Charges

In nature, there are two types of charged particles: protons, which carry a positive charge, and electrons, which carry a negative charge. These charges form the foundation of electrostatic phenomena, dictating how objects interact on both the microscopic and macroscopic scale. The governing principle is simple: unlike charges attract, like charges repel.

The labels "positive" and "negative" were introduced by Benjamin Franklin. The choice of names was arbitrary — what matters is how the charges behave. A positive and a negative charge brought together can cancel out, neutralizing one another algebraically.

Key Takeaways

• Protons carry positive charge, electrons carry negative charge; unlike charges attract, like charges repel.

• A neutral atom has equal numbers of protons and electrons; gaining or losing electrons turns it into an ion.

• The Law of Conservation of Electric Charge: total charge in an isolated system is constant — charge is transferred, never created or destroyed.

• The SI unit of charge is the coulomb (C); the elementary charge (magnitude on one electron or proton) is 1.6 × 10⁻¹⁹ C.

• Insulators (e.g., rubber) keep charge localized; conductors (e.g., copper, aluminum) let charge move freely.

Neutral Atoms and Ions

Most matter is electrically neutral, because charge is balanced at the atomic level. In a neutral atom, the nucleus contains protons (positive) and neutrons (electrically neutral, contributing nothing to net charge), surrounded by electrons (negative). The number of protons exactly equals the number of electrons, so the atom has no net charge.

This balance is essential to the stability of matter. If it's disrupted — if an atom gains or loses electrons — the atom becomes an ion, carrying a net positive or negative charge. Ions behave differently from neutral atoms, because the resulting charge imbalance introduces electrostatic forces that affect how they interact with other charged particles.

Conservation of Charge

Charges aren't fixed in place — they can be transferred from one object to another. This is captured by the Law of Conservation of Electric Charge: in any isolated system, the total amount of charge remains constant over time. Charge can neither be created nor destroyed, only moved from one location to another.

For example, if a negatively charged object touches a neutral object, electrons may transfer from the charged object to the neutral one. The total charge across both objects — before and after — stays exactly the same. Charge redistributes; it doesn't disappear or multiply.

Measuring Charge: The Coulomb

The SI unit of electric charge is the coulomb (C). The magnitude of charge carried by a single electron or proton — known as the elementary charge — is:

1.6 × 10⁻¹⁹ C

Protons carry this charge as positive, electrons as negative, but the magnitude is identical for both.

Insulators and Conductors

Materials differ in how easily they let charge move through them.

Insulators resist the movement of electric charge. When charge is placed on an insulator, it doesn't spread out — it stays localized wherever it was applied, and doesn't transfer easily to other objects. Rubber is a classic example: charge applied to a rubber object stays confined to that spot.

Conductors behave the opposite way. Charge placed on a conductor is free to move throughout the material and spreads across its entire surface. Metals like copper and aluminum are common conductors, which is why metal wires are used to transport electricity over distances — they let charge flow freely and efficiently.

MCAT Callout — Insulators vs. Conductors: Insulators hold charge in place — useful when you want to prevent charge flow, like the protective coating around an electrical wire. Conductors let charge move freely — essential when you want charge to flow, as in circuits and electrical grids.

Common MCAT Mistakes

• Treating "positive" and "negative" as more than labels. The names are arbitrary (courtesy of Benjamin Franklin) — they don't imply one charge type is more "fundamental." What matters on the MCAT is the sign and how like/unlike charges interact.

• Forgetting neutrons when tallying atomic charge. Neutrons contribute zero net charge. Only the proton count vs. electron count determines whether an atom is neutral or an ion.

• Assuming conservation of charge means charge can't move. Conservation of charge means the total charge in an isolated system stays constant — charge is free to transfer between objects, it just can't be created or destroyed in the process.

• Mixing up insulators and conductors. Insulators (like rubber) keep charge localized where it's applied. Conductors (like copper and aluminum) let charge spread freely across the entire surface — don't reverse these on test day.

MCAT-Style Concept Check

Question: An equal amount of negative charge is placed at a single point on two separate objects: one made of rubber, one made of copper. Which statement correctly describes what happens to the charge on each object?

• A) On both objects, the charge spreads evenly across the entire surface

• B) On the copper object, the charge spreads across the entire surface; on the rubber object, it stays localized where it was applied

• C) On the rubber object, the charge spreads across the entire surface; on the copper object, it stays localized where it was applied

• D) Neither object retains the charge — conservation of charge requires it to transfer immediately into the ground

Answer: B

Explanation: Copper is a conductor, so charge placed on it is free to move throughout the material and spreads across its entire surface. Rubber is an insulator, so charge placed on it resists movement and stays localized wherever it was applied. Option D misapplies conservation of charge — that law only requires that total charge in an isolated system stay constant; it says nothing about charge being forced to ground.

FAQ

What is electric charge?

Electric charge comes in two types: the positive charge carried by protons and the negative charge carried by electrons. Unlike charges attract each other, and like charges repel.

What is the elementary charge?

The elementary charge is the magnitude of charge carried by a single proton or electron: 1.6 × 10⁻¹⁹ coulombs. Protons carry this charge as positive and electrons as negative, but the magnitude is identical for both. The coulomb (C) is the SI unit of electric charge.

What's the difference between a neutral atom and an ion?

A neutral atom has an equal number of protons and electrons, so its net charge is zero. An ion is an atom that has gained or lost electrons, giving it a net positive or negative charge.

What's the difference between an insulator and a conductor?

An insulator, like rubber, resists the movement of charge — charge placed on it stays localized. A conductor, like copper or aluminum, lets charge move freely and spread across its entire surface, which is why conductors are used to carry electricity through wires.