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what is electricity? part 5: volts, amps, currents, resistance and final acceptance, almost

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A vintage Edison carbon filament light bulb – I stole this from Amazon.com, which every decent person should do

Jacinta: So the struggle continues, but I do feel we’re making progress, after having perused our previous posts. We’re perhaps being too hard on ourselves.

Canto: Yes, we’re geniuses actually, asking all the smart questions, not taking anything for granted. Anyway, we posted an image with part 4 of this series, which might help us to understand volts, watts and so forth. It tells us that volts are ‘a force that makes electricity move’, and that voltage measures ‘the potential difference between two points in a circuit’. I don’t fully understand this. It also tells us that watts are the product of voltage and current, P = VI, which we’ve already stated. I’m worried that we’ll be able to make calculations without really understanding the forces involved.

Jacinta: I suspect that our lack of hands-on experimental experience is hindering us. Even brilliant.org won’t really give us that.

Canto: This ‘potential difference’ concept is hard to grasp. Here’s another, apparently very different definition:

Voltage is the pressure from an electrical circuit’s power source that pushes charged electrons (current) through a conducting loop, enabling them to do work such as illuminating a light.

This takes us back to the safe ground of comparing electricity with water. But how does ‘pressure’ equate with ‘potential difference’?  Fluke.com goes on to introduce another headache. Voltage can come in two forms, alternating current (ac) and direct current (dc). We’re definitely not ready for that complication.

Jacinta: But further on this site gives an explanation of potential difference, again using the water analogy. Like water in a tank, voltage is more powerful (has more potential energy) the more water is stored, the bigger the tank etc. When the valve to the tank is opened, that’s like switching on the current, but there will always be resistance (ohms) in the dimensions of the valve or the pipe (the conductivity of the wire). And of course we’re talking of the ‘flow’ of electrons, but I seem to recall it’s more like the electrons are bumping against each other rapidly, a sort of knock-on effect. I may be wrong about that though.

Canto: Voltage is a measure of the potential capacity to do work – to push electrons into activity, whatever the detail of that activity is. I think that’s right. I don’t understand why it’s called potential difference, though, rather than potential energy, say.

Jacinta: Okay, I’ve just asked the internet that question. On Quora, someone with a PhD in theoretical physics says that it’s not actually potential energy, though somewhat related. There’s an equation, U = qV, in which U is potential energy, q is charge, and V is potential, or voltage.

Canto: Right, so voltage is very close to potential energy, because it’s the next letter in the alphabet.

Jacinta: Haha, your knowledge has always been too alphabetical. But apparently it has something to do with fields, and scalar and vector properties. Let’s not go there.

Canto: We might not be able to avoid it. Another Quora answer gives voltage the symbol E, apparently due to Ohm’s Law, I think because in Ohm’s day voltage was described as electromotive force.

E (or V) = IR (I being the current, R the resistance).

Jacinta: We haven’t really talked about electrons thus far, because we’ve been treating the subject historically and we’ve not got past the 18th century, but let’s jump to a modern understanding for a while. We now know that metals and other materials that can pass electrons from atom to atom easily are called conductors. Or rather, we now know that the reason metals are good at conducting electric currents is because of their atomic structure, where valence electrons, the electrons in the outer or valence shell, are loosely bound and can move or bounce from atom to atom within the atomic lattice. I think. Materials like glass and rubber are insulators for the opposite reason – tightly bound electrons.

Canto: So wires of good conducting material, such as copper, are insulated with rubber, to contain and direct the current. If these conducting materials don’t have a current connected to them, the valence electrons will move about randomly. Attaching an electric current to these materials pushes the electrons in a particular direction. Which raises the question – how does this happen? Where exactly does this force come from? 

Jacinta: It apparently comes from the voltage – but that sounds like magic. Of course, the source is a battery or some kind of electrical grid which is connected to households – a  sort of power storehouse. The source is a force.

Canto: Nice. But how does an electrical current move these electrons? For example, we know how water in a stream flows from the mountains to the sea. That’s the force of gravity. And I know how that works, sort of.

Jacinta: What is gravity? Will that be our next 50-part series?

Canto: In yet another intro to electricity I get the analogy of voltage and water pressure, which sort of explains how the force works, like water released from a tank, but it doesn’t explain what the force is. That’s the question – what actually is electricity.

Jacinta: But surely it actually is electrons flowing in a circuit, in a particular direction. Or in lightning. And here’s another definition – of a volt. It’s a joule per coulomb. A joule, in this definition, is a unit of energy or work, and a coulomb is a ‘group of flowing electrons’.

Canto: Fluids again. Anyway, the direction of the current seems to be described as positive to negative (though I’m wondering if the ac/dc distinction comes into play here), as in a small circuit connecting to those terminals in a battery. But why does a current ‘flow’ in this direction, assuming it does? Or are these just arbitrary designations, made up by Franklin?

Jacinta: And here comes another problem thrown up by one of these ‘explanations for dummies’. It distinguishes between a closed circuit, which enables ‘flow’, and an open circuit which prevents the electrons from flowing. I’ve never heard these terms before. Sounds counter-intuitive, but no explanation is given.

Canto: The meaning seems to be that you have to close the circuit to make the flow happen, between one battery terminal and the other for example. And that circuit might include light bulbs, heaters etc. Switching the bulb off means opening the circuit and stopping the flow, at least to that particular bulb. If it’s really a flow.

Jacinta: Well it does seem to be, according to this explainer. The claim is that electron flow is measured in coulombs or amps, because one coulomb equals one amp, though why they confuse us with two measures for the same thing is as yet a mystery. Apparently we can measure the flow of electrons as easily as we can measure the flow of water in a pipe. Which is surely bullshit. The explainer goes on to tell us that this electron flow is called current, and the unit of measure is an amp, or a coulomb.

Canto: Aren’t we going to find out about Monsieur Ampère and Monsieur Coulomb?

Jacinta: Not for a while. Our explainer tells us that one amp or coulomb equals the flow of 6242 plus fifteen more zeroes of electrons over a single point in the circuit in one second.

Canto: Hmmm. I’d hate to be the one counting that, especially within the time limit. Not so easy-peasy. But this is called a unit of electric charge, or electron charge, or elementary charge, and Britannica tells us this about it:

In 2018 the General Conference on Weights and Measures (CGPM) agreed that on May 20, 2019, the ampere would henceforth be defined such that the elementary charge would be equal to 1.602176634 × 10−19 coulomb. Earlier the ampere was defined as the constant current which, if maintained in two straight parallel conductors of infinite length of negligible circular cross section and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7newton per metre of length.

Jacinta: Clarity at last! I need a drink. Anyway, our previous explainer seems to distinguish the group of electrons (a coulomb) from their passing one point in a second (an amp). I think. But let’s move on to something else to be confused about. Electrical currents don’t have to pass through wires of course but they do in our everyday electric circuits. And all these wires have a certain level of resistance.

Canto: Ohm, I think I know where you’re going with this.

Jacinta: The longer the wire, the more the resistance. The thicker the wire, the less the resistance. So in everyday circuits we have to find a compromise. And resistance is also temperature-dependent. Our circuits often incorporate resistors to deliberately restrict electron flow, which seems to be essential for lighting. Resistance within materials occurs when electrons collide with atoms, apparently.

Canto: And ‘conduction’ involves dodging atoms?

Jacinta: Well, it’s just electrons, not a game of Red Rover. Incandescent lights work by incorporating resistors, because the collisions release energy, which heats up the resistant tungsten wire in the bulb, producing light.

Canto: Ahh, that’s a real light bulb moment for me. And that’s not even a joke, though it also is.

Jacinta: Sounds like a great note to finish on. We’ll camp here for the night. For the road is long and winding, and I fear has no end….

References

https://www.fluke.com/en-au/learn/blog/electrical/what-is-voltage

https://www.quora.com/Why-is-voltage-sometimes-called-potential-Its-not-potential-energy-so-what-is-it-the-potential-of-The-electric-field

How electricity works – working principle (video – the engineering mindset)

https://www.britannica.com/science/ampere

https://www.qrg.northwestern.edu/projects/vss/docs/thermal/3-whats-a-resistor.html

Written by stewart henderson

December 23, 2021 at 10:58 pm