# What happens to a DC current at the end of a wire?

This question might be a little more appropriate for the physics stack, but I'd like to test the water here first. I am just getting into electrical engineering, and as I go along, certain aspects grab my curiosity.

When you have a junction, like the letter "T" we could say that the top of the T is a serial bus wire or something, or just a wire carrying DC current. Then the base of the "T" is a connector that some peripheral can connect to, to use some current to power itself, however, it's not always plugged in.

So assuming nothing is plugged in to that wire at the moment, what exactly happens? Is the DC current somehow reflected back? Also this may come across as naive, but when a peripheral is plugged in, do the electrons from one wire actually physically move into the other conductive compound? Or does the electromagnetic energy from the end of the wire induce current into the attached conductor?

Thanks for feeding my curiosity. I've Googled endlessly for this kind of info but I always turn up empty-handed, maybe someone can point me to some good reading material!

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Are you also interested in what would happen with AC with a DC bias, such as a square wave? – Ignacio Vazquez-Abrams Apr 12 '14 at 2:10
You are at the point that Mr Maxwell go for sleep and Mr.Ohm wake up – GR Tech Apr 12 '14 at 5:09

A way to look at your question is to look at the "circuits" you are talking about. When a wire is not connected to another wire, i.e. exposed to the air, we call that "open circuit" and that means that no current is flowing through the wire. If there is no current, there is no DC current (I) and no power is wasted/transferred. You can think about the air gap between to unconnected connectors as a "resistor" with a very high resistance (R). From Ohm's Law:

We see why there is no current running in the "circuit" when the resistance is very large:

There is no power (or energy) transfer because electrical power (P) is:

When the current diminishes so does the power:

When you connect the connector, there is negligible resistance between the two conductors (wires) and the resistance of the circuit will be set by the device you are connecting to it (the Engineers designed the device to have the correct resistance for the application).

*Source: I'm an engineer.

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Yes that is very helpful. So then if i'm following you correctly, the peripheral device would also need then another contact point to a path of low resistance.. like ground for current to actually flow? and so maybe the connector would have a separate wire for this, or maybe even the device has it's own ground? – user40262 Apr 12 '14 at 2:53
Yes the device must have another wire or "ground" for the current to flow. But probably the connector has at least 2 independent conductors in it (e.g. one is the signal wire and the other is ground or reference wire) like you see in the power plug or inside a USB wire or in almost all other connectors. – Gomunkul Apr 12 '14 at 2:59

Current is the moving of energy from one voltage potential to another voltage potential along a conductive path. For example, 5V Positive to 0V Ground, through a resistor.

If there is no path for current to move, there is no current. Think of it like a pool of water. If there is no where for the water to move to, the water is stagnant. It stays still. Add a path, like a leak in the pool, and water moves. That is current.

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Actually, current is the flow of charge, not energy. The presence of a current does not require a change in voltage. The fact that we typically encounter a current when there is a change in potential does not change the definition of current. – Joe Hass Apr 12 '14 at 1:48
@JoeHass How can you have current without a change in voltage? – Passerby Apr 12 '14 at 1:51
Electrons traveling through free space constitute a current. Until they interact with an electromagnetic field or matter there is no change in energy, therefore no voltage difference along their path. You can argue that such a path is very rare or extremely unlikely in nature, but if it exists for a single electron for just one millimeter in the void of space then we must admit that it is possible to have a current without a voltage. – Joe Hass Apr 12 '14 at 1:56
@JoeHass Theoretically or Technically correct, but for practical purposes, that means no, right? – Passerby Apr 12 '14 at 2:01
Sorry, I was talking about the definition of current so "technically correct" means yes. The point is, the existence of a current does not, by definition, depend on a change in voltage. I think if you start with "Current is..." you should then provide a proper definition of current. You could say that a voltage is necessary to produce a current in typical real-world situations that involve non-ideal conductors, but the definition of current does not require a change in voltage. – Joe Hass Apr 12 '14 at 2:12

The current in a wire is electrons that are free to wander about wherever an electric or magnetic field (or accumulation of charge to put it another way) pushes them. Even at high currents they move along a wire very slowly because there are so many of them. These electrons are often referred to as a "sea of electrons" and are loosely bound outer electrons in metal atoms. They can shift from atom to atom easily. Yes, they can move from one metal to another. In an AC circuit they shake back and forth instead of drifting in one direction.

Your dangling wire can have more electrons at one end than the other of there is an electric field, say from another wire that ends nearby and is at a different voltage. But in a DC circuit, after it comes to equilibrium, there is no current flow - theoretically. In reality you can also think of the dangling wire as a blob where the conductor gets bigger in area. There can be some current flow crosswise in the wire. You can map the electric field around the physical layout, and beyond some very short distance down the T, the effect is insignificant.

When some device is connected to the T it will complete a circuit back to the negative side of the power source and this creates a potential, an electric field, along the wire and the electrons will move. Since Ben Franklin got the charge backwards, the way we use + and -, the electrons really flow from ground to +. Fortunately the math doesn't care. I have an EE textbook that does the entire intro to circuits in correct electron flow terms. I have the free instructor sample and don't know if anyone every used it in a class. They could have a mutiny when students found they paid \$120 for a textbook that does everything "backwards".

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