# Flow of charges in two different materials

Consider two cylindrical conductor rods: one copper rod and one aluminium rod. The rods contact at a circular face, aligned coaxially.

Their exterior circular faces are connected to some DC voltage. And of course some current will flow through this circuit.

Questions:

1. If electrons are really drifting from one point to another point, how will they drift at the contact? Will they drift from copper edge to aluminium edge?

2. I have read that electrons actually don't move from one place to another, they'll just travel a very small distance, during their travel they'll pass charge from one electron to another.

Also I have read that electrons vibrate at their position and their energy will transfer from one place to another (dominoes analogy). Which is true among these?

3. How do electrons flow inside the conductor? Please post any pictures so that I can understand clearly.

4. If we touch a high voltage positive terminal, do we definitely get an electric shock? Given that positive terminal attracts electrons, will electrons in my body get attracted to voltage? How do electrons behave in this situation?

1. If electrons are really drifting from one point to another point, how will they drift at the contact? Will they drift from copper edge to aluminium edge?

They will drift from one rod to the other just as they would if both rods were of the same material. The DC voltage source that you apply across the outer end-face of one and the outer end-face of the other rod sets up a high potential at one end-face and a low potential at the other end-face.

Charge will then drift from higher to lower potential.

A potential is just a fancy word for their tendency to move. Pack many electrons at one end and they will really want to move away from this spot due to their mutual electric repulsion force. If they could, then they would very much like to move to the other end where the repulsion is much lower.

• (Electric) potential energy $U$ is a word invented for this tendency for them to want to move somewhere else if they can. We say that the point of higher electric potential energy is the point they want to move away from. Just as gravitational potential energy is higher, when the ball is on the shelf than when it is on the ground - the ball wants to fall downwards, if allowed, and that is what the potential energy describes.
• (Electric) potential $V$ is the same, but per charge $V=U/q$. This is often easier for comparing different points.
• Voltage is a word for (electric) potential difference $\Delta V=V_2-V_1=U_2/q-U_1/q$ (sometimes denoted $V$).

That you have two rods of two different materials doesn't change the fact that the charges want to move from one end to the other. Those rods may introduce different resistances as well as a contact resistance (maybe so high that no current is able to flow), but the tendency will still always be from that same end to the other.

1. I have read that electrons actually don't move from one place to another, they'll just travel a very small distance, during their travel they'll pass charge from one electron to another.

Well, electrons do move, but not much. That is correct.

An electron is always in constant violent motion and bumping into to atoms around it all the time. This is just random motion. Overall, on average, it doesn't move anywhere. When a potential difference is established, the electron suddenly feels a tendency towards one direction. It still moves around randomly and violently, but it at the same time slowly drifts sideways. And this is what creates you current - the drift speed.

The drift speed is usually small - something like a few millimeters per second. But think of a company of soldiers. When the Lieutenant yells "March!", they all start at the same time. The first soldier doesn't get fast to the end - but the signal does reach the end right away.

This is a simplification of course, and it may be more fruitful for you to think of the electrons as a line of stones or billiard balls. When you strike the first one, it hits the next which hits the next etc. And that "hitting" propagates very, very fast to the end stone or ball, much faster than the first one moves. The same with electrons, and the propagation of their electrical force from the first to the proceeding ones is almost at the speed of light.

Also I have read that electrons vibrate at their position and their energy will transfer from one place to another (dominoes analogy). Which is true among these?

The vibration idea is mainly useful for AC circuits. The domino analogy is still useful for both DC and AC, but don't think of electrons as "vibrating" in DC; it is better to think of them as drifting. (In AC cases you also have drifting - just drifting that quickly changes direction all the time, and therefore the idea of vibrating).

1. How do electrons flow inside the conductor? Please post any pictures so that I can understand clearly.

The above explanation of the violent random motion of electrons should due. Search on Google for this and you'll get illustrations that visualize it.

1. If we touch a high voltage positive terminal, do we definitely get an electric shock? Given that positive terminal attracts electrons, will electrons in my body get attracted to voltage? How do electrons behave in this situation?

When touching a high-voltage positive terminal, the mobile charges in your body (not just electrons, but also ionic compounds and alike) will move slightly, but they will quickly even out your potential. Yes, you can get an electric shock, similar to when you rub your feet over a carpet and collect a net charge on your body. That gives your body a higher potential than your surroundings, and therefor you may feel an electric shock (and maybe even see a spark), when touching something conductive that can move all your excess charge away rapidly.

But that will quickly be over with. Your body will in an instant reach the same potential as what you are touching. Exactly how dangerous it is to touch a high-voltage cable and feel this effect, is not something I can answer directly.

The electric current passing through a wire is related to the charge moving through it in the following sense: fixing a point of the wire in space, how much charge passes through it as time goes on? In other words, you have the constitutive relationship (readily derived from electromagnetic theory) that the current flowing through a volume in space is equal to the variation in time of the charge enclosed in that volume, that is, for a one-dimensional wire,

$$I(x) = \frac{dQ(x)}{dt}$$

Note that the current depends on the variation in the charge present in the wire's position, not on the electron's precise movement. This is to say that the current flowing through a wire depends on the combined behaviour of all the charges within it, so the important characteristic is the bulk movement of the electrons and not each individual trajectory.

So, if you have a current inside a wire, what that means is that, on average, the electrons are moving in that direction, but each individual electron's motion is far more complicated than a linear translation through space, because it will also display thermal vibrations, collisions with other particles, etc.

For your last question, one thing to keep in mind is that it makes no sense to talk about voltages without a reference point. There is no such thing as an absolute voltage level, the only physical aspect of electric potential are voltage differences. So, if you touch a wire, you will only receive a shock if you're also touching another surface which has a different voltage from the wire (in which case you will act as a resistance between two different voltage points, so a current will pass through you). This is why birds can stand on electrical wires and not get shocked, in fact - they're not touching anything else that would allow a current to pass through them.