Understanding Conventions in Engineering: The Role of Rules and Consistency

[Trying2Learn]

TL;DR Summary

Stress sign conventions in engineering

(I admit this might be better in the mechanical engineering forum, but let me ask, and the moderator can decide)

What is a "convention?"

For example, in elasticity, we have a stress tensor. If a coordinate axis is in one direction, positive, we state the the convention is that the stress is positive outward on that face.

If we modeled tractions on a cube, where the normal to the face, is in the negative diretion, then the Traction also points in the negative, but we call it positive.

(I do not wish to argue or discuss that issue as relates to engineering.)

I want to know what a "convention" means (and I am not entirely sure I am asking it the right way).

Essentially, we arrive at a point, in the purity of mathematical analysis, and then must adopt a rule, that is "external" to our brain.

Ah ha! I DO have another example: using the right hand rule to create frames and to ensure consistency with, say, the cross product.

There, again, we adopt a right hand rule. We arrive at a point in math IN OUR BRAIN, but adopt a rule WE SEE IN THE WORLD, e.g.: when we turn a screw driver or insert a lightbulb.

Could someone please expound on this "singular moment" in the evolution of disciplines?
What is happening at the moment when we adopt a convention?

In other words, the word "convention" is one I now accept, blindly. How would you explain what we are doing, without using the word "convention?" I do NOT mind (in fact, I welcome it), if this evolves into a discussion on epistemology

 

[pbuk]

Generally we use the word convention in science when:

1. there is more than one way to define something
2. there is no overall advantage that one way has over all other ways
3. [Edited]it is important that everyone uses the same definition it is beneficial to a group of people if they all use the same definition

See:
https://en.wikipedia.org/wiki/Sign_convention
https://en.wikipedia.org/wiki/Orientation_(vector_space)

This is a fairly simple and unambiguous concept, there should be no discussion on epistemology required.

 

[Frabjous]

I am not sure that 3 is phrased correctly because there can be multiple conventions that different practioners use.

 

[pbuk]

I am not sure that 3 is phrased correctly because there can be multiple conventions that different practioners use.

Good point - edited

 

[Steve4Physics]

Note that different conventions are sometimes used for the same thing. Examples…

In optics there are several different sign conventions for the signs of focal length and object/image distances.

In maths and physics it is a convention to use "i" for the square root of minus one. But in electronics and electrical engineering "j" is used.

By convention, nuts/bolts/screws have right-handed threads unless you explicitly specify a left-handed requirement.

Any convention is simply a matter of convenience to facilitate communications between people working in a common area. There’s nothing ‘profound’ about adopting a convention.

If there is any ambiguity, the specific convention being used should be stated.

 

[sophiecentaur]

By convention, nuts/bolts/screws have right-handed threads unless you explicitly specify a left-handed requirement.

I find it unconsciously disturbing to watch many of the Movies, shown on Facebook. For some reason, they are shown 'mirror image' and people screwing things UP are unscrewing them - and vice versa. Our thread convention has got well into our brains. 'Clockwise' seems to have started with sundials and carried on into nearly everything.

 

[anorlunda]

An infamous convention we can blame on Benjamin Franklin. It became evident that charges come in two polarities. It would be natural to call those two positive and negative, + and -. Franklin arbitrarily chose a convention that makes the electron have negative charge. Because of that convention, electric current flows in the opposite direction of electron movement. Countless students of basic electricity curse the day that Franklin chose that convention.

So we all benefit from using the same convention, even if the choice of convention is unfortunate.

Note that natural language works only by conventions. The only way we understand the word convention in your question is a convention about what that word means. The meanings of words and the appearance of character symbols are all conventions.

 

[Trying2Learn]

I find it unconsciously disturbing to watch many of the Movies, shown on Facebook. For some reason, they are shown 'mirror image' and people screwing things UP are unscrewing them - and vice versa. Our thread convention has got well into our brains. 'Clockwise' seems to have started with sundials and carried on into nearly everything.

You know, this helps me a lot, too: it connects convention to experience of how they made sun dials (and then clocks)

Thank you.

 

[sophiecentaur]

we can blame on Benjamin Franklin.

Countless students of basic electricity curse the day that Franklin chose that convention.

Imo, he did us all a favour because, this way round, forces us to think about what's going on and discourages the water analogy and 'flowing electrons'.

 

[kuruman]

You know, this helps me a lot, too: it connects convention to experience of how they made sun dials (and then clocks)

Thank you.

There are many examples of conventions above already so I will draw your attention to the reason why conventions are adopted. Simply put, communication. Can you imagine the chaos that would ensue if there were no convention about which side of the street we drive our cars? People aren't stupid. The necessity for a driving convention has been recognized since ancient times. In the marble quarry of Carrara (I think) there are two sets of parallel tracks on the ancient road to the quarry. One, presumably leading away from the quarry, is much deeper than the other one. Fully loaded carts dug deeper into the road surface than empty ones.

Conventions are one instance where people cannot agree to disagree because they are agreements to agree.

 

[kuruman]

An infamous convention we can blame on Benjamin Franklin. It became evident that charges come in two polarities. It would be natural to call those two positive and negative, + and -. Franklin arbitrarily chose a convention that makes the electron have negative charge. Because of that convention, electric current flows in the opposite direction of electron movement. Countless students of basic electricity curse the day that Franklin chose that convention.

The urge to reply with this was irresistible.

 

[Dale]

What is a "convention?"

A convention means that there are at least two equivalent ways to do something and so we made a choice to do it one specific way.

in elasticity, we have a stress tensor. If a coordinate axis is in one direction, positive, we state the the convention is that the stress is positive outward on that face.

In this case, we could have chosen that stress is positive inward. As long as we did everything else consistently with that choice we would get the same result. We would still be able to tell how much load a truss can take before collapsing, even though all of our stress tensor components would be opposite. The choice is arbitrary, but once made must be used consistently.

 

[anorlunda]

Imo, he did us all a favour because, this way round, forces us to think about what's going on and discourages the water analogy and 'flowing electrons'.

Am I wrong? I think you grew up in the vacuum tube era like I did. That was the context where the conflict was most stark.

 

[sophiecentaur]

Am I wrong? I think you grew up in the vacuum tube era like I did. That was the context where the conflict was most stark.

A vacuum tube is one of the few instances where the electrons happen to have significant KE and KE is seldom any help in trying to understand Electricity in circuits.

Strange but the approach where I was taught the basics of Electricity was to keep Current as a flow of charge, rather than a flow of electrons. Any apparent conflict involving moving electrons was avoided because no one tried to 'make it easier for us' by introducing a tangible model for Current.
I guess we avoided a lot of those sorts of problems because we treated Current in an abstract way and 'did the maths' and, pragmatically, believed the results.

I read so many requests on PF for a 'Physical' explanation for phenomena in Physics and I'm convinced that that approach can be a lost cause.

 

[Dale]

the approach where I was taught the basics of Electricity was to keep Current as a flow of charge, rather than a flow of electrons

I agree with that approach. The phrase “conventional current” always makes me cringe.

 

[kuruman]

Now that I think about it, I believe that Ben Franklin is getting a bum rap. He came to a fork in the road and took it: The charge carriers in a a conductor are negative. People could have followed Ben's lead and sensibly defined positive current flowing in the direction of the charge carriers in a conductor. But nooo! Here comes John Ambrose Fleming. According to the Wikipedia link,

​

"Sir John Ambrose Fleming FRS (29 November 1849 – 18 April 1945) was an English electrical engineer and physicist who invented the first thermionic valve or vacuum tube, designed the radio transmitter with which the first transatlantic radio transmission was made, and also established the right-hand rule used in physics."​

It seems to me that if Fleming had established a left-hand rule instead, the current in the direction of electron flow would be consistent with the Lorentz force and Ben Franklin would be off the hook.

 

[pbuk]

It seems to me that if Fleming had established a left-hand rule instead, the current in the direction of electron flow would be consistent with the Lorentz force and Ben Franklin would be off the hook.

But current in the opposite direction to electron flow had already been established as the convention for 150 years, are you suggesting that Fleming should have tried to introduce the opposite convention for free electrons to that in a conductor or ionic solution?

 

[kuruman]

But current in the opposite direction to electron flow had already been established as the convention for 150 years, are you suggesting that Fleming should have tried to introduce the opposite convention for free electrons to that in a conductor or ionic solution?

I think it is safe to say that if we had the opposite current convention and a left hand rule, there would be no problem. Fleming did the best he could do. I am not sure who initially defined the current direction that we have know for conductors, but I have never heard Ben Franklin's name associated with it.

 

[sophiecentaur]

Fleming did the best he could do.

That makes the assumption that he was wrong about something. It really doesn't matter which direction of flow is chosen; the whole edifice is self consistent. That sort of choice occurs everywhere in Science and we 'deal with it'.
It's really not much of a struggle so what's all the fuss about? Is it just that people don't want the rigour that EE actually demands of us?

 

[Trying2Learn]

That makes the assumption that he was wrong about something. It really doesn't matter which direction of flow is chosen; the whole edifice is self consistent. That sort of choice occurs everywhere in Science and we 'deal with it'.
It's really not much of a struggle so what's all the fuss about? Is it just that people don't want the rigour that EE actually demands of us?

I certainly do not know enough about your discipline to comment on your post.
But I love your quote at the bottom, and have shared it.

 

[sophiecentaur]

I certainly do not know enough about your discipline

My personal discipline is very questionable but I think this is a valid 'do as I say' even if I don't always do it. The fact that I am so much against analogies as a way of teachings based on my experience of those who do and those who don't. Those who don't try to use analogies seem to produce more reliable results