Posts Tagged ‘current’
what is electricity? part 6: ohm’s law, electron flow and ac/dc, not explained
got it?
Canto: So we were getting into the behaviour of electrons in electrical field or currents, and the different ways electrons behave when voltage or electromotive force is applied to them, depending on the materials in which they’re embedded, whether that material is more or less conductive/resistant.
Jacinta: Which led to our light bulb moment. But we really need to look at electrons and their behaviour more closely, methinks.
Canto: I’ve noticed Quora questions such as ‘Why do electrons move against the electric field?’ and ‘Why do electrons experience force in direction opposite to electric field?’ Amongst other confusing things, responders note that there is simply a convention, created by Franklin. The convention being, I think, that electric fields/currents flow from positive to negative. This isn’t entirely clear to me, though I get the idea of conventional designations.
Jacinta: A number of responders point out, in different ways, that an electric current, say from a battery, flows from the positive terminal to the negative terminal. Electrons, being negatively charged, are repelled by the negative terminal and attracted to the positive terminal, due to the rule that like charges repel and opposite charges attract. So electrons flow in the opposite direction of the current/field. Which raises the question of why currents flow from positive to negative (presumably that’s just the convention).
Canto: So if the convention was turned around and we describe the flow as being from negative to positive, then we’d recognise the flow of electrons as going in that direction? I mean, which way do electrons flow really?
Jacinta: This might be a non-issue. A circuit attached to a voltage generator, such as a battery, sends the electrons in the direction of the current, which is arbitrarily designated as from the positive terminal to the negative one. Sounds like the electrons, negatively charged, are being pushed to the negative terminal, which would be expected to repel them, but that isn’t what’s happening. The electrons are just flowing in the direction of the current. Better to call the terminals A and B.
Canto: But if that was so, there’s an easy fix – we’d stop referring to those terminals as positive and negative. But I don’t think it is so. In one video I’ve watched, a battery is described as something which has two terminals, positively and negatively charged points, with a charge imbalance between them. The electrons are definitely described as being ‘pushed’ by the current from the negative point or terminal to the positive one, as you’d expect with opposite charges attracting. Though it also says that the flow of the charge is opposite to the flow of electrons, something to ponder. It also describes the negative point as a source, and the positive point as an attractor. The two-pronged electrical plugs use this system, one being the source, the other the attractor. And a ‘short circuit’ involves wires burning out because there is no resistance in the circuit – that’s to say, no appliances which work by applying resistance, which creates energy to run the appliance, as we saw with an incandescent bulb. Fuses act to prevent short circuits, cutting the current when the wire overheats.
Jacinta: Well, we seem to be learning something. This is better than a historical account it seems. But there are still so many problems. The ‘electricity explained’ video you’ve been describing says that the negative point is the source. So it’s saying negative to positive, simply ignoring the positive to negative convention. Perhaps we should too, but the video makes no mention of the convention, which confuses me.
Canto: Well, let’s push on. We’ll need to understand electrical fields, and of course the difference between ac and dc, and probably a host of other things, before we return to the historical discovery stuff, which of course is fascinating in its own quite different way.
Jacinta: Absolutely. And the relationship between electricity and magnetism, and Maxwell’s equations, haha. All without ever doing anything hands-on.
Canto: So I’m watching the apparently somewhat notorious recent Veritassium video on the subject, and I’ve learned in the first minute or so that a battery uses dc electricity whereas the grid connected to our homes uses ac. Though I knew that about the grid. Not that I know what it means exactly.
Jacinta: Yes, and he then says that in ac the electrons are just wiggling back and forth – as ‘alternating current’ suggests. But as mentioned earlier, I thought that was always the case – or, no, the electrons don’t flow, they just bump each other along, which obviously isn’t the same as ‘wiggling’. Each electron has moved, but only slightly. And I never thought of this in ac or dc terms.
Canto: So I’ve just watched the whole video, and I think I’ll pass on commenting at this stage. Obviously I don’t understand it all, nor do I understand the comments, many of them highly detailed.
Jacinta: Yes I think we should get our heads around the ac/dc stuff, and fields, then maybe get back to it.
Canto: This’ll probably take a lifetime, but we’ll start with direct current, dc. Your basic AA or AAA battery is a source of direct current. I’m looking at a typical AA 1.5 volt battery. It will provide 1.5 volts of, errr, voltage constantly in a circuit. Until it doesn’t. But also the circuit will have a resistance, measured in ohms, and we need to remember Ohm’s Law, from part 5, V (or E) = IR. That’s to say, the voltage is the current (I, in amps) multiplied by the resistance. I don’t know why that is, of course, but in any case a circuit connected by a certain voltage of battery will produce a particular current depending on the size of the resistance.
Jacinta: Like the dimensions of a pipe through which water flows. If you have one part of the pipe with a narrowed channel, that will effect the whole flow. The same with a resistor. And of course any wire will have resistance, depending on its conductivity. So why do we multiply the current by the resistance?
Canto: Ohm’s Law can be expressed as I = V/R. Here’s an elaboration of this:
This equation, i = v/r, tells us that the current, i, flowing through a circuit is directly proportional to the voltage, v, and inversely proportional to the resistance, r. In other words, if we increase the voltage, then the current will increase. But, if we increase the resistance, then the current will decrease.
I think it means that voltage will need to be increased to overcome the resistance, which reduces the current. It would be worthwhile to think of this in the brilliant.org way, to solve some simple problems. I’ve used study.com here, a site for engineers and such. Suppose you have a 10 volt battery connected to a light bulb with a resistance of 20 ohms. What is the current in the circuit?
Jacinta: So we have an equation with three variables. The current, in amps, is the voltage divided by the resistance, in this case 10/20, so the current should be 0.5 amps? Wow, I think I done some maths!
Canto: So if we double the voltage in this circuit, we double the current. Now, the great Khan, of Khan Academy fame, describes voltage as electric potential, as we’ve described before, or even energy potential. Think of a closed tap with potential energy. Open it, and you release kinetic energy in the flow. Current is measured as the flow of ‘electricity’, or electrical charge, per unit of time, I = Q/t. But then he confuses me with coulombs, which I’m not ready for. Q means charge (possibly measured in coulombs), and I’m not sure of its relation to V.
Jacinta: We’re equally confused. Let’s focus briefly on ac electricity. Alternating current involves this ‘wiggling’ of electrons mentioned before. Apparently electrons can be made to wiggle back and forth at particular rates, measured in cycles. Each cycle involves the electrons moving forward and then back to their starting points. In some grids, the electrons wiggle like this at 50 cycles/second, in others, e.g in the US, at 60 cycles/ second, or 60 hertz. How electrons can be made to do this I’m not sure – it presumably involves pulses of force? From both ends? Anyway, this form of electricity is apparently safer because it doesn’t heat up the wires so much. I can’t clearly see why though. But then you need transformers to connect the wires to your house, which uses direct current, I think. And as far as I know, a transformer is, like – here, a miracle happens.
Canto: So, more questions than answers here. What, exactly, is a transformer? How does it work? Why doesn’t ac electricity heat up the wires so much? How exactly is ac electricity created? Does every home need a transformer, or is it one transformer per street, or district….? it just goes on and on…
References
https://www.quora.com/Why-do-electrons-move-against-the-electric-field
What is electricity? – Electricity Explained – (1) , video from Into the Ordinary
Introduction to circuits and Ohm’s law | Circuits | Physics | Khan Academy
Alternating current, direct current & what is frequency? | Physics | Khan Academy
what is electricity? part 5: volts, amps, currents, resistance and final acceptance, almost
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
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