The strength of the electromagnetic force

[Mario F.]

I've read a couple of hard to digest claims from Neil deGrasse Tyson, a renowned american physicist. They concern the strength of the electromagnetic force compared to gravity and clearly illustrate how powers of ten are such difficult quantities to mentally grasp.

Claim 1:

If you managed to extricate all the electrons from a cubic millimeter of atoms in the nose of the space shuttle, and if you affixed them all to the base of the launchpad, then the attractive force would inhibit the launch. All engines would fire and the shuttle wouldn’t budge.

Claim 2:

And if the Apollo astronauts had brought back to Earth all electrons from a thimbleful of lunar dust (while leaving behind on the Moon the atoms from which they came), then their force of attraction would exceed the gravitational attraction between Earth and the Moon in its orbit.

To be sure, the electrical force of attraction between a proton and an electron is 10⁴⁰ stronger than their gravitational attraction. But despite all, I still find all this just hard to accept.

[Epy]

I think it's easier to accept once you take into consideration that the reason stuff like that doesn't happen is that electrons in normal everyday life on planet earth are part of atoms, and the EMF is basically "busy" keeping those atoms together rather than having the raw attraction of free electrons.

[Sebastiani]

I've read a couple of hard to digest claims from Neil deGrasse Tyson, a renowned american physicist. They concern the strength of the electromagnetic force compared to gravity and clearly illustrate how powers of ten are such difficult quantities to mentally grasp.

Claim 1:


Claim 2:


To be sure, the electrical force of attraction between a proton and an electron is 10⁴⁰ stronger than their gravitational attraction. But despite all, I still find all this just hard to accept.

If you were to extract all of the electrons from one cubic millimeter and confine them to the same volume there would be a non-negligible ionic pressure exerted upon any matter immediately surrounding it, but it wouldn't be *that* great. Now confine them to a volume equal to their individual volumes combined, then yes, you'd have a localized field with a strength comparable to something much greater (just as a laser can be focused onto a very small point to achieve extremely high temperatures), but it would nonetheless be *very* localized and would never be able to actually attract a large object, just from the inverse square law alone.

[grumpy]

The claims are false anyway. Electrons do not exert a net force on matter that holds no charge. And they exert a repulsive force on each other.

[Sebastiani]

Electrons do not exert a net force on matter that holds no charge.

Any given volume of matter (or brane, if you will), assuming that it is essentially homogenous, will have either a net positive or negative charge. A "pocket" of pure electrons will exert a repelling pressure on eachother and any surrounding negative branes and an attractive force upon positive branes. So "no charge" has very little practical meaning here. Also, electrons can interact with the magnetic moment of neutrons so even certain so-called neutral particles are not completely free from their influence.

[MutantJohn]

Huh, I think I did that first problem in one of my lectures in physics O_o

I swear, we had to do something with a rocketship and we took a bunch of electrons from the nose and I'm pretty sure it was that exact problem.

And if you have trouble visualizing it then do math. Or consider chemistry. There are many ionic and covalent bonds, not gravitational ones. In fact, we only really see the effects of gravity on a macro scale or in individual stars.

[Sebastiani]

Huh, I think I did that first problem in one of my lectures in physics O_o

I swear, we had to do something with a rocketship and we took a bunch of electrons from the nose and I'm pretty sure it was that exact problem.

And if you have trouble visualizing it then do math. Or consider chemistry. There are many ionic and covalent bonds, not gravitational ones. In fact, we only really see the effects of gravity on a macro scale or in individual stars.

Okay, so what's the rationale behind the claim then? There are lot's of hypothetical scenarios in physics that are false simply due to the fact that the overall description of interactions is incomplete. Even a very strong electromagnet can only affect relatively nearby objects due to the inverse-square law. The same holds for charges; to have any real effect on the furthest end of the rocket would require a substantial volume of tightly-confined electrons (nevermind how one is to accomplish that for very long!). In reality, these "tractor beam" scenarios just works out to a bunch of nonsense.

[Mario F.]

Okay, so what's the rationale behind the claim then?

All electrons are somehow removed from a cubic milimeter of atoms, leaving only the positively charged core.

If these were hydrogen atoms, you would be left with a cubic millimeter of H⁺ atoms at the space shuttle nose and the electrons at the base of the platform would be attracted to them. Since they were somehow stuck to the platform they couldn't join the hydrogen atoms and the force of attraction exerted would be greater than that exerted by the space shuttle thrusters.

To be sure, the escape velocity needed to escape Earth gravity pull is 11 Km per second. The exercise simply tells you that at the distance between the space shuttle nose and it's launch platform, the electromagnetic force exerted by a cubic millimeter worth of atoms is still stronger than the escape velocity of the space shuttle. In other words, gravity is an extraordinarily weak force in comparison.

I never considered the claims false. Just hard to believe, in the sense that when put as real-life examples, we really get to appreciate EMF strength.

[MutantJohn]

While it may be true that in a lot of hypotheticals things are omitted it is because the effects of those interactions are not on a scale that matters. Neutral matter would indeed be affected.

But those field interactions are small. In the first scenario, you have these two giant fields and a bunch of little ones. You should only really care about the two giant fields first as those will be your largest terms.

To explain the first one, you first have to know that the field flows from positive to negative so that's the direction of the force in this case (Field line - Wikipedia, the free encyclopedia).

How many electrons are in a cubic millimeter? Simple. Consider the material. It's most likely a metal and probably has a high amount of electrons. Given the density of the material, you can easily derive the mass and with the Periodic Table you can even calculate the amount of electrons. Using this, you can calculate the field between the two poles. The field emitted from the positive pole, E = k (+q) / r ^ 2 and knowing that force, F = qE so then you have F = -k(q/r)^2. The idea is that this force is much larger than the force of propulsion of the rocket.

Remember, the fields interact as positive to negative meaning the force is downward, directly against the force of propulsion. It's just a simple thought experiment to give you an idea of how strong electricity can be.

[megafiddle]

The difference between electrical force and gravitational force is enormous.
It is only the force itself that falls off with distance. The difference between
the forces remains enormous.

Consider the gravitational attraction of a couple of bowling balls. Virtually unmeasurable.
Now place a static charge on the balls. There is now a measurable force between them.
And that's just with a very tiny mass of electrons, also virtually unmeasurable.

-

[brewbuck]

The claims are false anyway. Electrons do not exert a net force on matter that holds no charge. And they exert a repulsive force on each other.

Free charges do exert force on uncharged matter. A negative charge (for example) will polarize nearby matter, pulling its positive components closer and repelling the negative components away. The positive components, being nearer to the charge than the negative ones, are attracted more strongly than the negative ones are repelled, resulting in a net attractive force.

This is why a charged comb will attract a nearby stream of water, even though the water is flowing out of a grounded faucet and certainly has no net charge.

[megafiddle]

The claims are false anyway. Electrons do not exert a net force on matter that holds no charge. And they exert a repulsive force on each other.

Free charges do exert force on uncharged matter. A negative charge (for example) will polarize nearby matter, pulling its positive components closer and repelling the negative components away. The positive components, being nearer to the charge than the negative ones, are attracted more strongly than the negative ones are repelled, resulting in a net attractive force.

This is why a charged comb will attract a nearby stream of water, even though the water is flowing out of a grounded faucet and certainly has no net charge.

I believe the examples given also leave the positively charged protons behind when the electrons are removed.

-

[stahta01]

Yeah, what you say is true; but, the premise is that what you say is false.

Edit: To sum it up, If this impossible thing happens and keeps happening then this hard to believe thing will happen.

Tim S.

Free charges do exert force on uncharged matter. A negative charge (for example) will polarize nearby matter, pulling its positive components closer and repelling the negative components away. The positive components, being nearer to the charge than the negative ones, are attracted more strongly than the negative ones are repelled, resulting in a net attractive force.

This is why a charged comb will attract a nearby stream of water, even though the water is flowing out of a grounded faucet and certainly has no net charge.

[MutantJohn]

Inducing a dipole is well-known but I'm not sure if a neutron will ever be effected by charge.

[Elkvis]

To be sure, the escape velocity needed to escape Earth gravity pull is 11 Km per second. The exercise simply tells you that at the distance between the space shuttle nose and it's launch platform, the electromagnetic force exerted by a cubic millimeter worth of atoms is still stronger than the escape velocity of the space shuttle. In other words, gravity is an extraordinarily weak force in comparison.

A minor, but relevant point: the space shuttle never achieves escape velocity. If it did, it would leave orbit and go out into the general area that is our solar system. The space shuttle, and most manned orbiting spacecraft, achieve a speed of about 17,500 MPH, or 28,157 KPH, or about 7.8km/s.