I got my head around something "that opened a door" it turned out there were even more closed doors after that. And then is the problem, what path to follow. There is not one path, there are many running parallel and they are forking out constantly too. You need a very high drive to follow a lot of paths parallel Now you talk about a simple restive load and that is already hard, but in real there is not just resistance, there is "complex resistance" called impedance and split out in reactance and resistance. You need to know this. But to know this you need to know a lot about inductance and capacitance. A real resistor also has inductance (and capacitance to other parts of the circuit) A trace on a pcb is a resistance for DC but has transmission line behavior for AC. Without that knowledge you can understand a lot of things practically, the rest is called voodoo by many and they can do a lot without knowing that part. Knowing that makes life more easy. And that also brings in a lot of math You are now struggling to learn maybe 0,001% of the fundamentals. By far the easy part and a lot of people do not understand more then this and still can do a lot. ;) |
| hamster_nz:
Maybe a car analogy would help (not very likely, but I'll try :D ). Think of a car as being an electron (all identical cars, because all elections are exactly identical), and motorways are wires. You pick a reference point across the motorway and count each car as it passes, and how fast it is traveling. In this analogy, current is the count of cars crossing that line in a standard unit of time. (this is a good analogy, as one amp is 6.242×10^18 electrons passing in one second). Voltage could be thought of as the average speed of cars (this part isn't a very good analogy, as voltage is more a 'difference in pressure' between two points). But what is clear in this analogy is that the two things (count of cars & average speed of cars) can be independent of each other - you can have a few cars moving slowly (low voltage and low current), or a few cars moving very fast (high voltage and high current), or you could lots of cars moving slowly (high current, low voltage), or lots of cars moving fast (high current high voltage). It all depends on the local conditions at the time - there might be heavy fog with snow and ice, or it might be a clear day. So when you measure 2A on your plugpack, that is measuring 12,484,000,000,000,000,000 electrons entering the meter every second (and the same number exiting too). The confusing bit is the voltage, and it is not just because of my broken analogy. The plugpack is engineered such that it will at most only let a given number of electrons flow per second, regardless of the voltage setting. So should the plugpack is set to 12V and more than 2A wants to flow, then the electronics in the plugpack reduces the output voltage (the energy per electron) until only 2A will flow. |
| PA4TIM:
http://wiki.c2.com/?SpeedOfElectrons Electrons moving through a cable is a big simplification on its own. There is a drift speed and a thermal velocity. The electrons move slow but the charge moves fast. From the link: --- Quote --- For example, for a copper wire of radius 1 mm carrying a steady current of 10 Amps, the drift velocity is only about 0.024 cm/sec! --- End quote --- |
| alanb:
Mr D. It sounds from your original question that you are connecting your multimeter directly to the power supply whilst in the current mode. This is something that you should not do. In current mode you should connect the meter in series between the power supply and the load. With the meter connected directly to the power source in current mode the current is only limited by the maximum current capability of the supply and this can blow the fuse in the meter or worse. In your case the maximum current from the supply appears to be 2 amps thats why you get the same result on both voltage ranges. |
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