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Current is the amount of electrons passing through a wire. Can we say that voltage is the speed of those electrons?

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    \$\begingroup\$ Voltage is more like pressure that drives the current. Speed is not the speed of the electrons (which move in mm/s) but the speed of the electric field (more like the speed of light). \$\endgroup\$ – Transistor Jul 12 '16 at 10:37
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    \$\begingroup\$ Voltage is more like the pressure of electrons. \$\endgroup\$ – user253751 Jul 12 '16 at 17:06
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    \$\begingroup\$ electrons want to be evened out (more accurately is to say that they repel each other). if you pile a bunch of them up in one place, and have an absence of them near to it, they will really really "want" to move to the empty place. the bigger the difference between the presence of them in one spot, and the absence of them in the other, the more they will "want" to move. the "wanting to move" is the voltage (as others said, pressure). if this "wanting to move" gets strong enough, the charge can travel through something it would normally not be able to, like a lightning bolt through the air. \$\endgroup\$ – Dave Cousineau Jul 12 '16 at 17:13
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    \$\begingroup\$ Current is not the amount of electrons passing through a wire. Instead it is the amount of charge passing through the wire per unit time. \$\endgroup\$ – nidhin Jul 12 '16 at 18:26
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    \$\begingroup\$ You might be interested in vacuum tubes, most notably the x-ray tube. The voltage between cathode and anode accelerates electrons to an energy of voltage*electron-charge. Also note that 1 A = 1 C/s while 1 V = 1 J/C, i.e. while current denotes charge per time (as you mentioned), voltage simply gives you the energy that charge has. \$\endgroup\$ – Tobias Kienzler Jul 13 '16 at 7:01

13 Answers 13

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Is voltage the speed of electrons?

No, it's not the speed of the electrons moving within the conductor.

The voltage unit is potential energy per charge:

voltage definition formula


An example...

Imagine we have a ball of mass M = 10 kg.

This ball exists in a conservative gravitational field (the Earth's gravitational field). If we want to raise it by a height of 1 meter, we must - somehow - supply an X amount of energy, that gives the ball enough speed to move 1m above its surface.

We will give the ball this amount of energy in terms of kinetic energy (speed). So we throw the ball upwards with some speed, and as the ball moves upward, its speed decreases; and its potential energy increases until the it stops and all the kinetic energy is converted to potential energy.

The following picture shows the amount of potential energy for a ball of mass M = 10 kg at different heights above sea level:

energy at different height levels

But what if we want to make a generic scale?
For any ball of an arbitrary mass, at any height, we can get the amount of energy for every 1 kg in it (Energy per mass):

energy per mass at different height levels

Now we can say that, at a height of 3 meters above sea level, any object of mass X will have an amount of energy equals 29.4 joules for every 1 kg of mass. This is due to the earth's gravitational field.

Voltage, or electric potential, is the amount of potential energy (joules) that any "charged body" within an electric field will have, for every 1 coulomb of electric charge in it.

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  • \$\begingroup\$ One might add that the potential energy does directly translate into kinetic energy if there is only negligible "friction", for example in an (evacuated) cathode ray tube. The kinetic energy of an electron is indeed measured in "electron volts", eV, the energy an electron gains or loses when moving through a potential difference of 1 Volt. \$\endgroup\$ – Peter - Reinstate Monica Jul 14 '16 at 11:24
  • \$\begingroup\$ It's not that isolated right? Since With V = I/R a V increase also forces a I increase. So the number of coulombs also increase by the same amount. \$\endgroup\$ – Cojones Dec 2 '18 at 6:40
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Voltage is a property of an electric field.

An electric field behaves a little like a gravitational field. Objects in a gravitational field are pulled together. Drop a stone in a gravitational field and it will accelerate downwards, taking energy from the field.

Electric fields, unlike gravitational fields, have polarity. Drop an electron in an electric field and it will accelerate in the direction of positive charge. The electron does not have a voltage, it has a charge: \$1.6×10^{−19}\$ coulombs.

How much force is applied to the electron depends on the voltage of the positive and negative sides of the field and their distance apart.

That's all in free space. What about inside a wire? The situation there is much more like a tube filled with balls than a free space. Apply a force to the ball at one end and it will push the ball at the other end out. Apply a voltage to a wire and the electrons will move, forcing out the one at the positive end. The amount of force applied corresponds to the voltage applied to the wire.

The key thing about this model is that the force travels much faster than the balls/electrons that are transmitting it - it doesn't require a ball/electron to go all the way through, it just requires it to push its neighbours along.

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  • \$\begingroup\$ This is a good analogy, but it is important to note that the electrons flow out of the negative side, not out of the positive side. \$\endgroup\$ – DerStrom8 Jul 12 '16 at 12:06
  • \$\begingroup\$ Sorry, inadequate labelling by me there: if you have a DC power source, the electrons will leave the wire connected at its positive side and enter the power source. \$\endgroup\$ – pjc50 Jul 12 '16 at 12:45
  • \$\begingroup\$ Voltage is not about the force (= energy/displacement). Voltage is about the difference in energy potential. That is field strength which, multiplied by charge, produces force. \$\endgroup\$ – Incnis Mrsi Aug 30 '16 at 9:24
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Take a Real time scenario,

We can take it water as analogy.

Lets consider a overhead tank and a water tap which is supplied from this over head tank.

Now,

Whenever open a tap water will come through this tap.

The amount of water which is coming through is equivalent to the current

At what Pressure is coming, that is voltage

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    \$\begingroup\$ The problem with this analogy is higher pressure imparts the water with greater velocity, which is likely why the asker's confused - this is one place where the popular analogy of water to electricity breaks down. It's a good, intuitive way to explore many aspects of electricity, as long as you don't look at it too carefully. \$\endgroup\$ – talrnu Jul 12 '16 at 20:39
  • \$\begingroup\$ Yes @talrnu, if we consider the velocity we will get confused. Its not exact electricity analogy, Just i took two phenomena of the water pressure and quantity to easily grasp the what is the voltage and current \$\endgroup\$ – Photon001 Jul 13 '16 at 5:24
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    \$\begingroup\$ Agree, this answer is problematic because the speed of the flowing water increases with pressure, while the speed at which an electron propagates through any particular medium is constant even if the "pressure" (voltage) is increased. I think what the OP is really asking is why that's the case. \$\endgroup\$ – aroth Jul 13 '16 at 7:33
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No, the voltage is the "potential energy" given to electrons. Like as if you take a stone and lift up. Until you do not connect a load the electron don't go anywhere.

If you let it falling down the stone (or connect a resistor at your voltage source) the energy move the stone (electrons).

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Is voltage the speed of electrons?

No

Voltage is a measure of how much energy is delivered to charge. At its most basic, an electron (basic charge) is imparted 1.602×10−19 joules when moved through an electric potential difference of one volt. An electron is then said to have an energy of 1 electronvolt.

So voltage is energy divided by charge.

You can start with power and multiply it by time to get energy:

Energy = Power × time = VI × time.

Now substitute Q (charge) for current × time and you get:

Energy = VQ or V = Energy/Q.

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This is actually a physics question. I don't believe there is an experimental method available in the confines of the electrical engineering discipline to answer this question credibly.

Having said that, it is commonly believed that the speed of electrons in a conductor experiencing current flow is actually quite slow compared to the speed of light. This is often referred to as the "drift speed" of the electrons. However, the effects of voltage and current on the electrons is propagated thru the conductor at nearly the speed of light. The usual analogy is a pipe filled with marbles. If you push the marble at one end of the pipe the marble at the other end will experience the push nearly instantaneously even though none of the intermediate marbles moved.

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    \$\begingroup\$ Not sure if "nearly the speed of light" is the right expression - it's around half that in an ordinary PCB and 2/3 in common coax. \$\endgroup\$ – pipe Jul 12 '16 at 11:51
  • \$\begingroup\$ @pipe I think the difference is that a single electron may travel at, say, half the speed of light, but considering the ball in a tube analogy, the response time between pushing in the first ball and the last ball falling out is nearly instantaneous (approaching the speed of light). \$\endgroup\$ – DerStrom8 Jul 12 '16 at 12:10
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    \$\begingroup\$ @DerStrom8 No, the response time is the signal velocity here, which is slowed down by the dielectric in the PCB and the cables. It only approaches speed of light in a bare wire. A single electron travels much much slower than half the speed of light. \$\endgroup\$ – pipe Jul 12 '16 at 12:15
  • \$\begingroup\$ Hmm, I'm not convinced but I won't argue it. Physics class was a long time ago =P \$\endgroup\$ – DerStrom8 Jul 12 '16 at 12:17
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    \$\begingroup\$ @IncnisMrsi Indeed, calculating it I get more like 1.08E5 m/s at 300K. \$\endgroup\$ – Spehro Pefhany Aug 30 '16 at 11:42
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Voltage is the pressure that pushes electrons around a circuit. It says nothing about their speed. If you take a 1.5V battery and don't connect it to anything, then there's still 1.5V present, even though no electrons are flowing anywhere.

Further, voltage is the pressure difference between two points. You can only measure the voltage between one point and another. That's why it's also called "potential difference".

It is possible to calculate the average electron speed if you know the current, the physical properties of the wire (particularly its cross-sectional area) and the properties of the material the wire is made from (the spacing between the atoms, and how many free electrons there are per atom).

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  • \$\begingroup\$ I'll not talk about pressure. It's a really different concept, IMHO. \$\endgroup\$ – Antonio Jul 12 '16 at 12:07
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    \$\begingroup\$ @Antonio Pressure and voltage are very similar concepts, if not identical. \$\endgroup\$ – endolith Jul 12 '16 at 20:10
  • \$\begingroup\$ @endolith, now my professor of physics may be turning in his grave. :-) \$\endgroup\$ – Antonio Jul 12 '16 at 23:31
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    \$\begingroup\$ @Antonio Attach a dynamo and generate some voltage :D \$\endgroup\$ – endolith Jul 13 '16 at 1:51
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    \$\begingroup\$ @endolith I always use the water flow and pressure as analogy for current and voltage. KCL and KVL works perfectly well. \$\endgroup\$ – winny Jul 13 '16 at 13:43
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No, voltage is not the speed of electrons through a wire, but current (almost) is.

You said, "Current is the amount of electrons passing through a wire," but this is not quite correct. Current is the amount of electric charge (electrons) passing through a conductor per unit of time. The ampere, our unit of measure for current, is defined as 1 coulomb of electric charge per second. Current is a rate value.

For the water pipe analogy, charge (coulombs) is analogous to the volume of water (gallons), current (amps) is analogous to flow rate of water (gallons per minute), and voltage is analogous to the water pressure that is causing the flow.

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    \$\begingroup\$ Current is not the speed of electrons through a wire, it's the rate at which they flow past. If the channel is wider, they will flow slower to produce the same current. \$\endgroup\$ – endolith Jul 12 '16 at 20:10
  • \$\begingroup\$ @endolith Speed, rate, close enough. :) I changed the wording slightly. Better? The point is that current is that change over time that I believe the OP is asking about. \$\endgroup\$ – Ben Miller - Reinstate Monica Jul 12 '16 at 20:26
  • \$\begingroup\$ Yes, better. :) \$\endgroup\$ – endolith Jul 12 '16 at 21:12
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    \$\begingroup\$ @JohnPeters I think it is a little too simplistic to say that current is "the amount" of electricity. Current is the amount of electrical charge that passes a point in a unit of time. In that sense, it is the rate (or speed, if you like) of charge. \$\endgroup\$ – Ben Miller - Reinstate Monica Jul 13 '16 at 13:26
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    \$\begingroup\$ @JohnPeters What is the speed of electricity? is it the speed of electrons in the conductor or the speed of voltage change? \$\endgroup\$ – Crowley Jul 13 '16 at 16:13
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Voltage isn't a property of electrons. Electron are the 'subjects' as it is. A voltage (or potential difference) is the 'ability' to transport a certain charge. In electronics, this charge is generally carried by electrons. A higher voltage is able to carry more electrons, hence induce a higher current.
Another way of looking at it is that the voltage is the amount of potential energy that an electron gains or looses by traveling from one potential to another potential. In this way, voltage is very similar to potential energy in kinetics - if I lift a ball, the ball's properties doesn't change but it gains potential energy.

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  • \$\begingroup\$ A good start, but slipped deeply into crap very soon. Ever heard of superconductors? Voltage has nothing to do with “ability to transport”. Voltage is, rather, energy output for a unit of charge transported. \$\endgroup\$ – Incnis Mrsi Aug 30 '16 at 9:30
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If an electron was a marble, Voltage is like the height of the slope that the marble is at the top of.

It might be a really tall slope - miles high. It might be a tiny rise - just a couple of centimetres. That's what's determined by the voltage.

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    \$\begingroup\$ I feel like this analogy could be expanded into a great alternative for other related concepts too. If voltage is the height, what's the angle of the hill correspond to? Maybe resistance could be represented by grass or mud. Then you've got the number of marbles, the horizontal distance from hilltop to base (which is going to relate with height and angle just as the corresponding electrical concepts do)... \$\endgroup\$ – Dan Henderson Jul 13 '16 at 22:17
  • \$\begingroup\$ I agree, but I'm on lousy internet atm. :-) \$\endgroup\$ – Euan M Jul 13 '16 at 23:32
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The speed of electrons depends on the density of the wire. It also depends on the number of free atoms in the conductor.

Think of it like pushing sand through stones. The more dense the stones are, the harder it becomes pushing the sand through it.

The more sand (free electrons) is inside, the less distance you'll need to push for the same amount of sand dropping out at the other end.

For details, you may read about drift velocity. The actual speed of an electron in the example there is just as little as 23µm/s.

In fact, the voltage will influence the speed of electrons: in the given formula, replace I by U/R and you'll see that the velocity will increase with the voltage.

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A lot of good information here to hopefully clarify your question.

The voltage can be thought of as the energy difference between two points within a network (potential difference), think about the voltage dropped across a resistor. Different at each end due to the power dissipated across the resistor itself.

If your where to consider the supply voltage to a circuit (EMF, electromotive force), it can be thought of as the pressure forcing current through the circuit.

a note about electron flow

The convention is taken to be that current moves from + to -, this however electron flow is - to +. The formulas etc of course will work with this convention, as usually we dont care about electron flow, unless we are into semiconductor stuff, however its important to remember they actually flow from - to + (the electron being a negitive charge carrier).

I hope this along with the many other comments helps. Tony

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  • \$\begingroup\$ Electron does neither necessarily nor usually “move from + to −”. It gains energy moving from − to +. \$\endgroup\$ – Incnis Mrsi Aug 30 '16 at 9:42
  • \$\begingroup\$ @Tony. "The convention is taken to be that electroncs move from + to -, ..." No, the convention is that current flows from + to -. In conventional circuit theory we don't care what the actual charge carriers are or the direction of their movement. \$\endgroup\$ – Transistor Jan 20 at 18:45
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No. The simplest answer possible is that voltage is the density of electrons. That is, the "pressure" required to push them together against their repulsive force. Of course, this is complicated by other factors such as the medium in which they are moving.

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protected by W5VO Jul 12 '16 at 19:04

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