Comment by gugagore | Hacker News Reader

Comment by gugagore | Hacker News Reader

There are two possible mechanical analogies to electrical circuits (https://en.wikipedia.org/wiki/Mechanical%E2%80%93electrical_...). In domains (electrical, acoustic, thermal, mechanical, ...) there are two kinds of quantities, sometimes called "across" (voltage, temperature difference, ...) and "through" (current, heat, ...).

My first guess was that the analogy here appears to be velocity is voltage and force is current, but I think I have that backwards. The battery, which I was taking to be a ideally a voltage source without internal resistance, appears to be a constant-torque mechanical device. Connecting it in series to different resistances means it spins at different speeds (different current is drawn).

But the battery will also spin if it's not connected to anything... so I'm struggling to keep the analogy straight while thinking about how these parts behave ideally and non-ideally.

Looking at the ammeter, it's definitely velocity = current.

Edit: and finally direct evidence

> But the most practical place for [ground] to be is anywhere there is zero force (i.e., voltage) on the chain.

Interesting, I'm curious why this analogy was chosen, and not the other way around. Current as torque and voltage as (rotational) velocity would seem to allow the junctions to be much simpler -- just a gang of gears on a single axle, rather than the differential mechanism used here -- since torques (like current) add at an axle/junction, while rotational velocity (like voltage) transmits through an axle/junction.

Edit: I suppose a/the major reason is to get the resistance = friction analogy down. With current as torque, friction acts as conductance. E.g., a constant-torque/"current" motor would need to be held still (high friction) to prevent its speed/"voltage" from growing, whereas with the equivalent electrical circuit, you need to short the terminals (low resistance) of a current source to do the same.

I suppose the reason for this seeming discrepancy is that, in an electric circuit, wires are separated by high resistance. But in a physical circuit, "wires" are separated by low friction (= not touching). Flipping around the natural behavior of a junction allows you to take the dual of the entire circuit, thus causing the behavior of resistance and friction to line up.

All natural systems are spring+damper. That means they're effected in the same way by a difference in potential (spring) and also resistance to rate (damper).

This isn't so much analogies, more that the physics of natural systems means they're governed by 2nd order differential equations, so really do behave the same.

More importantly, they're all forms of energy and transferable. Power=IV=Fv=TO=PQ.

I don't think I understand the point you are making. Power = product of two quantities. You have these two quantities in various domains: I and V, F and v, T and O, P and Q, ...

The analogizing comes from saying "I is like P and V is like Q", or vice versa.

2nd order ODEs are very common, and I agree that it is unifying to see different systems modeled by the same equation. But I think more fundamental than that is the understanding of "through" and "across" variables, the notion of a "port" [1], and series/parallel toplogies for combining two one-port components to form a new one-port component.

You could still make an analogy between domains, even if you don't have second-order ODEs. This is clear even from your comment because, note that a spring+damper is going to be a first-order ODE. You would need moving mass to store kinetic energy, in addition to the spring to store potential energy.

[1] https://en.wikipedia.org/wiki/Port_(circuit_theory)

If you scroll way far down on the campaign page, there's actually a table of Spintronics Physical Units:

Spin Volt = 0.1 newtons

Spin Coulomb = 10 meters of chain

Spin Ampere = 10 meters of chain per second

Spin Ohm = 0.1 newton-seconds per meter

Spin Farad = 100 meters per newton

Spin Henry = 0.01 newton-second-seconds per meter

Spin Watt = 1 newton-meter per second

> My first guess was that the analogy here appears to be velocity is voltage and force is current, but I think I have that backwards.

For at least planar circuits there is a dual circuit that is equivalent with voltages and currents switched. So maybe both analogies are correct.

https://basicelectronicsguide.blogspot.com/2018/08/duality.h...

When you choose to call one spintronic component a capacitor and another a inductor, you've determined which of the two analogies is the correct one. You could swap the names for the components, and you would swap which analogy is correct.

Reply on news.ycombinator.com