Where do all these electrons come from? [duplicate]

This question already has an answer here:

I'm a high school student and I'm fairly familiar with basic electronics, but I've always wondered one thing.

So how generators make electricity from motion is the move a magnet around or through a coil of conductive wire, which causes the electrons from the metal to move in the direction of the magnet. But that could theoretically happen forever. When does the metal wire run out of electrons? Are electrons being formed? Is matter being created?

Also if you want to help me out a bit more, please explain how transformers work and how voltage and amperage are related.

marked as duplicate by heather, Jon Custer, John Rennie, mpv, Brian MothsNov 16 '16 at 18:11

• No, electrons are not being created. It's a closed electrical circuit. The electrons go around and around in the wires in the closed electrical circuit just like water (coolant) in your car's engine is pumped in a closed circuit or the blood in your body is pumped in a closed circuit. – Samuel Weir Nov 16 '16 at 4:36
• Try to ask only one question per question; if you ask multiple questions in a question then usually only one of them gets answered. – Eric Lippert Nov 16 '16 at 13:42
• Imagine you have a pump with the input hose in the ocean and the output hose in your bathtub. The pump causes water to move. This could happen forever. So, now answer your questions: when does the hose run out of water? Is new water being formed? Is matter being created? – Eric Lippert Nov 16 '16 at 13:48

Electrons are not being created nor they are destroyed. The electrons already exist in the metal.

The metal consists of layers of positive ions (depicted by orange circles in the picture) in which the electrons (depicted by cyan colored circles in the picture) are in continuous motion. Hence, the charge on the metal is overall neutral.

Where did the electrons come from?

The electrons escape from the metal atom leaving the metal atoms positively charged. The delocalized electrons form a "sea" of electrons in the metal.

What is electric current?

Electric current is defined as the amount of charge moving across a cross-section of a conductor per unit time.

$$I = \frac{\Delta q}{\Delta t}$$

To have a current in a metal, you need to have a net movement of charge.

What causes electric current? In figure one, the circuit is open. The electrons are free to move around randomly (due to thermal energy). The electrons keep bumping into atoms and keep changing directions. There is no orderly motion of electrons. Therefore, on average, there is a net zero movement of charge.

In the second figure, the circuit is closed, i.e: the metal conductor has been connected to a voltage source (electric generator or a battery). The potential difference across the conductor exerts force on the electrons which causes the electrons to move in order from a region of lower potential to a region of higher potential. Of course, the electrons still collide with the atoms but the potential difference (technically, the electric field) tries to get the electron back to the orderly motion. Therefore, there is a net charge flowing in one direction which constitutes the current.

Therefore, no charge is created or destroyed rather they are just moved. It is similar to a water in a hose. The water in a pipe does not flow unless there is a pressure/height difference across the ends of the pipe. However, the water molecules inside the pipe are in continuous motion.

An important point to note here is that the electrons move really fast (millions of meters per second) but at the same time, they collide with many atoms. Therefore, the net velocity averaged over time is very small. We call this velocity drift velocity. In copper, it is of the order of millimeters per second.

$$I = ne v_d A$$

where $n$ is the number of electrons per unit volume of the conductor, $v_d$ is the drift velocity and $A$ is the area of cross section of the conductor.

What does a generator do?

An electric generator basically creates an E.M.F (voltage/potential difference) across a conductor which produces a current.

How does a generator work?

An electric generator works on the principle of electromagnetic induction.

The law states that if the flux through a coil of wire is changing, an E.M.F (voltage) is induced in the coil.

Magnetic flux is the amount of magnetic field lines passing through a surface.

The magnitude of E.M.F induced is given by Faraday's law,

$$E.M.F =\int\vec{E}.d{\vec{l}} = \frac{d\phi_B}{dt}$$

where $\phi_B$ is the magnetic flux through the coil.

The E.M.F induced is such that the induced current produces a magnetic flux of its own which opposes the changing magnetic flux. This is known as Lenz's law.

You can go even deeper to explain why a changing magnetic flux induces an electric current. A changing magnetic flux produces an electric field.

This electric field drives the induced current.

If you are new to magnetism, refer to this answer which gives an easy way to understand magnetostatics if you are good with electrostatics

How are voltage and current related?

In simple DC circuits, they are related by Ohm's law,

$$V = IR$$

where R is the resistance of the conductor.

• You may want to specify that the current is the ratio between the variation of the charge in the elapsed time looked at, namely $i=\Delta q/\Delta t$. – gented Nov 16 '16 at 9:24
• You should emphasize more that electrons are not produced, but simply moved, similar to water in a hose; and that that movement comes to a quick stop when there is not a closed circuit, as in a capacitor. I also find your figures misleading: when a circuit is closed, electron density is the same everywhere; they move in a circle, and the field forces equal the "friction". There is no row without electrons as in fig.2; that figure actually symbolizes an open circuit where the field does lead to an electron imbalance (the electrons would be standing still though). – Peter A. Schneider Nov 16 '16 at 11:25
• The one left to answer is "When does the metal wire run out of electrons?". So all the electrons in your picture reach "+" and it's still positively charged. Now what? – Nordik Nov 16 '16 at 11:52
• Don't use a battery, use the generator principle mentioned by the OP: The Lorentz force on charges moving through a magnetic field. Then it boils down to a closed loop wire. (The theory behind it can be ignored; we can just postulate it.) – Peter A. Schneider Nov 16 '16 at 11:58
• Don't underestimate your audience. – Peter A. Schneider Nov 16 '16 at 12:01

Yasha's answer provides an excellent physics description of how the process works.

But from reading your question you seem to understand what electric current is and how it arises. The thing you seem to be missing is that a generator doesn't generate electrons, it's a bit a misnomer if you think of it that way. It generates an electromotive force (a voltage)

The generator (or battery) is not a source of electrons, it's a pump to move electrons that are already there in a wire. This is why a circuit needs to be a closed loop to work. View this animation here.