Paying Ohmage

[From the last episode: We looked at some ways of optimizing neural-network models so that they run better at the edgeThis term is slightly confusing because, in different contexts, in means slightly different things. In general, when talking about networks, it refers to that part of the network where devices (computers, printers, etc.) are connected. That's in contrast to the core, which is the middle of the network where lots of traffic gets moved around..]

We’re going to cover one more interesting development in the world of AIA broad term for technology that acts more human-like than a typical machine, especially when it comes to "thinking." Machine learning is one approach to AI., but in order for it to make any sense, we’re going to have to start by covering some super basic concepts. For anyone with the slightest electrical background, this will be very much a review. But if you haven’t had this, it will hopefully help you understand electricity better – and it’s important for the thing we’re going to talk about in a few weeks.

Let’s talk about three – actually four – basic electrical concepts. They’re easiest to understand if you think about flowing water, since there are a lot of parallels.

The Basics

The three basic concepts are voltageVoltage is what gets electrons to flow. It's analogous to water pressure, which gets water to flow. Voltage is measured in units of "volts.", currentThe amount of electrical flow. Measured in amperes or amps (A)., and resistanceForces that tend to reduce the amount of flow or current. Measured in ohms (Ω).. The fourth one is powerThe rate of energy consumption. For electricity, it’s measured in watts (W)..

• You can think of voltage as pressure. It’s what makes electricity want to move. It’s like the water pressure in your pipes at home. But, instead of those pipes, imagine a tank of water with a hose coming out. The higher you put that tank, the farther the water is going to move, and the higher the pressure will be. If you live with a well on a larger property, you know that the water pressure will be higher farther down from the well. Same thing with voltage. The height of the water tank is equivalent to the amount of voltage applied to a circuit. The higher the voltage, the “more” things move. We measure it in volts, with V being the symbol.
• You can think of current as the amount of flow. With water, that would be gallons per minute. With electricity, it amounts to how many electronsA fundamental particle found outside atoms. It carries a negative charge. It can move easily in a conducting material, which gives rise to electrical current. flow per minute; same basic notion. We measure it in amperes, or amps, with A being the symbol.
• Resistance is a little harder to picture with water. It’s what makes things want to stop flowing. With water, there is friction on the inside of the pipe. Or, when the pipe changes directions with elbows and such, that slows the water down as well. The amount of energy that water has when it comes out of the hose is less than the energy of the water that went into the hose, because some of that energy was lost through the pipe. We measure it in ohms, which we designate by the omega symbol Ω.

They Work Together

These three concepts are related. If you increase the height of the water tank, water will come out faster – you’ve increased the current. If you make the hose wider, then you’ve reduced the friction and resistance, and so you get more water for the same pressure.

This works with electricity as well, and we can summarize it in something called Ohm’s Law. It’s a really simple relationship:

V = I*R

That is, the product of the current and the resistance gives the voltage – or pressure. You can raise the voltage by running more current or by raising the resistance. Which probably feels backwards, since we tend to feel like the pressure is the “cause” of things moving. So it may look more natural if we move a couple things around to give the equivalent relationship:

I = V/R

This gives us a picture of how the flow depends on the pressure and resistance. The higher the voltage, the more current you get. The lower the resistance, the more current you get. That’s probably an easier way to think of it, even though it’s mathematically equivalent to the prior way of looking at it.

While resistance is “what makes things slow down,” it’s kind of a glass-half-empty way of looking at it. You can look at the glass half-full by thinking not of what makes it slow down, but what lets it go faster. We call this conductanceThe literal opposite of resistance. Rather than how hard is it to push the flow through, it looks at how easy it is. Measured in mhos., represented by G (for whatever reason), and it’s the inverse of resistance:

G = 1/R

The bigger the resistance, the smaller the conductance, and vice versa. And – you’re not going to believe this – but the unit for the conductance is the mho – yup, that’s “ohm” spelled backwards. We’ll need this conductance concept in a few weeks. We can rewrite the second version of Ohm’s Law with conductance instead of resistance:

I = V/R = V*1/R = V*G

Power Play

The last quick thing is power, which we measure in watts (using the symbol W). It also has a simple relationship to two of the other concepts:

P = V*I

People get confused about what power really means. It’s often associated these days with how “power-hungry” a chipAn electronic device made on a piece of silicon. These days, it could also involve a mechanical chip, but, to the outside world, everything looks electronic. The chip is usually in some kind of package; that package might contain multiple chips. "Integrated circuit," and "IC" mean the same thing, but refer only to electronic chips, not mechanical chips. or circuit is.

But here’s the thing: power is the rate at which you use up energy. We can measure it in a unit called the joule, so power means how many joules you’re using up in a time period, like joules per minute. The critical thing here is that it’s energy that’s being consumed. How fast it’s consumed is the power. So high power means you’re using up energy really fast – which can be bad for a battery, since it will go dead faster.

But you’ll see phrases like power-hungry or references to power consumption. Those are incorrect. Nothing is hungry for power (except a politician); no power is being consumed, since power isn’t a substance that can be consumed. It’s a rate of consumption. Things are hungry for energy, and energy is consumed. So it’s an important thing to remember – even though engineers are sloppy with this language all of the time, and, for many of them, reading this paragraph might be an eye opener. (Or, more likely, they would shrug and say, “Oh well, everyone says it that way, so I’m going to as well.”)

Once we’re done with neural networksA type of conceptual network organized in a manner inspired by our evolving understanding of how the brain works., we’ll start talking about power and energy consumption – a very important thing for the IoTThe Internet of Things. A broad term covering many different applications where "things"  are interconnected through the internet.. But… all in good time.