What's a Joule?

That's a good model: the voltage on a conductive object represents the density of electrons, where zero volts means the number of electrons just equals the number of protons.

So voltage is charge density on a conductor. Measured in volts.

Current is the rate of flow of charge from one conductor to another, in amps. If there's a conductive path between the charged things, the voltage difference tends to push charges along that path; more voltage difference, more push.

Power is the current flow multiplied by the voltage difference between the conductors involved; watts.

Energy is the time integral of power; watt-seconds, aka joules.

Two nearby conductors at different voltages constitute a charged capacitor, which stores some joules of energy. If current is allowed to flow between them long enough for the voltage difference to vanish, the stored joules will have been dissipated as power over the time it took to discharge the system. If a resistor was used as the conductive path, those joules would have heated up the resistor, which is where the stored energy went.

John

or, the way I prefer to think of it,

I = dQ/dt

Instantaneous change in Coulombs divided by the instantaneous change in time. (When it is expressed using finite values, then these must by definition amount to average quantities and are never really precise for any instant in time.)

Joules per Coulomb, yes. But... Think of it this way....

Set up two plates in a vacuum, somewhere in an otherwise empty universe. Provide 1 Volt of potential difference across these plates (we'll figure out what that is in a moment) and set a Coulomb's worth of electrons over near the surface of the one that is more negative, so that they are accelerated towards the other plate. Eventually, they will travel across the intervening space and will strike the other plate. When they do, the net energy of the impact will be

1 Joule. Release two Coulombs to start and the net energy of the impact of that many will be two Joules. And so on. And it doesn't matter how far apart the plates are moved. If they are close together, the electrons will be accelerated more and take less time to arrive, but their final velocity will be the same when they hit and the resulting energy of the impact will be the same. Further away, they will take much longer and be accelerated more slowly, but they have much further distance over which to gain their velocity and they will arrive again with the same impact energy. Hence, Volts is expressed as Joules/Coulomb and doesn't need to include the distance between the plates.

Don't forget that:

Ohm = Joule-second/Coulumb^2 = Watt/Amp^2 = Henry/second

Watt = Joule/second

Henry = Joule-second^2/Coulomb^2 = Joule/Amp^2 = Ohm-second

Farad = Coulomb^2/Joule = Coulomb/Volt

If you then look at an "RC time constant," this is:

R*C = (Joule-second/Coulomb^2)*(Coulomb^2/Joule) = second

Nice, eh? Time, as you might expect. Also, take a look at the SQRT(L*C), which is:

SQRT(L*C) = SQRT((Joule-second^2/Coulomb^2)*(Coulomb^2/Joule)) = SQRT(second^2) = second

Once again, time! Dimensional analysis to the rescue.

Jon

By the way, this "final velocity" won't be the same if you were propelling a Coulomb's worth of protons instead of electrons, as they are nearly 2000 times as massive and the force will impress a much lower acceleration on them. But the net energy at impact should be the same. Since potential energy is proportional to M*V^2, I'd anticipate that the velocity of the protons at arrival would be about the square root of the ratio of the mass of the electron to the mass of the proton, or about sqrt(1/1836.15266) = .02334 times as fast. A little more than 2% of the speed that the electrons would achieve. (Assuming neither case was approaching relativistic speeds, of course.) Same energy, though.

Jon

"Chris Williams" schreef in bericht news: snipped-for-privacy@posting.google.com...

Chris,

Voltage is often compared with pressure and it's not a bad comparison. But I guess your description of pressure is not correct so you will fail to understand voltage as well. Pressure has nothing to do with amount. The "potential" of water depends on its height (due to gravity) no matter the amount of water. It may be either a raindrop or a reservoir. Only if you look at the energy the amount becomes important.

Power has to do with the available energy per second. Even a filled reservoir can deliver little energy/second if you have only a 1" pipe. But when the weir breaks... Another example is a flashlight battery. Its amount of energy is comparable with the energy of a thunderflash but you need at least minutes to empty a battery. A flash delivers its energy in a split second.

petrus bitbyter

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Nope. Electromagnetic energy (Joules) are nothing like a "push."

Voltage is not pressure, but it's connected. Voltage is more like the depth of water. Potential difference between conductors is like the height difference between two lakes: connect them and there will be a flow. Voltage is also like the height of a hill up which we roll a boulder, with the charge being the boulder's mass. In that case the potential energy of the boulder is "Joules." The higher the boulder (i.e. the higher the voltage,) the more Joules are stored as potential energy... and the greater the mass, the greater the energy. Joules = height * mass, and Joules = Volts * Coulombs.

Another note: whenerver circuits are involved, the energy is electromagnetic energy, and it's made of e-fields or b-fields (or both.) In other words, the Joules have a particular location. The Joules stored in a capacitor are stored in the insulating gap. The Joules transmitted along a wire are in the space surrounding the wire.

What goes Erg! ? A dyn' centimeter, of course.

--
Rich Webb   Norfolk, VA

Ok I believe I understand correctly now--I was a bit off.

Floyd's description of "potential energy" was throwing me off as, how do you evaluate the total potential energy of a mass? If you have a boulder just sitting there on the ground, how much energy does it "potentially" have? Well that kind of depends on what you do with the boulder afterwards (like rolling it up a hill.) I was mistakenly thinking of the boulder (or water) in it's starting state, instead of it's final moment of impact.

So:

Voltage = Current * Resistance Voltage is the product of the speed and the resistance. So in water terms, you will have greater pressure by using a smaller pipe (increasing resistance) just as you will need a greater amount of pressure to increase the flowthrough if the size of the pipe stays the same.

Voltage = Energy / Quantity Energy = Voltage * Quantity The greater the amount of force on a certain body, the greater the amount of energy we will have _at the time of use_. Similarly, a larger mass being delivered at the same amount of force will have more energy _at the time of use_.

Does this seem correct?

-Chris

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------------ I note that now the ampere is now defined as the basic element in the SI system - the coulomb is then an ampere-second. I got jumped on once for using the coulomb. Check out:

--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer

Yes, I'm aware of it. I still prefer to think in terms of countable things, though. Penchant of mine. Still, there are a variety of commensurate systems to the SI system. One I like is a relativistic mapping, which makes length and time the same, describing everything in a 'pure' number of light-seconds.

Jon

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------- Basically OK

----------

---- OK but "quantity" needs further refinement.

-

----- You are getting closer but you are mixing mechanical and electrical terms-as well as mixing up energy and power concepts. You can have a force on a body but if it doesn't move- no energy is involved. You can have a voltage source but if it is open circuit (no current) then there is no energy involved. If you have a current due to this voltage, then you will have power =V*I but power is the time rate of change of energy so the energy will depend on both power and time. Mechanically a force times a velocity is power but velocity is distance/unit time so energy includes a time factor.

Voltage or potential difference between a and b is defined in terms of the energy per unit charge to move the charge from a to b in an electrical field. This is energy or work and is expressed in Joules (1 watt-second=1 Joule). It can exist whether or not there is a current. Certainly, in a given field, it will take 2 times as much energy to move 2 Coulombs as to move 1.

The analogy would be the energy needed per unit mass to move a mass from point a to point b in a gravitational field. This also is expressed in Joules. More energy is needed to lift 2kg than to lift 1 kg.

In either case, it doesn't matter how long you take or what path you take- the energy depends on the end points. In the case of a mass being lifted from a to b, the mass could be lifted up directly against gravity or slid up a long ramp- the energy change depends on the initial and final heights in the gravitational field. Ditto for the movement of a charge in an electrical field. Small force over a long time or higher force for a shorter time =same energy

Current at a point is the rate of change of charge passing that point. Coulombs/second A voltage or potential is needed to produce a current (except in true superconductors which aren't available at Radio Shack)

An analogy is the total flow rate in a river at some measuring point- say gallons/minute. This flow is produced by a pressure difference or head (eg. potential difference).

-- Don Kelly snipped-for-privacy@peeshaw.ca remove the urine to answer