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This is what was given in my textbook:

"The net charge on every component in the system is always zero. Thus no component can collect a net excess of charge, although some components can hold equal but opposite separated charges."

I can't quite understand why this is so. For example, what would happen if the two plates of a capacitor had unequal opposite charges?

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    \$\begingroup\$ Because the net charge starts at zero so every time you move one electron from one plate to the other, it makes it more negative at the destination and more positive at the origin by one charge. \$\endgroup\$
    – DKNguyen
    Jan 13, 2020 at 14:11
  • \$\begingroup\$ @DKNguyen You make it sounds as though electrons are moving through the gap between the plates, which is not generally true. (Exceptions include quantum tunneling and dielectric breakdown.) \$\endgroup\$
    – Elliot Alderson
    Jan 13, 2020 at 14:15
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    \$\begingroup\$ @ElliotAlderson That was not my intention. They take the scenic route through the wires. \$\endgroup\$
    – DKNguyen
    Jan 13, 2020 at 14:17
  • \$\begingroup\$ Ideal Conductors do not store charges unless you consider relative to adjacent floating wires. All real conductors have resistance. Ideal Insulators are also Ideal Capacitors with ideal dielectric and visa versa always have equal and opposite plate charge. Yet all real capacitors have "low" effective series resistance (ESR) (some lower than others) and "high" parallel resistance that causes leakage current at rated voltage. Caps always have charge displaced equally however voltage drop from electrode ESR may cause self-heating from ripple current \$\endgroup\$
    – Tony Stewart EE75
    Jan 13, 2020 at 14:30
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    \$\begingroup\$ "The net charge on every component in the system is always zero" is a simplifying assumption, not a statement of fact. \$\endgroup\$
    – Spehro Pefhany
    Jan 13, 2020 at 16:53

1 Answer 1

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The assumption of charge neutrality is based on our notion of common electric circuits, where charge is free to move under the influence of electric fields.

If the amount of positive charge on one capacitor plate did not equal the amount of negative charge on the other plate then the electric field between the plates would attract and draw charge as needed to establish neutrality.

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