# How can metal objects become electrically charged, if current flow means that an equal number of electrons enter/leave the object?

I am trying to answer to the question in the title. I am restricting my question to metal objects only.

Here is my logical reasoning:

1. Current is the flow of charge over time.
2. In a circuit (simple series circuit, let's say), the flow of current is the same at every point in the circuit.
3. Therefore, the same # of coulombs of charge is flowing at every point in the circuit.
4. Electrons are the "material" of the charge that is flowing.
5. Therefore, equal flow of charge at every point in the circuit must mean equal flow of electrons at every point in the circuit.
6. Therefore, current can never cause a metal object to become positively or negatively charged, because the net number of electrons in the metal object will never change due to current flow. (!)

Of course, objects CAN become electrically charged, gaining or losing electrons. So something is wrong with my reasoning or my premises. I just don't know what it is. Where am I going wrong?

If you have a complete circuit, every piece of metal will gain and lose the same number of electrons and will not have a net charge. If you connect two plates, one to each end of a battery, the battery will take charges from the plate connected to the positive terminal and send charges to the plate connected to the negative terminal until the voltage between the plates equals the voltage of the battery. At this point, no more current will flow, but each plate will have been given a net charge.

What you have described is the reason that conductors (such as metals) will act to reduce any polarisation within their structure. Polarisation is an imbalance of charge. In an object with a neutral charge overall, one region of the object may have a surplus of electrons, and thus have a net negative charge, and therefore another region will have a deficit of electrons, and thus have a net positive charge.

Now, what you say about metals is true, they conduct electricity and therefore current will flow in order to neutralise any imbalance in charge. However, this does not prevent them losing electrons by friction. The standard classroom experiment where you rub a balloon against your shirt in order to pull electrons from one to the other will still work with metals. Copper, for example, is often used as a conductor in wire form, but a copper rod may also be rubbed against another object to transfer electrons. This is called the Triboelectric Effect.

Also, although conductors when left on their own will act to spread charge out across their structure through current flow, it is still possible to polarise them (i.e. create imbalances of charge) using an external means. One such method is called Electrostatic Induction. In the picture below, moving a positively charged object near to the conductor in the electroscope pulls the electrons toward the top end, and as a result causes a local net negative charge here, and a local net positive charge at the bottom.

"Electroscope showing induction" by Sylvanus P. Thompson - Downloaded from Sylvanus P. Thompson (1881) Elementary Lessons in Electricity and Magnetism, MacMillan, New York, p.16, fig. 12. Licensed under Public domain via Wikimedia Commons.

Now imagine you were to suddenly cut the conducting material in the electroscope in half. The top half would be a completely independent object, and would have more negative charge than positive charge, a surplus of electrons. The bottom half would be similar, but would have more positive charge than negative charge, a deficit of electrons.

I hope this helps.

• This is the most satisfying response I have received on the 3-4 forums where I've asked this question. So essentially what it boils down to is that electrons are transferred through a means other than current (triboelectric effect), therefore it is possible for the charge to become imbalanced? Aug 28 '14 at 18:54
• Well technically current is defined as the flow of charge, so even in the triboelectric effect you could say current is flowing from one material to the other as the electrons are transferred over, albeit very briefly. Essentially though you were correct in saying that current will flow to neutralise any imbalance in charge in a material, but it can only do this if nothing prevents it. With the triboelectric effect and electrostatic induction there is an external influence causing the electrons to behave differently, flowing to one end of the material or leaving it altogether.
– Apoc
Aug 28 '14 at 21:49

There's a problem in that you are assuming that all current takes place in a circuit. But in some circumstances, like in a lightning strike or other form of electrostatic discharge for example, a current exists for a while, but it does not take place in a circuit.

• But, there IS a circuit in ESD, including lightning strikes. It's just not constituted by physical wires... Aug 28 '14 at 18:52