• Jeremy Rutman

You Oughta Know by Now - Things Every Inventor of Devices Should Know

Updated: 14 hours ago

There are a few engineering rules-of-thumb that any erstwhile inventor should keep in mind:

Friction is for Reals

Pretty much every physical object rubs up against friction, which slows and eventually stops mechanical motion unless there is some external energy input. Pendula stop swinging, clocks stop ticking, rolling stones eventually gather moss.

Even the earth will eventually stop turning on its axis, due to tidal locking (this article for more)

left, tidally locked moon; right, non-rotating moon

And it will also stop going around the sun, due to random collisions with space dust and meteors, gravitational radiation and a few other spoilers.

Lest we be accused of grinch-like curmudgeonry, we hasten to point out that there do seem to be some exceptions, to wit:

  1. The electron(s) of an atom, it would appear, will forever orbit its/their parent nucleus.

  2. A superfluid will circulate without friction forever if left undisturbed.

  3. Similarly a superconductor will conduct a super-current forever if it remains in the superconducting state and external fields don't inhibit the flow.

If that tends to encourage the perpetual enthusiasm of those into perpetuum-mobiles, we have some spoilers: 1. The perpetuum mobile is specifically prohibited by the USPTO , and 2. magnets won't help:

Thermo - The Carnot Limit and More

Any heat engine (including internal combustion engines, external combustion engines like a Stirling engine, and other devices running on expansion of a heated fluid or gas) runs up against a limit on its efficiency called the Carnot limit.

where T1 is the high temperature (e.g. the temperature of combustion), and T2 is the exhaust temperature (e.g. room temperature or the exhaust gas temperature), both measured in degrees above absolute zero (aka Kelvin) .The graph below shows the maximum efficiency for an exhaust of 20C for different working temperature differences.

From this graph you can already appreciate that low temperature differences are rather parsimonious sources for running heat engines, which torpedoes a lot of 'beginner inventions' such as "lets use the exhaust heat from the airconditioner radiator to produce energy!!!". If the exhaust heat is at 20C above ambient, we have from the graph a maximum efficiency of about 5%. The actual efficiency you may reach will generally be some small fraction (e.g. half) of this theoretical maximum.

Conservation of energy

Energy can neither be created nor destroyed; we and everything around us (pretty much) spend our days converting one form of energy to another .

Check yourself before you wreck yourself

If it’s an obvious idea, chances are that someone’s thought of it before, and there’s a good reason why it won’t work. This is not a reason to give up, but rather a reminder to check the idea out first - if you are reading this, you have access to the greatest information source in history (the internet, not this blog).

Throwing bad money after good

Be flexible enough to drop losing propositions. Give it up once if you find its not workable - physically impossible, economically infeasible, etc..


Extrapolations from everyday experience may be good for cases near STP (standard temperature and pressure) but may well lead you astray in other conditions (relativistic velocities, quantum superpositions, etc). Likewise computer games, action movies, and fantasy literature may be good for inspiration but there's a lot of detail to get right before one of these dreams becomes reality (see upcoming blog on SF inspired patents) .

KISS: Keep it simple, stupid.

Occam’s razor.

The simplest explanations/solutions are usually best.

A corrolary is Hanlon's razor: Never attribute to malice that which is adequately explained by stupidity.

A counter-saying is Hickam's Dictum: "A man can have as many diseases as he damn well pleases."

Carbon dioxide and water are products, not reactants

Q: Why can't you burn water - can't everything burn?

A: Its already burnt!

If you get energy out of a system, as in an automobile engine, you can’t get much more energy out of the products. You'll have to add energy to do anything chemically with carbon dioxide and water. Raising it to height is another matter (see upcoming blog on water storage at height).

The Ideal Gas Law

The ideal gas law is

PV = nRT

where P is pressure, V is volume, n is an amount of gas (generally in moles) , R a constant, and T the temperature. This comes across pretty well in the video below, so I won't belabor the point.

Rules of Thumb


The density of a gas is about 1/1000 that of a solid.

Density of water is 1g/cc = 1kg/liter = 1ton/cubic meter

Density of air is about 1/1000 that of water, namely 1g/liter = 1kg/cubic meter

Solar radiation

Power from the sun is about 1kW/m^2 at solar noon.

A full day of sunlight is equivalent to 5 hours of 'full power' at solar noon.


A year is about π*10^7 seconds = 3.14*10^7s

The speed of light is about 1ft/ns

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