Electric current

Question

I am here to talk about current. I have read many papers about this, but it simply doesn't help me.

When I think about current in a metal conductor I always think about the physical: the crystal, the valence bond, and the electrons moving, and the CONVENTIONAL CURRENT going in the opposite direction.

I known from the math it is equivalent, but thinking about conventional current, should I forget about the crystal and the valence bond, and see it just as a positive charge moving around a metal conductor, and don't have anything around when the conventional current goes through? I think it is like the mathematical model, and don't think about the physiscal.

Can somebody help me? I have been stuck with this for a long time and I need to get through this.

Answer 1

Conventional current is a crutch for electrical circuit design. For this task, conduction physics is irrelevant. And having the "current flow down from high potential to low potential" makes intuitive sense for circuit design.

Besides, real physical current doesn't just flow "opposite" to conventional current. It flows in all direction at insane speeds. The charge carriers just zap about chaotically constantly. Even large forced currents are just a tiny tiny alteration to the still-random motion.

Answer 2

It's unfortunate that when Ben Franklin flipped the coin, it landed on the wrong side, and it's far too late to correct the meanings of + and - now.

Electrons actually move in the opposite direction to the current flow.

Think of a long line of people waiting to be served. The first one leaves the line and goes to the teller. The second person moves to where the first one was. The third replaces the second. And so on.

The people are slowly moving forward, but from a distance, what you actually see is an empty spot that quickly travels from the front of the line to the back of the line.

This is analogous to electrons lined up, being pushed by the voltage, but not being able to move anywhere until an empty spot opens up in front of them. It's that apparently moving empty spot that corresponds to current flow and direction.

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Another analogy that helps understand how things work is to think of electricity as water, and electronic components as mechanical devices.

Water stored at high elevation (battery) provides pressure (Volts), narrow pipes (resistors) restrict the flow (Ohms), storage tanks (capacitors) allow excess water to accumulate in reserve (Farads), and turbines with flywheels (coils) provide inductance (Henrys).

And from there, transformers are simply a flywheel with turbines on two different circuits. Flow in one circuit turns the flywheel, which turns the other flywheel, which causes flow in the other circuit. If the the first turbine is larger, in the second circuit the water pressure (voltage) is larger and the flow (current) is smaller.

Mechanical valves correspond to vacuum tubes, and later to diodes, transistors, etc.

Answer 3

The conceptof electronic current has had many debates. The problem arises because there are two forms of charge, positive and negative.

In this answer I will treat current and flow with the same meaning. Current is the flow of charge.

If a compass needle is used to detect the magnetic field generated by a moving charge, then negative charges moving to the left will deflect the compass in the same direction and amount as the same quantity of positive charge moving to the right.

Moving the compass around a circuit (keeping the same orientation and distance to the charge flow) such as the one shown in the original post, the compass must deflect exatly the same amount everywhere along the circuit. The charge carriers in the external wire are negative electrons. The charge carriers in the battery are negative and positive ions flowing in opposite directions.

The magnetic field is then used to define a direction for electric current, not charge flow. Positive and negative charges flowing in opposite directions produce the electric current in the same direction.

Physically the electric current in a stationsry wire is carried by electrons. In transistors, holes and electrons contribute to electric current. In chemical solutions (voltage cells), both positive and negative ions contribute to electric current.

I always think about the physical, the crystal and the valence bond and the electron moves, and the CONVENTIONAL CURRENT go to opposite.

The convention is about how both electron and proton charge flows can produce the same electrical current including direction and magnetic field.

Including velocity \$v\$ and charge density \$\lambda\$ in the definition of current: $$i=\frac{dq}{dt}=\frac{dq}{dx}\frac{dx}{dt}=\lambda v\tag{Equation 1}$$

The charge density can be positive or negative as can the velocity. So positive charge having a positive velocuty produces a positive current as does a negative charge having a negative velocity.

A negative current is produced by either a positive charge with a negative velocity or a negative charge with a positive velocity.

The lesson that I learned was to connect electric current to the magnetic field to get the sense of direction. I had to stop thinking that conventional current direction is some virtual construct that came from a poor choice. It is a necessary result that came from observation of currents and magnetic fields. I do my best to avoid using the phrase "current flow" although we all do it (even Ampere). Charge flow, electron flow, ion flow are better phrases.

Use Equation 1 for the meaning of electric current.

Answer 4

Conventional current is an abstraction used in circuit analysis. The fact that somewhere there are physical charge carriers that "implement" this current doesn't really enter into the picture. When you deal with current in circuit analysis, the quantities are continuous and the are no discrete charges in the model at all. In that sense, charge - in circuit analysis - is like water, not like little rubber balls.

Now, plenty of continuous semiconductor device models have been derived while keeping the quantum effects front end center. And they are crucial for understanding how semiconductor devices work. But by the time you get to use those models, the charge carriers are "long gone" - they remain at a different level of abstraction, below the (perhaps piecewise) continuous models used in circuit analysis.

The charges are used to figure out how to model semiconductors, but not how to analyze circuits built out of them (with some caveats that don't matter much at the introductory level).