Electric charge is a basic property of matter that explains electric forces, static electricity, circuits, and atomic structure. There are two kinds of charge, positive and negative, and like charges repel while opposite charges attract. Most everyday objects contain huge numbers of positive protons and negative electrons, so they are often neutral overall.
Understanding charge helps explain why a balloon sticks to a wall, why sparks jump, and how electrical devices work.
Objects usually become charged by gaining or losing electrons, because protons are locked inside atomic nuclei while electrons can move between materials. If an object gains electrons, it becomes negatively charged, and if it loses electrons, it becomes positively charged. Charge is quantized, meaning it comes in whole-number multiples of the elementary charge e.
In any isolated system, total electric charge is conserved, so charge can move from one object to another but cannot be created or destroyed.
Key Facts
- Like charges repel, and opposite charges attract.
- A neutral object has equal total positive and negative charge.
- Elementary charge: e = 1.60 x 10^-19 C.
- Charge is quantized: q = ne, where n is an integer.
- Net charge equals total positive charge plus total negative charge.
- Conservation of charge: total charge before = total charge after in an isolated system.
Vocabulary
- Electric charge
- A property of matter that causes objects to exert electric forces on one another.
- Proton
- A positively charged particle found in the nucleus of an atom.
- Electron
- A negatively charged particle that can move between atoms or objects.
- Quantization
- The rule that electric charge occurs only in whole-number multiples of the elementary charge.
- Conservation of charge
- The principle that the total electric charge of an isolated system remains constant.
Common Mistakes to Avoid
- Thinking positive objects gained protons. This is wrong because ordinary charging happens by moving electrons, while protons usually remain fixed in atomic nuclei.
- Treating a neutral object as having no charges inside it. This is wrong because a neutral object has positive and negative charges that balance to a net charge of zero.
- Using any decimal value for the number of excess electrons. This is wrong because charge is quantized, so the number of gained or lost electrons must be a whole number.
- Forgetting the sign of electron charge in calculations. This is wrong because electrons have charge -e, so gaining electrons makes net charge more negative.
Practice Questions
- 1 An object gains 5.0 x 10^12 electrons. What is its net charge in coulombs?
- 2 A small metal sphere has a net charge of +3.2 x 10^-9 C. How many electrons did it lose?
- 3 Two identical neutral metal spheres touch. One sphere is then given extra electrons and separated from the other. Explain how conservation of charge applies to the two-sphere system.