In short
Electric charge is an intrinsic property of matter, responsible for one of the four fundamental forces (electromagnetism). Three empirical facts, now elevated to principles, govern it:
- Two kinds. Charges come in exactly two varieties — called positive and negative by convention (the naming traces to Benjamin Franklin, 1752, and could equally well have been "red" and "blue"). Like charges repel, unlike charges attract.
- Quantisation. The magnitude of any observable electric charge is an integer multiple of the elementary charge e = 1.602\,176\,634 \times 10^{-19}\,\text{C} (exact by 2019 SI definition). No isolated object has ever been observed with a charge of 0.5e or \pi e. Quarks carry \pm e/3 and \pm 2e/3, but are permanently confined inside hadrons (protons, neutrons); quark charges cannot be isolated.
- Conservation. The total charge of an isolated system is invariant in time. Every physical process — friction on a dry comb, nuclear decay, electron-positron annihilation — produces and destroys equal amounts of positive and negative charge. \sum Q_{\text{before}} = \sum Q_{\text{after}}, always.
Unit: coulomb (C). One coulomb is the charge of roughly 6.24 \times 10^{18} electrons. Transfer of charge happens by three routes: friction (tribocharging when two materials are rubbed — the origin of the spark off a plastic manjha, or the zap off a polyester sari in winter), conduction (direct contact transferring charge through a conductor), and induction (a nearby charge rearranges charges in a neutral conductor without any physical contact).
On the pre-dawn of Makar Sankranti in Ahmedabad, you stand on a rooftop winding plastic manjha onto a spool. The line has been freshly oiled with the crushed-glass paste that makes kite fights brutal. You pull a length between your fingers, and in the dim light, you see it: a tiny crackle of blue-white sparks, each flash accompanied by a faint snap you can barely hear over the neighbouring music. If you had pulled the line in a pitch-dark room and watched carefully, you would have seen a tiny thread of light jumping between your fingertip and the string.
What you are watching is electric charge. The same thing that zaps you when you touch a metal tap after walking across a synthetic-carpeted hotel lobby in Gurgaon. The same thing that makes your polyester kurta pop and snap as you peel it off in the dry January air. The same thing, scaled up by a factor of billions, that connects the underside of a monsoon cloud to the ground above Powai with a stroke of lightning so hot it briefly ionises a narrow channel of air and sends a pressure wave you hear as thunder. Every one of these is a transfer of electric charge — the same quantity, obeying the same laws.
The goal of this article is to turn the familiar into the precise. You already know that rubbed plastic picks up a charge, and that charged objects repel or attract each other. You may have heard "opposites attract", "like repels like", and you may have a fuzzy sense that electrons are negative and protons are positive. What you need, before you can set foot into Coulomb's law, electric fields, and Gauss's law, is a clean set of principles: exactly two kinds of charge, one numerical unit, one conservation law, three methods of transfer. This article gives you all of them, with the experimental evidence for each, in a form that a JEE Advanced problem will actually ask you to use.
Two kinds of charge — the empirical starting point
Rub a plastic comb on dry hair. Bring the comb near a thin stream of water from a tap. The stream deflects toward the comb. Now rub a glass rod on silk. Bring the glass rod near the deflected water stream. The stream deflects too — also toward the rod. But here is the critical observation: bring the comb and the glass rod near each other and they attract. Rub two combs, each on dry hair, and bring them together — they repel. Rub two glass rods, each on silk, and they also repel.
The pattern that emerges from such experiments — Robert Symmer demonstrated them to the Royal Society in 1759, and they reproduce identically in an Indian physics classroom today — has exactly two states. Call the state of the rubbed comb "charge type A" and the state of the rubbed glass rod "charge type B". Then:
- Two A's repel each other.
- Two B's repel each other.
- An A and a B attract.
No third state exists. No one has ever rubbed two objects and found that they attract other A-charged and other B-charged objects both. The same three rules govern every case.
Benjamin Franklin, in 1752, named the two states "positive" and "negative", guessing that the "positive" state was an excess of some electric fluid and the "negative" state a deficit. The choice was arbitrary: he picked glass-rubbed-on-silk as positive. With the modern understanding — that charge carriers are electrons (negative) and atomic nuclei (positive) — Franklin's convention turns out to be slightly inconvenient (current in metals is actually carried by electrons moving against the conventional current direction), but by the time this was known the convention was locked in. Every formula in every Indian physics textbook uses Franklin's sign choice. If he had picked the other way, every expression involving current would have a flipped sign and nothing else would change.
Why the sign convention really does not matter for physics
A JEE problem might give you +5 \mu\text{C} and -3 \mu\text{C} charges and ask for the force between them. The answer is attractive, with magnitude set by Coulomb's law. Had you swapped the signs (Franklin in an alternate universe picked the other convention), the charges would read -5 \mu\text{C} and +3 \mu\text{C}. The product q_1 q_2 is still negative, the force is still attractive, the magnitude is unchanged. The convention picks a label; the physics lives in the product.
Quantisation — charge comes in discrete lumps
Take any macroscopic object — a comb, a rod, a piece of plastic kite string — and measure its charge. The measurement, no matter how precise, always returns an integer multiple of
a number called the elementary charge. Never 1.5e, never 0.1e, never \pi e. The charge of an electron is exactly -e, the charge of a proton is exactly +e, and the charge of any neutral atom is exactly zero (electrons balance protons). This fact is known as quantisation of charge.
The number e was first measured to good precision by Robert Millikan's oil-drop experiment (1909–1913), which Indian physics textbooks cover under "Millikan's oil-drop experiment" and which remains one of the cleanest quantitative demonstrations of quantisation in all of physics. Drop a fine mist of oil between two horizontal metal plates. Turn on a voltage; the oil drops, which have picked up a few electrons from frictional contact with the sprayer, feel an electric force. Adjust the voltage so that the electric force exactly cancels gravity for a chosen drop, and read off the charge. The results — for droplet after droplet, millions of drops over the years of the experiment — always come out as integer multiples of 1.6 \times 10^{-19} C. Never fractional. The elementary charge is not a statistical average; it is an exact unit.
As of 20 May 2019, the definition of the coulomb in the SI system was inverted: e is now defined to have exactly the value 1.602\,176\,634 \times 10^{-19} C, and the coulomb is defined as whatever is needed to make this come out. Quantisation is no longer an experimental result to be measured; it is built into the definition of the unit.
How large is one coulomb? A quick intuition
One coulomb is the charge of
That is roughly 10 billion billion electrons. Macroscopic objects in everyday life carry charges vastly smaller than one coulomb. A comb rubbed on your hair picks up perhaps 10^{-8} C — about 10^{11} excess electrons on the comb. A Van de Graaff generator in an IIT Bombay demonstration might build up to 10^{-5} C on its dome. A bolt of lightning transfers perhaps 10 C. The coulomb is a big unit — so big that charges of a few coulombs, concentrated on macroscopic objects, produce forces enormous enough to rearrange lives.
Quarks, and why quantisation is about what you can observe
Peek inside a proton or a neutron with enough energy and you find quarks — particles carrying charges of +\tfrac{2}{3}e (up quark) or -\tfrac{1}{3}e (down quark). A proton is two ups and a down, total charge +\tfrac{2}{3}+\tfrac{2}{3}-\tfrac{1}{3} = +e. A neutron is one up and two downs, total charge +\tfrac{2}{3}-\tfrac{1}{3}-\tfrac{1}{3} = 0. So at the deepest level, charge comes in units of e/3, not e.
Yet no experiment has ever isolated a quark. The strong force — the force that binds quarks — has the peculiar property (called confinement) that pulling two quarks apart costs more and more energy until it becomes cheaper to spontaneously create a new quark-antiquark pair from the vacuum, which latches onto the originals and heals the tear. Quarks live only in combinations whose net charge is a whole integer multiple of e: protons (+e), neutrons (0), pions (\pm e, 0), and so on. The statement "observable charge is quantised in units of e" is empirically airtight. The deeper statement, "the most fundamental unit is e/3", is structural but operationally invisible.
Conservation — the total charge of an isolated system never changes
Across every process yet observed — chemical reactions, nuclear reactions, particle decays, electron-positron pair production — the total charge of an isolated system before the process equals the total charge after. This is the law of conservation of charge.
A few concrete examples make the content sharp.
Example A — friction. Rub a plastic comb against dry hair. Before: comb neutral, hair neutral, total = 0. After: comb has charge -q (it gained electrons), hair has charge +q (it lost the same electrons), total = -q + q = 0. No charge was created from nothing — it was merely moved from hair to comb. The ionic conservation is exact.
Example B — nuclear beta decay. A free neutron decays as n \to p + e^- + \bar{\nu}_e. Before: Q_n = 0. After: Q_p = +e, Q_{e^-} = -e, Q_{\bar{\nu}_e} = 0. Total after: +e - e + 0 = 0. Conserved.
Example C — pair production. A high-energy gamma photon (Q = 0) near a nucleus can convert into an electron-positron pair: \gamma \to e^- + e^+. Before: 0. After: -e + e = 0. Conserved.
Example D — electron-positron annihilation. Reverse of Example C. An electron and a positron meet and convert their rest energy into two photons: e^- + e^+ \to 2\gamma. Before: -e + e = 0. After: 0 + 0 = 0. Conserved.
In every case, the total charge on the two sides of the arrow is identical. This is not a coincidence — in the language of modern theoretical physics, it is a consequence of a deep symmetry (gauge symmetry of electromagnetism, via Noether's theorem), exactly as conservation of energy follows from time-translation symmetry. For the purposes of this article, treat it as the empirical law it is: in an isolated system, the algebraic sum of all charges never changes.
What "isolated" means
Conservation applies to an isolated system — one across whose boundary no charge can cross. Your palm-held charged comb is not isolated: if you touch a metal tap, charge flows from the comb through your body into the plumbing and the Earth, and the comb's charge goes to zero. Total charge is still conserved — the Earth simply absorbed it — but the comb alone is not an isolated system. Draw the boundary around the comb-plus-you-plus-plumbing-plus-Earth, and the total is invariant.
This is the same caveat as for energy conservation: \Delta U = Q - W only if you include every channel of energy exchange. Charge conservation is analogous and, if anything, cleaner, because charge has no analogue of "lost to heat" — it does not degrade, it just moves.
Methods of charging — three ways to move charge
Given that charge cannot be created or destroyed, how do neutral objects become charged? Three mechanisms cover every case.
1. Friction (tribocharging)
When two different materials are rubbed together, electrons transfer from one to the other. The direction of transfer depends on the material pair — an empirical ordering called the triboelectric series exists, listing materials from "most eager to lose electrons" (hair, wool, cat fur) to "most eager to gain electrons" (PVC, teflon, silicon). Rub any two materials whose positions in the series differ, and electrons move from the more-eager-loser to the more-eager-gainer.
For kite flying on Makar Sankranti, the plastic manjha (strong electron-gainer) rubbed against cotton thread or silk gives the plastic a net negative charge; for the polyester sari coming off in dry air, the polyester is a strong electron-gainer and the wearer's skin donates electrons. The "zap" is the sudden equalisation of charge when you touch a grounded metal object.
The key point for charge conservation: every time material A becomes negatively charged by friction, material B becomes positively charged by exactly the same amount. The combined A+B system is still neutral. This is why, if you keep the comb in contact with the hair it rubbed, the pair as a whole exerts no net electric force on a distant test charge.
2. Conduction
A conductor (a metal like copper, or a human body in mildly humid conditions) allows charges to move freely through it. Touch a charged sphere with a neutral sphere; when they separate, both are charged, each carrying half the original charge (if the spheres are identical). More generally, after contact, the charge redistributes until the two spheres are at the same electric potential — a condition that, for identical spheres, means equal charge, and for different-sized spheres, means proportional-to-radius charge (derived in the capacitance article).
The direction of charge flow in conduction is governed by potential, not by the "amount" of charge directly: charge flows from higher potential to lower potential until both bodies are at the same potential. For two identical spheres starting at Q and 0, the final state is Q/2 each — but this is a consequence of the potentials equalising, not a division rule about charge itself.
3. Induction
Induction is the most elegant of the three, and the one most likely to trip up a student. A charged object can induce a charge redistribution in a nearby neutral conductor without touching it.
Bring a positively charged rod near one end of an isolated neutral conducting sphere. Free electrons in the sphere (always present in a metal) are attracted toward the rod's side and pile up there; the opposite side of the sphere, having lost electrons, is left positively charged. The sphere's total charge is still zero — conservation is respected — but the charge is now separated within the sphere. The near side is negative, the far side is positive.
If you now touch the far side (the positively charged side) with a grounded wire, electrons flow from the Earth onto the sphere to neutralise the positive side. Remove the wire. Remove the charged rod. The sphere is now left with a net negative charge. You have charged the sphere without ever touching it with a charged object. This is charging by induction.
The beauty is that induction leaves the inducing object's charge unchanged. A single charged rod can be used to induce charges on many neutral objects in sequence, without losing any of its own charge.
How does the induced charge depend on distance?
The strength of the induction effect falls off sharply as the inducing charge moves away. For a point charge +Q at distance d from the centre of a small neutral conducting sphere (radius a \ll d), the induced surface charge on the near side has a magnitude that scales, to leading order in a/d, like
You can read this off the interactive below: vary the distance d between the inducing point charge and the centre of a small neutral sphere and watch how the induced dipole magnitude on the sphere grows as d decreases.
A note on insulators
Metals have conduction electrons that move freely through the material; rubber, glass, and plastic do not. Charging a rubber rod by friction leaves the acquired electrons stuck in place — they do not spread across the rod the way they would on a copper sphere. Touch a copper sphere with a charged rubber rod and some charge transfers (conduction), but most of the rubber rod's charge stays put because the electrons on the rod cannot easily travel to the point of contact. This is why insulators hold a localised charge for long periods, while conductors dissipate their charge quickly to whatever neutral body they are in contact with.
Worked examples
Example 1: Millikan-style drop — extracting $e$ from a suspended droplet
An oil droplet of mass m = 3.27 \times 10^{-15} kg hangs stationary in the uniform electric field between two horizontal parallel plates, the upper plate positive and the lower plate negative. The field strength is E = 2.0 \times 10^5 N/C. (a) Find the charge on the droplet, signed. (b) How many excess or missing electrons does this correspond to?
Step 1. Write the equilibrium condition.
For the droplet to hang stationary, the vertical forces must sum to zero. Gravity pulls down with magnitude mg. The electric force has magnitude |q|E and direction depending on the sign of q: for q negative and \vec E pointing downward (from + plate to − plate), the force on the droplet \vec F = q\vec E points upward. Equilibrium:
Why: the only two vertical forces on the droplet are gravity and the electric force. Stationary equilibrium requires them to cancel.
Step 2. Solve for |q|.
Use g = 9.8 m/s²:
Why: plug into the force balance. The enormously small numerator (a sub-nanonewton gravitational force on a tiny droplet) divided by the large denominator (a strong field) gives a charge at the elementary scale.
Step 3. Determine the sign.
The field points from + (top) to − (bottom), so \vec E is downward. For the electric force to point upward (to cancel gravity), q must be negative. So q = -1.60 \times 10^{-19} C.
Step 4. How many electrons?
The droplet carries exactly one excess electron. Why: the measured charge is (within experimental precision) the elementary charge e. No experimentally accessible charge comes out as 0.5 e or \pi e; only integer multiples — the heart of quantisation.
Result: q = -1.60 \times 10^{-19} C; the droplet carries one extra electron. If the droplet had a mass twice as large but still hung at the same field, it would require |q| = 2e — two excess electrons — to balance.
What this shows: Millikan's oil-drop experiment works because the electric force and gravity can both be made to compete at the elementary-charge scale for a microscopic oil drop. Repeat this measurement for hundreds of droplets, and the charges always come out as integer multiples of e, never fractional — quantisation becomes observationally inescapable.
Example 2: Charge conservation in a spark — Makar Sankranti kite line
A plastic manjha of length 1.0 m is drawn briskly through a cotton cloth. The friction transfers n = 6.24 \times 10^{10} electrons from the cloth onto the string. A moment later, the string's tip sparks into your fingertip, neutralising its charge. (a) What was the charge on the string immediately after rubbing? (b) What was the charge on the cotton cloth? (c) After the spark, where did the charge go? Comment on the conservation ledger.
Step 1. Charge on the string after rubbing.
n electrons transferred onto the plastic, each of charge -e:
Why: each electron contributes -e to the charge; n electrons contribute -ne. Multiplying gives a tiny but macroscopically measurable charge.
Step 2. Charge on the cotton cloth.
The cloth lost n electrons, so it gained charge +ne:
Why: the electrons had to come from somewhere. By conservation, if one body gains -ne, the other must gain +ne. Before rubbing the total was zero; after rubbing the total is still zero (q_{\text{manjha}} + q_{\text{cotton}} = -10 + 10 = 0 nC).
Step 3. After the spark.
When the manjha's tip reaches your fingertip, the electric field between the two is large enough to ionise the thin air gap between them, opening a conducting channel. Electrons flow from the (negatively charged) manjha to your (grounded, via your body) fingertip:
The charge went into your finger, through your body, and ultimately into the Earth (which is so large that a few nanocoulombs are undetectable).
Step 4. The conservation ledger.
Before any rubbing: cotton 0, manjha 0, hand-and-Earth 0. Total: 0.
After rubbing: cotton +10 nC, manjha -10 nC, hand-and-Earth 0. Total: 0.
After spark: cotton +10 nC, manjha 0, hand-and-Earth -10 nC. Total: 0.
At every stage, the total charge of the isolated system (cotton + manjha + you + Earth) is exactly zero. Charge never appeared; it only moved.
Why: this is the point of conservation. Local charge changes, but the global sum over the isolated system is invariant. In practice, the Earth's enormous reservoir makes it convenient to treat "grounded" as "neutralised" — the Earth simply absorbs any charge without noticeably changing its own state.
Result: q_{\text{manjha, after friction}} = -10 nC; q_{\text{cotton}} = +10 nC; after the spark all charge has flowed to ground. Total charge of the full isolated system = 0 at every stage, as required by conservation.
What this shows: The sparks of Makar Sankranti, or the zaps from a synthetic sari, are not "creation" of charge — they are redistribution. The electrons existed in the cotton cloth before you touched it; friction moved them onto the manjha; the spark moved them again, through your body, to Earth. Charge conservation is cleaner and more exact than almost any other conservation law you will meet.
Common confusions
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"Positive charges flow in a wire." In almost every practical conductor (copper wire, human body, saltwater in a pipe), the mobile carriers are electrons — negative particles. The "conventional current" direction (from + terminal to − terminal through the external circuit) is the direction positive charge would flow; electrons actually move in the opposite direction. This is a historical legacy of Franklin's sign choice and does not change any physics.
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"A body can have \tfrac{1}{2}e of charge if it is small enough." No. Every isolated, observable object — from a single atom to a galaxy — has a charge that is an integer multiple of e. Quarks have \pm e/3 but are never isolated.
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"Friction creates charge." Friction transfers charge (electrons) from one material to another. The total charge of the two materials is conserved. If you separate the two after rubbing, each has equal and opposite charge.
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"Induction requires a conductor." Insulators also undergo a related but weaker effect called polarisation — the bound charges within an insulator's molecules shift slightly, even though no charges move across the material. This is why a charged comb attracts a neutral piece of paper (paper is an insulator): the paper polarises, the nearer side gets the opposite sign of the comb, and the net force is attractive.
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"Grounding destroys charge." Grounding transfers charge to the Earth, which is so large that its total charge is essentially fixed. The charge is not annihilated; it is spread over a reservoir so vast that its effects are undetectable anywhere. From the local point of view, grounding removes charge; from a global point of view, charge is still conserved.
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"Quantisation is just an experimental result that might break at higher precision." Modern tests of charge quantisation have confirmed integer-e to better than 1 part in 10^{21}. And since 20 May 2019, the coulomb is defined by setting e = 1.602\,176\,634 \times 10^{-19} C exactly — quantisation is built into the SI.
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"A neutral body exerts no force on anything electric." A neutral overall body can still be polarised by a nearby charge and thereby exert an attractive force. The classic demonstration: a charged balloon sticking to a neutral wall. The wall's electrons shift slightly, creating a thin layer of induced opposite charge on the wall's surface under the balloon; that induced layer attracts the balloon.
Stop here if you wanted the empirical principles and a solid grip on charge transfer. What follows is a sketch of where these principles come from in deeper physics — Noether's theorem, gauge symmetry, and a look at why the proton charge and electron charge are exactly opposite in magnitude.
Noether's theorem and charge conservation
Emmy Noether's 1918 theorem, one of the deepest results in mathematical physics, states: every continuous symmetry of a physical system's action corresponds to a conservation law. Time-translation symmetry ("the laws are the same today as they were yesterday") gives rise to energy conservation. Space-translation symmetry ("the laws are the same in Mumbai as in Chennai") gives momentum conservation. Rotational symmetry gives angular momentum conservation.
What continuous symmetry gives charge conservation? The answer is the gauge symmetry of electromagnetism: the physics is invariant under a redefinition of the electromagnetic potentials (\phi, \vec A) \to (\phi - \partial_t \lambda, \vec A + \nabla \lambda) for any smooth function \lambda(x, t). (This is a "local" symmetry because \lambda can vary from point to point.) By Noether's theorem, the conserved current associated with this symmetry is the electric current \vec J, and its time component is the charge density \rho. The statement \partial_t \rho + \nabla \cdot \vec J = 0 (the continuity equation) is charge conservation at a local level: charge cannot disappear from one region without flowing out as current.
This is why charge conservation, empirically so solid, is really a consequence of the gauge structure of electromagnetism itself — a structure that sits beneath Coulomb's law, Ampere's law, and everything you will meet in the next several chapters of the wiki.
Why proton charge = -(electron charge), exactly
A neutral hydrogen atom consists of one proton and one electron. Laboratory measurements have set the upper bound on |q_p + q_e| at less than 10^{-21} e — i.e. the two charges cancel to at least 21 decimal places. If they cancelled only approximately, bulk matter (which is made of vast numbers of protons and electrons) would have a macroscopic net charge proportional to the mismatch times the atom count, and this would produce enormous electric fields around ordinary objects. The fact that an apple exerts no measurable electric force on a neighbouring apple bounds the mismatch below the sensitivity of laboratory experiments.
Why is the cancellation exact? In the Standard Model of particle physics, it is a consequence of a condition called anomaly cancellation — a mathematical requirement that the gauge theory be internally consistent. Roughly, the electroweak gauge theory (which includes electromagnetism) can only be quantum-mechanically consistent if certain sums of charges over all particle types vanish. When you write those sums out for the observed particles (up quark, down quark, electron, neutrino), they do vanish — but only because the electron's charge is exactly -e and the proton's (two ups plus a down) is exactly +e. This is not a coincidence; it is the condition that makes the theory make sense.
Fractional charges in condensed matter
In specific exotic materials (fractional quantum Hall systems in high magnetic fields, certain topological insulators), quasi-particles can appear with effective charges of e/3, e/5, etc. These are not fundamental charges — the underlying electrons still carry -e each — but collective excitations of the electron fluid that behave, in their reduced environment, like particles with fractional charge. Indian groups at TIFR Mumbai, IISc Bangalore, and the Harish-Chandra Research Institute have contributed to this field. The existence of such quasi-particles does not violate quantisation in the usual sense; it broadens the question of what "charge" means at the emergent level.
Milliken's experiment and the precise value of e
Millikan's own 1913 value was e = 1.592 \times 10^{-19} C, about 0.6 % low. The error came from his use of an outdated value for the viscosity of air (needed to convert droplet terminal velocity into droplet mass). When the correct viscosity is used, his data return values within 1 % of the modern exact value. The result in his Nobel-winning paper is a case study in how a measurement can be off by a systematic error whose source only becomes visible decades later, when independent techniques (X-ray diffraction, Josephson-effect metrology) give an independent pin on the constant.
As of 2019, the measurement problem is gone — e is now defined, not measured. Any future precision improvements will redefine the coulomb around the same e, not revise e itself.
Charge conservation in cosmology
The observable universe contains approximately 10^{80} electrons and approximately 10^{80} protons — equal to stunning precision. The imbalance, if any, is smaller than 1 part in 10^{26} on cosmological scales. This global neutrality is another empirical confirmation of charge conservation: if, at the Big Bang, some process had produced a charge imbalance, it would persist forever by conservation, and its gravitational and electromagnetic effects would be visible today. They are not, so the initial state was neutral, and the current state is too. The conservation law runs all the way back, 13.8 billion years, to the earliest moments of cosmological thermodynamics.
Where this leads next
- Coulomb's Law — the quantitative force law between two point charges, F = kq_1q_2/r^2, the first thing you will compute with the charges defined in this article.
- Superposition of Electrostatic Forces — extending Coulomb's law from two charges to many, by vector addition.
- Electric Field of a Point Charge — the field picture, which makes forces easier to think about in complicated geometries.
- Gauss's Law — the integral form of the inverse-square law, which turns multi-charge problems into symmetry exercises.
- Forces in Nature — an Overview — electromagnetism sits alongside gravity, the strong force, and the weak force as one of the four fundamental interactions; charge is the property that couples matter to electromagnetism.