Electric Charge, Conductors, and Insulators

Rub a balloon on your hair on a dry winter day and the strands stand on end, reaching toward it as if summoned. There is no magic here — you have moved a vanishingly small fraction of the electrons in your hair onto the balloon's surface, and that tiny imbalance is enough to move hair against gravity. Electricity and magnetism, the subject of this entire course, trace back to a single property of matter: electric charge.

What charge is

Every proton carries a fixed positive charge and every electron carries an equal-magnitude negative charge, called the elementary charge, e = 1.60 × 10⁻¹⁹ C, where the coulomb (C) is the SI unit of charge. Ordinary matter is electrically neutral because atoms carry equal numbers of protons and electrons; an object becomes charged only when electrons are added or removed, leaving a net surplus or deficit. Because electrons are the particles that move (protons are locked inside nuclei), every charging process — rubbing, touching, or the flow of current in a wire — is really electron traffic, never proton traffic.

Two rules govern every charge interaction you will ever analyze. First, like charges repel and opposite charges attract, with a force you will quantify in the next lesson. Second, charge is conserved: the total charge of an isolated system never changes. Rub the balloon on your hair and the hair is left positive by exactly the amount the balloon is left negative — nothing was created, only redistributed.

Quantization of charge

Because charge always comes packaged in whole protons and electrons, any object's net charge is an integer multiple of e: Q = n e for some whole number n. This is called charge quantization, and it means a charge of 1.5 × 1.60 × 10⁻¹⁹ C is physically impossible — charge, unlike position or time, is fundamentally lumpy rather than continuous. In everyday charging experiments the numbers n involved are enormous (billions of electrons), which is why charge feels continuous at the macroscopic scale even though it is not.

Conductors and insulators

Materials differ enormously in how easily charge moves through them. In a conductor — metals, saltwater, the human body — outer electrons are only loosely bound to their atoms and roam freely through the material, so charge placed anywhere on a conductor quickly spreads out. In an insulator — glass, rubber, dry air, plastic — electrons stay bound to their home atoms, so charge placed on one spot largely stays put. This is exactly why rubbing works on a balloon (an insulator) but would fail on a metal rod held in your hand (a conductor, connected through you to the vast neutral reservoir of the Earth).

Charge can also be moved without contact, by induction: bring a charged rod near a neutral conductor and its free electrons shift away from (or toward) the rod, polarizing the conductor into separated positive and negative regions even though its total charge is still zero. Ground the far side while the rod is still near and electrons can escape (or arrive) entirely, leaving the conductor with a genuine net charge opposite that of the rod — without the two ever touching.

Worked example: counting electrons

A charged plastic comb has picked up a net charge of −3.2 nC after being dragged through dry hair. How many excess electrons does it carry?

n = Q / e = 3.2 × 10⁻⁹ C / 1.60 × 10⁻¹⁹ C
  = 2.0 × 10¹⁰ electrons

Twenty billion electrons sounds like an enormous number, and it is — yet it represents a charge imbalance of only a few billionths of a coulomb, an amount too small to feel as a shock but easily large enough to snap and crackle in dry air or attract bits of paper.

Why this matters

Every phenomenon in this course — the force between charges, the fields they create, the currents that light a circuit, the magnetism that current produces — starts from this chapter's two facts: charge is conserved and it is quantized. Photocopiers and laser printers deposit toner using induction and static charge; ESD-safe electronics assembly exists because a stray spark from your body can destroy a microchip; lightning is nothing more than these same rules operating on a planetary scale, as separated charge in a storm cloud finally overwhelms the insulating strength of air.

Curriculum aligned with OpenStax's University Physics Volume 2; all lesson text is original to Syllabus.