Electric Charge

Intuition

The thing that electricity is made of

Everything around you is built from atoms, and atoms contain particles that carry a property called charge. It comes in a positive kind, carried by protons, and a negative kind, carried by electrons, and one short rule governs both: like repels like, opposites attract. Bring two electrons close together and they push apart; bring an electron and a proton close together and they pull.

Most matter is neutral because it holds equal numbers of both, and their effects cancel exactly. Electricity is what happens when that balance is disturbed — when charge is separated, or made to move.

You already know what separated charge feels like. Rub a balloon on your hair and it pulls a few electrons off; the balloon now has slightly more negative charge than positive, and it will stick to a wall or make your hair stand up. That is the same charge that runs a phone, only in far smaller quantity and going nowhere.

Because charge can be counted, it needs a unit: the coulomb. One coulomb is a specific quantity of charge, in the same way a litre is a specific quantity of water. What the charge is doing then decides which part of the subject you are in. Charge standing still gives you static electricity and the shock from a car door; charge in motion gives you electric current; charge pushed somewhere it does not want to be gives you voltage. The rest of electronics is built on those three.

Practitioner

Coulombs, currents and battery capacity

The coulomb (symbol C) is the SI unit of charge, and charge itself is written with the symbol Q. It is a large unit for everyday electronics: the charge on a small capacitor is measured in microcoulombs or less, while the charge stored in a phone battery runs to a few thousand coulombs.

Charge and current are two views of the same thing. Current is the rate at which charge passes a point, so charge is current multiplied by the time it flows:

That equivalence is why battery capacity is almost never printed in coulombs. Manufacturers use ampere-hours — usually milliampere-hours for portable cells — because the numbers land in a friendlier range, and because what a user wants to know is "how long will it run", not "how many coulombs". Converting between the two units takes a single multiplication by the number of seconds in an hour. Capacity in ampere-hours still says nothing about energy on its own; that needs the voltage as well.

Charge also appears wherever something stores it rather than passing it along. A capacitor's whole purpose is to hold separated charge on two plates, and its capacitance states how much charge it holds per volt applied. At the other extreme, charge that accumulates on an insulating surface with nowhere to go is what makes electrostatic discharge a hazard on a workbench.

All of that rests on conservation. No circuit creates or destroys charge: a battery does not manufacture it — it moves charge around a loop, converting chemical energy in the process — so whatever leaves one terminal returns to the other. The working consequence is that charge in a circuit never gets used up. The current entering a component equals the current leaving it, and what the component consumes is energy rather than charge.

Engineer

Quantisation, carriers and conservation

Charge is not a continuous fluid. It comes in whole multiples of a single indivisible step, the elementary charge, written e or q_e — the magnitude of the charge on one electron or one proton. Since the 2019 revision of the SI, that step is not measured at all: it is a defining constant, fixed at exactly 1.602 176 634 × 10⁻¹⁹ C, and the ampere is defined from it. Every charge you can ever measure is a whole-number multiple of that value.

Charge conservation is one of the deepest results in physics and the reason Kirchhoff's current law works: in an isolated system the total charge is constant, so whatever flows into a junction flows out of it. Chemistry and semiconductor physics obey it too. When a diode's junction recombines an electron and a hole, no charge disappears — a negative carrier and the absence of one cancel.

The carriers that actually move differ by material, and confusing them for the charge itself causes real errors:

• In metals, current is carried by a sea of loosely bound electrons drifting through a fixed lattice of positive ions. Only the electrons move; the positive charge stays put. See Electrons & Atomic Structure.
• In electrolytes and in the human body, charge is carried by ions of both signs, moving in opposite directions at once.
• In semiconductors, conduction is described using both electrons and holes — the vacancies electrons leave behind, which behave as positive carriers with their own mobility. That two-carrier picture is the basis of the whole of semiconductor materials.
• In a gas discharge or an arc, both free electrons and ionised atoms carry the current.

Conventional current was defined as flowing from positive to negative long before anyone knew that metals conduct by electrons moving the other way. Since the physics comes out identical either way, the convention was never revised, and every schematic ever drawn still carries it. What it does mean is that in a wire the carriers travel opposite to the arrow. Conventional vs electron flow covers the consequences.

Charge is neither energy nor current. A capacitor holding a given charge stores an energy that depends on the voltage it reached, while a wire carrying a given current transports charge without storing any of it. Keeping coulombs, joules and amperes distinct removes most of the confusion beginners have with batteries and capacitors.

Professional

Where charge is the working quantity

For a practising engineer, charge stops being a physics-lesson abstraction in the places where it — and not current — is the quantity actually specified.

Battery gauging. Coulomb counters integrate current over time to track how much charge has left a cell, which is the same operation as the worked example above run continuously. They are accurate over short intervals and drift over long ones, because integration accumulates the offset error of the sense amplifier as well as the signal. Practical fuel gauges therefore combine coulomb counting with voltage-based state-of-charge estimation and periodic recalibration at full and empty. Cell capacity itself is temperature- and rate-dependent, so the coulombs you can extract are not a fixed number — see battery types.

Gate charge. A MOSFET datasheet specifies total gate charge Q_g in nanocoulombs, not gate capacitance, because the capacitance is strongly voltage-dependent and charge is not. Switching time follows directly from the drive current: a device with a gate charge of 20 nC driven by 1.0 A switches in 20 ns. Halve the drive current and the switching time doubles, along with the switching loss. That is the calculation that sizes a gate driver — see MOSFET gate drive.

Charge as the signal. Piezoelectric sensors, photodiodes in integrating mode and radiation detectors all produce charge rather than voltage or current, and are read by charge amplifiers that convert coulombs to volts. The units here are picocoulombs and femtocoulombs, and what spoils a reading is leakage, triboelectric noise from cable flexing, or dielectric absorption in the feedback capacitor.

Charge where you did not want it. Leakage that would be irrelevant as a current becomes decisive when a node must hold charge: a sample-and-hold droops, a high-impedance input drifts, and a DRAM cell forgets what it was holding. Board contamination and flux residue open leakage paths measured in picoamperes, which is still enough to empty a small holding capacitor in seconds. Static charge accumulated on a person or a chair is the ESD event that damages a part — modelled, tested and controlled as described in ESD basics, with on-board protection provided by devices such as the TVS diode.

In each case charge is the quantity specified because it persists rather than flows. Where a value has to be held or counted rather than merely passed along, coulombs are the primary unit and current the derived one.