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Bit of a strange question, but what is it? My physics teacher said it was kind of like a "push" that pushes electrons around the circuit. Can I have a more complex explanation? Any help is much appreciated.

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9 Answers 9

up vote 61 down vote accepted

Your teacher was right.

Current is electric charges (usually electrons) moving. They don't do that by themselves for no reason, no more so than a shopping cart moves across the floor of a store by itself. In physics, we call the force that pushes charges the electromotive force, or "EMF". It is almost always expressed in units of volts, so we usually take little shortcut and say "voltage" most of the time. Technically EMF is the physical quantity and volts is one unit it can be quantified in.

EMF can be generated several ways:

  1. Electromagnetic. When a conductor (like a wire) is moved sideways thru a magnetic field, there will be a voltage generated along the length of the wire. Electric generators like in power plants and the alternator in your car work on this principle.

  2. Electrochemical. A chemical reaction can cause a voltage difference. Batteries work on this principle.

  3. Photovoltaic. Crash photons into a semiconductor diode at the right place and you get a voltage. This is how solar cells work.

  4. Electrostatic. Rub two of the right kind of materials together and one sheds electrons onto the other. Two material that exhibit this phenomenon well are a plastic comb and a cat. This is what happens when you shuffle across the right kind of carpet and then get a zap when you touch a metal object. Rubbing a balloon against your shirt does this, which then allows the balloon to "stick" to something else. In that case the EMF can't make the electrons move, but it still pulls on them, which then in turn pull on the baloon they are stuck on.

    This effect can be scaled up to make vary high voltages and is the basis for how Van de Graaff generators work.

  5. Thermo-electric. A temperature gradient along most conductors causes a voltage. This is called the Siebeck effect. Unfortunately you can't harness that because to use this voltage there is eventually a closed loop. Any voltage gained by a temperature rise in part of the loop is then offset by a temperature decrease in another part of the loop. The trick is to use two different materials that exhibit a different voltage as a result of the same temperature gradient (different Siebeck coefficient). Use one material going out to a heat source and a different coming back, and you do get a net voltage you can use at the same temperature.

    The total voltage you get from one out and back, even with a high temperature difference is pretty small. By putting many of these out and back combinations together, you can get a useful voltage. A single out and back is called a thermocouple, and can be used to sense temperature. Many together is a thermocouple generator. Yes, those actually exist. There have been spacecraft powered on this principle with the heat source coming from the decay of a radio-isotope.

  6. Thermionic. If you heat something high enough (100s of °C), then the electrons on its surface move so fast that sometimes they fly off. If they have a place to land that is colder (so they won't fly off again from there), you have a thermionic generator. This may sound far fetched, but there have also been spacecraft powered from this principle with the heat source again being radio-isotope decay.

    Electron tubes use this principle in part. Instead of heating something so that electrons fly off on their own, you can heat it to almost that point so that they fly off when a little extra voltage is applied. This is the basis of the vacuum tube diode and important to most vacuum tubes. This is why these tubes had heaters and you could see them glow. It takes glowing temperatures to get to where the thermionic effect is significant.

  7. Piezo-electric. Certain materials (quartz crystal for example) generate a voltage when you squeeze them. Some microphones work on this principle. The varying pressure waves in the air we call sound squish and squash a quartz crystal alternately, which causes it to make tiny voltage waves as a result. We can amplify them to eventually make signals you can record, drive loudspeakers with so you can hear them, etc.

    This principle is also used in many barbecue grill igniters. A spring mechanism whacks a quartz crystal pretty hard so that it makes enough of a voltage to cause a spark.

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Thanks to all of you for some really wonderful answers! It makes complete sense now. This is only my second question on this site, and although I have a lot of experience with stackoverflow.com, this site is all very new. So thank you all once again for all of your help :) –  imulsion Dec 10 '12 at 17:44
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Olin answer as usual is very complete, but may miss some special cases. In an atom the electron will keep moving around and around without an emf. This can give the atom a magnetic field. –  russ_hensel Dec 10 '12 at 19:43
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This is an nice little image that's been helpful in getting a basic idea of voltage, current and resistance. –  KronoS Dec 13 '12 at 6:35
    
@Kronos for some reason the image doesn't display –  imulsion Jan 25 '13 at 10:37
    
@imulsion works fine for me. –  KronoS Jan 25 '13 at 14:24

Using a fluid analogy, Voltage is pressure, Current is Flow rate.

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The fluid analogy is really good. Imagine a wire as a pipe (that can't leak). Imagine a capacitor as a stretchy membrane that completely covers the pipe. A resistor is a narrowing in the pipe. An inductor is a heavy flywheel that interferes with the flow until it has spun up, and helps it along afterwards. Voilá, suddenly it's easy to visualise what might happen in specific set-ups! Like the fact that a capacitor allows water to flow only until the membrane is stretched enough to counteract the pressure, at which point the flow is blocked. –  romkyns Mar 15 '13 at 23:30

A voltage appears whenever there is an imbalance of electrical charge (i.e. electrons). Since like charges repel and equal charges attract, any collection of electrically charged particles creates some kind of force on each other. If there is an imbalance of negative to positive, a kind of "pressure" or "push" is formed. In conducting materials, electrons are free to flow through the material, as opposed to being fixed in atoms, and will therefore flow to the point of least "pressure".

Some complicating considerations:

  • Electricity and chemistry are closely connected. In a battery, for example, a chemical imbalance creates an electrical imbalance (voltage) across the terminals, by forcing charged particles to one side. Chemistry also affects electrical conditions in other ways.
  • Current (I) is the flow of electrons, however, electrons (since they are negative) flow in the opposite direction of the "current". The current is then the conceptual flow of positive charge, even though the actual flow is negative, but in the other direction. This demonstrates that a negative "push" is the exact same as a positive "pull".
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This is the only answer that answers the question. While the others talk about how voltage is created or what it does, this answers what voltage is. –  Rob Jul 5 at 12:48

Actually we can't.

Electrostatic force is proportional to the potential gradient but not directly to potential. Force on a one coloumb of charge is proportional to the potential gradient.

\$ F= Q \times {d[V]\over dl } \$

Actually ,1V mens if you 1Joule of electrical energy will be transfered into mechanical energy on a +1coloumb charge [so it will accelerate , or increase it's 1/2mV^2 by 1J ]. It's actually analogous to energy,

But anyway this answer will confuse a newbie.

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why downvote? This may confuse a newbie, but this is the truth. –  Standard Sandun Dec 11 '12 at 17:33
    
upvoted! I'm actually not confused xD –  imulsion Dec 11 '12 at 17:37

Adding to what Gunnish said:

Voltage at point A is literally a measurement of the work you would expend if you were to push a positive charge from 0V (usually either defined as infinitely far from A, or ground) to A.

Voltage is important in electronics because if we start with a positive charge at point A, it is able to DO that same amount of work getting to 0V (ex. turning on an LED in the process).

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A definition I've heard is:

Voltage is the potential (for charge) to do work.

In other words, voltage is the energy given to a unit of charge, i.e., \$ V = {dE \over dQ} \$, where \$ E \$ is energy and \$ Q \$ is charge.

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"Voltage" is a derived quantity. It is hard to understand its Physical meaning without understanding the quantities it is derived from.

It all starts with the force between two point charges. Let the charges of the points \$ P_1 \$ and \$ P_2 \$ be \$ q_1 \$ and \$ q_2 \$. Let the distance between them be \$ r \$. The fundamental theorem says that, the force between these two charges are proportional with the amount of charges, and inversely proportional with square of the distance between the charges. That is:

\$ F = k\frac{q_1 q_2 }{r^2} \$

Let the location and the charge of \$ P_1 \$ be fixed. Now the force depends on the location and charge of \$ P_2 \$. So we define a vector field called "Electrostatic Field". Direction of the vector field is the same with direction of the field of the force between \$ P_1 \$ and \$ P_2 \$ when \$ q_2 \$ is positive unit charge. And magnitude of the field is the force per charge \$ q_1 \$ when \$ q_2 \$ is unit positive charge. That is:

\$ \bar{E} = \lim \limits_{q_1 \to 0} \frac{\bar{F}}{q_1} \quad \mbox{(} q_2 \mbox{ is unit positive charge)} \$

We make \$ q_1 \$ approach to zero in order to neglect some other electromagnetic effects disappear; don't let it confuse you so much.

Now we come to see that these quantities we defined are very similar to some other Physical quantities we know. For example, the force above is very similar to the force between the Earth and an space object like the Moon. And the \$ \bar{E} \$ field is very similar to the gravitational field of the Earth.

Then the idea of defining electrical potential arises which is similar to the potential of a space object with respect to the Earth. Potential of a point in the space around Earth is energy per unit mass to bring an object (which has unit mass) from infinity to that point. When we define it in Electrostatics, the potential of the point \$ P_2 \$ becomes:

\$ V_2 = - \int \limits_{\infty}^{P_2} \bar{E} d\bar{\ell} \$

Then the potential difference between two independent points (\$ P_2 \$ and \$ P_3 \$) in the space within the \$ \bar{E} \$ field (caused by \$ q_1 \$) is:

\$ V_2 - V_3 = \left(-\int \limits_{\infty}^{P_2} \bar{E} d\bar{\ell}\right) - \left(-\int \limits_{\infty}^{P_3} \bar{E} d\bar{\ell}\right) = \int \limits_{P_3}^{P_2} \bar{E} d\bar{\ell}\$

Note that electric field is curl-free, which means it can always be represented as gradient of a scalar field (\$ \bar{E} = - \bar{\nabla} V \$). The these line integrals are independent of path.

So, this is the true definition of the potential field. A point will always have a potential even if there is no charge on it. Thing it as of "the energy needed to bring a unit charge to there from infinity". Potential difference between two points is similar; it is the energy needed to carry a unit charge from one point to another. Or think it on a more concrete example like for celestial bodies. Potential difference between 100km height and 200km height above Earth's surface is nothing but differences of potential energies between two 1kg objects at the given heights.

When we come to real world, potential of a point is some of all individual potentials caused by the charges around (theory of superposition applies).

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What is pushing the elections is a difference in potential energy, much like the way you are being pushed/pulled to the earth by gravity. This generates a favorable probably for the electrons to move one way over another, this also partly explains why the electrons move "randomly" in a wire.

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The quickie, 1st approximation, rule-of-thumb answer: voltage is electrical pressure.

But expanding on that: Voltage is not like pressure, not exactly. Instead it's a physics concept called "Potential." It's more like altitude in a gravity field, where each electron or proton is like a boulder. If a boulder is at the top of a hill, it's at a high-Potential location, this means the boulder is storing PE, and will release this energy as KE if it's allowed to move downhill (move to a low-Potential location.)

More precise: voltage is electric Potential. It is not force (it's not like the weight of the boulder or like the Newtons of force upon an electric charge in an e-field.) Also it's not potential energy, since gravity, altitude, and Potential still exists even when the boulder is not present. Potentials are part of the field itself.

Voltage is a way of describing/visualizing/measuring electric fields. We can draw flux-lines between opposite electric charges. Or instead we can draw the pattern of voltage, the iso-potential surfaces, perpendicular to the flux lines.

What is voltage? It's a pattern of concentric onion-layers which surround any charged object, with the onion-layers running perpendicular to the flux-lines of the electric field. So, voltage is one way of describing an e-field. Flux lines are the other more common way.

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With regard to the pressure analogy, it's useful to recognize that while there is a concept of absolute voltage (as with pressure), in many cases far more meaningful to think in terms of relative voltage. For example, a typical pipe organ may be said to operate with a pressure of 7mm Hg. One could in theory use a barometer to measure the pressure inside as 764mm Hg, and the pressure outside as 757MM Hz, and conclude that the pipes saw a pressure difference of 7mmHg, but it would be easier and more accurate to measure the difference in pressure between the inside and outside. With voltage... –  supercat Dec 13 '12 at 16:25
    
...the difference between the "baseline" and the typical differential voltages people deal with are usually many orders of magnitude larger. Think about trying to measure a man's stature by measuring the distance from the center of the earth to to of his head, and from the center of the earth to bottom of his feed, and subtracting. Measuring absolute voltage would be even worse than that. –  supercat Dec 13 '12 at 16:27
    
I just want to thank all of you once again for some really amazing answers - I never thought I would get a silver badge for such a simple question! :) –  imulsion Dec 13 '12 at 17:47

protected by Olin Lathrop Dec 31 '12 at 17:47

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