Does current flow between a positively charged conductor and a neutral conductor?

Question

Having some trouble understanding the concept of voltage.

Suppose an initially neutrally charged wire is touched only to the positive terminal of a 9V battery, but not the negative terminal. The positive terminal of the battery has an accumulation of positive charges, and the wire has no accumulation of charge. Since voltage is related to how much charge is separated between two points, shouldn't the potential difference between the + end of a battery and a neutral wire be greater than zero, therefore causing a flow of current between the positive terminal and the neutral wire? Or is the negative end of the battery always required to create a flow of current?

I know that there is no 'absolute voltage' but what exactly is the voltage between a positively charged conductor and a neutral conductor, and why? Does voltage only exist between + and - regions?

Answer 1

You have a 9V battery => You have a device which generates an electric field between its terminals.

You connect a metal wire to the +terminal => the field draws free electrons from the wire towards the +terminal until the 3 forces find a balance. They are:

• the attraction from the metal atoms of the wire
• the field of the battery
• the random motion phenomena which is called "heat" (this can be omitted if we do not assume remarkable temperature differences)

If the wire happens to be long, the lack of electrons in the distant end is effectively a +charge which generates a field around the wire. So the field generated by the battery can be noticed and utilized as far as your wire enters without disconnecting it from the battery. This is why we generally have wires.

The electric field is around a charge, there's no need to have any other pole nor negative charge.

Voltage is a practical measure for electric field. It's in use because batteries and other electricity sources generally cannot keep a certain electric field strength, but they can give a certain energy to the electrons that they put in motion. In batteries this is because the chemical reactions release a certain energy if the current is allowed to flow. 9V battery gives to moving electrons energy = 9 Joules per one coulomb charge or as well 9 electronvolts per one electron. To get that energy into use you must have a current loop, because every electron that the +terminal sucks must be also removed from the battery. They come out from the -terminal and the circulation prevents charge buildup, which would stop the current as soon as the balance is found.

The strength and spatial form of the electric field that the battery generates is highly dependent on the forms and placements of those parts which are connected to the battery terminals. Battery cannot keep certain electric field, it can give only a certain energy to the electrons. The voltage is the measure for that ability.

Without a conducting current loop the current stops as soon as the balance is found.What is the balance, do we have any measures for it? Yes, we have. We can calculate the capcacitance between two conductive pieces or as well from one piece to the infinity in otherwise empty space. The capacitance tells how much charge a certain voltage can place to the conductive piece which is not connected to both terminals of a battery.

If you happen to have two separated metallic pieces and you connect +terminal of the 9V battery to piece A and the -terminal to piece B, the current flows until the balance is found. Let's assume the 9V battery can push 9 coulombs (exactly -9C) of electrons to B and respectively pulls as much out of A until all forces are in balance. What do you have?

Congratulations! you have one farad capacitor and it's charged to 9V. It's not especially little.

Answer 2

It's all relative. So relative to a third reference (Conductor C), conductor A might be positive and conductor B might be neutral (aka equal to conductor C). But to conductor A (i.e. if you use conductor A as your reference) then Conductor B and C are negative (while conductor A is neutral). So if you label some conductors amongst many as neutral, what you are really saying is "I choose this as my reference" or "this conductor is equal to my chosen reference" (which may be an implicit and unstated that there is a reference but it's there).

The wire when touched to the positive terminal (and only the positive terminal) of a battery DOES accumulate a charge, though it may be too small to measure when removed (everything has a free space capacitance that can store charge...think static shock).

Current (or more technically charge) does can and does flow into a dead-end wire, but it is a transient current and not a steady state current as you might have in a loop. It's a transient so it occurs for some time and then decays away. It is not a steady state current flow since there is no loop. It's the same difference as pouring and draining water from a pipe when one end is plugged versus pouring or draining water through a pipe where both ends are open. The flow eventually stops in one case but can continue to flow in the other.

The charge travels down the wire at significant fractions of the speed of light and there is not a lot of charge needed to be pumped into the tiny piece of wire for everything to reach equilibrium, so by the time your slow multimeter and even slower human reflexes can register what is happening the transient is already over. The charges have equalized so the current flow has already ended and no current flow = no potential difference/voltage to measure between the ends of the wire.

BUT if you measured it with a VERY fast oscilloscope and use a very long piece of wire (since most oscilloscopes we have are still not fast enough) so that it takes more time for the charge to reach the end to the wire and requires more charge to be pumped into the wire to reach equilibrium, then you would see the transient voltage and current waveform of the charge flowing into the wire. This is the transmission line effect when the assumption can no longer be made that current flows down wires instantaneously.

Do that a few million times a second in both directions and you get an antenna.