# Plank's Constant for laymen.

## Recommended Posts

If a layman were to ask what is Planck's Constant and what does the figure 6.626 x10^-34 Js mean how could you describe what it means?

##### Share on other sites

A very small layman?

It's the quantum of angular momentum in quantum physics; all changes in angular momentum come in steps no smaller than this.

It's also the conversion factor from frequency to energy.

##### Share on other sites

Thanks but sadly our layman is just looking at you with a very very blank expression

##### Share on other sites

Things at the quantum level behave like they spin. If the spin vector is pointed in one particular direction, or has a particular magnitude, it can't smoothy change to another value.

It would be as if a motor could only go at 100 rpm, 200 rpm or 300 rpm, etc. but no value in between, or could only be aligned in a limited number of directions.

Planck's constant would be the equivalent of the 100 rpm increment. Except the "spinning" isn't a physical object actually spinning. (QM is quite weird)

This also applies to things behaving like they orbit, except in QM they don't really orbit like a planet orbits. You simply wouldn't be allowed to have a planet in any old orbit. Mars is OK, Earth is OK, but the spaceship orbit getting from one to the other isn't permitted.

##### Share on other sites

OK, mist is beginning to lift

So what do the units Js mean? I can fully understand what miles per hour or meters per second means but 'Joules second' is a tad mysterious.

##### Share on other sites
OK, mist is beginning to lift

So what do the units Js mean? I can fully understand what miles per hour or meters per second means but 'Joules second' is a tad mysterious.

Swansont has mentioned angular momentum several times, rotational momentum. Think about what combination of basic units would be appropriate to measure THAT and you will see that Js is a handy concise way of expressing it.

Ordinary linear momentum is measured in kg m/s

and the rotational momentum just equals that multiplied by the radius or lever arm, so kg m2/s

For example think of a half-meter radius wheel with lightweight axle and spokes and a massive 10 kg rim that is going 3 m/s at the rim. so there is 30 kg m/s of ordinary linear momentum, concentrated out at a radial distance of 1/2 meter. both factors, the rim's linear momentum and the radius, contribute to the angular momentum. So you multiply them and get

15 kg m2/s

But if you write a Joule in basic terms, it is just kg m2/s2. That is essentially just the definition of Joule. So multiplying by s

gets you Js as a nice concise way of writing the unit of angular momentum.

===============

there are other ways to think about it, like the Joule is also the unit of torque----"twist force"-----and the angular momentum you put on freely rotating body like a wheel is proportional to the torque you apply multiplied by the the length of time you apply it

##### Share on other sites
OK, mist is beginning to lift

So what do the units Js mean? I can fully understand what miles per hour or meters per second means but 'Joules second' is a tad mysterious.

The amount of angular momentum is the linear momentum multiplied by the moment arm length (and a trig function if those aren't perpendicular). Linear momentum is mv, or kg-m/s, and multiply by the moment arm (m) to get kg-m^2/s. Well, a kg-m/s^2 is a Newton (force = ma) and a kg-m^2/s^2 is a Joule (work or energy)

So a kg-m^2/s is a J-s

Angular momentum (L) is also related to torque (T), T = dL/dt which gets you to the same point

(it can be misleading that torque and energy have the same units, because energy does not always involve rotation, so this unit analysis isn't generally used for macroscopic objects. Torque is usually left as N-m rather than represented as a Joule)

##### Share on other sites

It is the measure of the minimum 'grainyness' of the universe. It is the resolution of reality. Think of pixels on this computer screen. The plank constant is the minimum size of the pixels in our universe.

Is that lay enough?

##### Share on other sites
It is the measure of the minimum 'grainyness' of the universe. It is the resolution of reality. Think of pixels on this computer screen. The plank constant is the minimum size of the pixels in our universe.

Is that lay enough?

You may be confusing the planck length with planck's constant. (the former being the scale where you'd need a quantum theory of gravity because general relativity breaks down)

##### Share on other sites
You may be confusing the planck length with planck's constant. (the former being the scale where you'd need a quantum theory of gravity because general relativity breaks down)

I thought the plank constant was the mathematical function used to account for the plank length when attempting calculations regarding the sub-atomic realm.

##### Share on other sites
I thought the plank constant was the mathematical function used to account for the plank length when attempting calculations regarding the sub-atomic realm.

Planck's constant is one of the terms in the Planck length, but they aren't the same thing. The different units tell you that some other terms must be involved, and in the case of the Planck length G is involved (which is small), and so is c (in the denominator, cubed) which makes the terms smaller still.

##### Share on other sites

swansont

Except the "spinning" isn't a physical object actually spinning. (QM is quite weird)

How do you explain the anomalous magnetic moment if the particle does not spin?

##### Share on other sites
swansont

Except the "spinning" isn't a physical object actually spinning. (QM is quite weird)

How do you explain the anomalous magnetic moment if the particle does not spin?

http://en.wikipedia.org/wiki/Magnetic_moment#Magnetic_moment_of_electrons

No one said they didn't have angular momentum.

##### Share on other sites

It's a QED correction

http://en.wikipedia.org/wiki/Anomalous_magnetic_dipole_moment

(if course, this link is contained in the one Klaynos gave)

##### Share on other sites

PHILOSOPHY AND YOUR RANDOM THOUGHTS ARE NOT SCIENCE DO NOT POST THEM AS SUCH

One could reply that it is equally true to say that mathematical prediction is not science but, such comments are not really helpful. Instead I would point out that there is an alternative view derived from Stochastic Electrodynamics. The crux of the debate is exactly what should be included within the particle, and what should be considered as part, or all; of some external field. Mac Gregor lays out the case in 'The Enigmatic Electron' If the magnetic field of a particle is within the particle then the four different electron radii (as explained by Mac Gregor) can be shown to be distinct parts of a vacuum/anti-vacuum field.

##### Share on other sites

If anyone asks me a question how to explain a quantum physics phenomenon to a layman I direct him to read Mr Tomkins in Paperback, by George Gamow. I certainly couldn't explain it better than he.

## Create an account or sign in to comment

You need to be a member in order to leave a comment

## Create an account

Sign up for a new account in our community. It's easy!

Register a new account