Electron-Volts, a unit of energy?

[Bob Scott]

I finally figured out why nuclear scientist use the Electron-Volt as a unit of energy. Maybe I'm just SLOW. It just does not LOOK like a unit of energy.

A Joule is a small unit of energy.
Joules = Watt-Seconds

E = P * t
P = V * I
E = V * I * t
I = Q / t
E = V * (Q / t) * t
E = V * Q
E = Q * V

E is expressed in Joules. V is Volts. Q is expressed in Coulombs. 1 Coulomb is the charge on 6.2415 × 10^18 electrons. I know this is true 'cause I read it in Wikipedia.

1 Joule = 6.2415e+18 Electron-Volts

So it takes 6,241,500,000,000,000,000 Electron-Volts to make up 1 Joule.

An Electron-Volt is a pretty tiny hunk of energy! 100 million Electron-Volts wouldn't even tickle your tongue.

Now if I wanted to, I could calculate how far and how fast an electron moves in a 18 gauge copper wire in an hour with an Amp of DC current flowing through it......maybe some other time...or how many copper atoms an electron passes through in half an 60 Hz AC cycle, through 18 gauge lamp cord, whilst driving a 60 Watt bulb.

Bob

[psycho]

Now if I wanted to, I could calculate how far and how fast an electron moves in a 18 gauge copper wire in an hour with an Amp of DC current flowing through it......maybe some other time...or how many copper atoms an electron passes through in half an 60 Hz AC cycle, through 18 guage lamp cord, whilst driving a 60 Watt bulb.

I would be interested in that

Kevin

[dacflyer]

just make sure that the right colored atoms are moving thru the right coloured wire..lol

[haklesup]

An Electron-Volt is a pretty tiny hunk of energy! 100 million Electron-Volts wouldn't even tickle your tongue.

That's why it isn't used to describe bulk electricity or human scale energy. It's primarily used to describe the energy of individual sub atomic particles. It's better understood from the perspective of E=MC^2 and is roughly proportional to the mass and velocity of a particle.

While a particle with 100Me-V wouldn't tickle your tounge, it could cause cancer if it shattered a DNA molicule as it shot through your body.

[jimmy101]

An Electron-Volt is a pretty tiny hunk of energy! 100 million Electron-Volts wouldn't even tickle your tongue.

But it may well cause cancer of your tongue.

Like haklesup said, it depends on your point of view. 100 million Electron-Volts is about the energy of a single subatomic particle ejected from the nucleus of an atom during a radioactive decay. All that energy is concentrated in very tiny particle that is moving at a significant fraction of the speed of light (for alpha and beta particles) and at the speed of light for gamma radiation.

When a single U235 atom decays it releases about 160 Million Electron-volts (MeV) of energy. For comparison, to rip an electron off a neutral atom takes about 10 eV (0.000010 MeV). The decay of a single Uranium atom releases enough energy to rip one electron off about 16 Million other atoms.

That single Uranium atom decay can potentially do truly amazing amounts of damage to the genome of a single cell. Indeed, worse case scenario would put the damage level at one nucleotide out of every ~200 or so, across the entire ~3 billion nucleotides in the genome. (In actuality the damage is much less since most of the energy ends up just heating up the water around the DNA.)

At the macroscopic level an eV is tiny. At the atomic and chemical level it is pretty large.

[Bob Scott]

While a particle with 100Me-V wouldn't tickle your tounge, it could cause cancer if it shattered a DNA molicule as it shot through your body.

I already had cancer. I'm interested in getting a sense of scale from molecules and atoms. I can zoom in on the picture of a house starting from an image of the entire Earth from space, using Google Earth. I'd like to continue zooming down to particle level, to get an intuitive sense of scale.

Don't geiger counters count actual high energy particle impacts with the counter's detector? What about the person holding the geiger counter and background radiation? Certainly we have high energy particles zooming through our bodies all the time. Apparently there must be a safe limit, with some damage repairable by our automatic immune systems.

Psycho: I plan to answer in this thread about the speed of electrons in wire. I think it's just fun to get a sense of scale of some things that are so small we can't see them.

dacflyer: ?? colored atoms? coloured wire?

[jimmy101]

Don't geiger counters count actual high energy particle impacts with the counter's detector? What about the person holding the geiger counter and background radiation? Certainly we have high energy particles zooming through our bodies all the time. Apparently there must be a safe limit, with some damage repairable by our automatic immune systems.

Yep, each "click" of a Geiger Counter represents a single particle (usually a beta) or packet of electromagnetic radiation (X-ray or gamma ray). The Geiger-Muller tube of a Geiger counter is a high gain amplifier so that a single event creates a signal strong enough electrical signal to create the "click". The high energy particle knocks electrons off many gas molecules in the GM tube which are then accelerated across a 500V ~ 1KV potential to create a current.

You've got background radiation (mostly beta and gamma) zooming through, and being absorbed by, your body all the time. As to there being a "safe limit", radiation health experts don't consider there to be one. There is an unavoidable level of radiation, but that isn't necisarily considered to be safe.

There are a repair mechanism in most organisms. Mostly it is the DNA that is actively repaired. RNA, proteins, sugars, lipids and most other biological molecules don't have much of a repair mechanism, though some molecules are tagged for "recyling" when their level of damage becomes apparent to the organism. Damage to DNA is usually the most problematic long term and occurs at background radiation levels. The problem with the DNA repair mechanism(s) is that it occasionally can't tell what the repaired DNA is supposed to be and the repair ends up being incorrect, which can be even worse than the orginal damage. Background radiation does contribute to things like the cancer rate in humans.

[jwax]

For a sense of scale:
http://micro.magnet.fsu.edu/primer/java ... owersof10/

[haklesup]

Most high energy particles zoom right through the space between the atoms. Whether one strikes an atom and transferrs its energy to the molicule is statistical in nature. Whether that damaged molicule is DNA is also a statistical calculation as well as if that damaged DNA is put out of action alltogether or continues making protiens that are deformed and whether those deformed protiens will lead to misshapen (cancerous) cells.

The safe level of radiatoin is defined by a very low statistical chance that things will go wrong. It's reasonable to believe that at least some cancers are caused by background radiation. In most cases doctors can only guess at the cause for many cancers.

Some theories for cancers depend on a particular body's ability to repair of kill damaged cells through aptosis (cellular level suicide). Radiation is not the only way cells get damaged and cancer arises. UV radiation, chemicals and genetic reasons also apply. Epigenetic theories say that one has a genetic predisposition but only if triggered or catalyzed by a third (and generally unknown) factor. That's why only some people get sick who have certain genetic markers.

There was a Simpsons episode where the opening scene did just a zoom from the human level to the cosmic scale and back down to sub atomic level. Let me try to find out which one. Did you know many of the Simpsons writers had science degrees

[Bob Scott]

Now if I wanted to, I could calculate how far and how fast an electron moves in a 18 gauge copper wire in an hour with an Amp of DC current flowing through it......maybe some other time...or how many copper atoms an electron passes through in half an 60 Hz AC cycle, through 18 guage lamp cord, whilst driving a 60 Watt bulb.

I would be interested in that
Kevin

Someone shoot me! Math is hard on a Windoze calculator!

One mole of copper has 6.02e23 molecules.
The atomic weight of copper is 63.55.
Copper has 63.55g / mole.

Standard lamp cord is 18 gauge.
#18 bare copper wire weighs 4.9166 lbs / 1000'
#18 bare copper wire weighs 0.186 gram / lineal inch.
Therefore #18 copper wire has 0.186 / 63.55 = 0.00293 moles per lineal inch.
Therefore #18 copper wire has .00293 * 6.02e23 = 1.762e21 molecules / lineal inch.

The copper molecule has 1 atom
The copper atom has 1 electron in its outer shell.
Therefore there is 1 freely moving electron per molecule.

Therefore #18 copper wire has 1.76e21 electrons per lineal inch.
1 Coulomb has 6.24e18 electrons.
Therfore #18 copper wire has 1.76e21 / 6.24e18 = 282 Coulombs per lineal inch.

1 Amp = 1 Coulomb / second.
AT a current of 1 Amp, it takes 282 seconds for electrons to move 1 inch in #18 wire.
Speed of electrons at 1 Amp in #18 wire is 1 / 282 = 0.00355 inches / second or 3.55 mils / second.
For a 60 watt bulb at 120VDC, current is 1/2 amp, speed would be half that.


Please correct any mistakes I may have made. I calculated it in a differently convoluted way and got a different answer that varies by about 50%.

Bob

[jimmy101]

IIRC, that method of calculating isn't correct.

The speed of current through a conductor is basically the speed of light in that material. The electric field propagates at the speed of light. To get an electron "off the end of the wire" you push an electron onto the other end, the field propagates through the wire at the SOS and a different electron pops off the other end. A particular electron doesn't have to move the length of the wire, it's electric field does.

Another way to look at it; You push an electron onto the end of the wire. As that electron is entering the wire a long chain of electrons shift forward one atom at the same time. Basically, the movement time adds up as if it is in parallel. The time to get an electron off the far end is the time for one electron to move one atom plus the time for the field to propagate through the conductor.

Things really aren't that simple though. The electron really isn't "owned" by a particular atom. The atomic orbitals of the atoms combine into crystal and/or molecular orbitals. Those orbitals can be very large, even as large as the entire hunk of metal. In metals, the bonding orbitals are not completely filled with electrons and it is very easy to not only add an electron to the system but also have that electron "appear" at the far end of the wire at the SOS since it is a wave function that is changing and not a descrete particle moving.

IIRC, the SOS in copper is something like 2/3 the SOS in a vacuum. So, roughly 120,000 miles/second (400 million MPH).

[MrAl]

Hello,


For a 1mm diameter wire with 3 amps flowing through it i get a drift velocity of about 0.011 inches per second.
That's roughly the same result Bob got. The electric field is set up very quickly, but the electrons dont
flow freely, they hobble around and 'drift' from one side to the other for DC current. The average motion
results in the drift. An electron would also 'drift' out of the end of the wire.
I never tried to measure this however

[Bob Scott]

The electric field is set up very quickly, but the electrons dont
flow freely, they hobble around and 'drift' from one side to the other for DC current. The average motion results in the drift. An electron would also 'drift' out of the end of the wire.
I never tried to measure this however

Al, do you think this movement at an angle or "drift" is reponsible for the velocity factor being lower than the 100%?

[MrAl]

Hi Bob,


Im not an expert on this, but the way i understand it is that the electron moves in three dimensions,
and some of that movement is from side to side and some is in the direction of current flow.
The side to side and possibly some backward motion results in the awkward movement that
overall slows down the electron. When we think about it, the voltage that causes this motion
must be quite small, so there is probably as much motion from side to side as forward, maybe
even more. The electron doesnt have too much reason to move in one 3d direction than another
except for the gentle field influence. It's no doubt then that it would drift rather than speed toward
some goal. It may also be that the electron speeds from one atom to the next, but still the overall
forward motion is slower like a drifting motion.

This isnt an exact analogy, but imagine having a large diameter (1m) pipe standing up on one end on a
planet with very low mass, then spilling a big box of ping pong balls into the open end of the pipe.
On the way down they will bump into each other and the walls, bumping back and forth sideways
and gradually with the influence of that very low gravity they would work their way down.
The larger the pipe diameter the easier it is for them to move downward too.

[jimmy101]

The only problem is that an electron isn't a particle. At least not when it is present in a conductor. It's a wave function. It doesn't exist at any particular place. "Drift velocity" requires the electron is located at a particular place at a particular time. This is an artifical construct that doesn't have much to do with reality.