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.
Understanding Physics: Electric Charge and Conservation
Charge produces an electric field in the space around an object. Another charged object responds to that field without needing to touch it. The strength of the interaction depends on two main things.
Larger amounts of charge produce a larger force. Greater separation produces a smaller force, and the change is especially rapid because the force follows one over the distance squared.
Doubling the distance makes the force only one quarter as large. This helps explain why tiny charge imbalances can have noticeable effects when surfaces are close together.
There are several ways to transfer charge. Rubbing two materials together can move electrons because different materials hold their outer electrons with different strengths. This is called charging by friction.
Direct contact can transfer electrons too. A charged metal object touching another metal object may share its excess charge until the charges reach a more balanced arrangement. Charging by induction works without contact.
A nearby charged object shifts mobile electrons inside a conductor. If the conductor is briefly connected to Earth, some electrons can enter or leave. Removing the Earth connection at the right time leaves the conductor with a net charge.
Conservation is best understood as careful bookkeeping. Suppose electrons move from a cloth to a plastic rod. The rod has extra electrons, while the cloth has exactly the matching shortage.
Looking at only one object can make charge seem to appear, but the full system shows no missing amount. Earth is so large that it can accept or supply many electrons with almost no detectable change in its overall state. For this reason, grounding is often treated as a way to remove charge, even though the charge has really moved into the ground.
In circuits, electrons already exist in the metal wires before a battery is connected. The battery uses chemical processes to separate charge and create an electric potential difference. This produces an electric field through the circuit, which causes the mobile electrons in the wire to drift.
Charge does not get used up by a lamp or motor. Energy is transferred to the device, often becoming light, motion, or heat, while charge continues around the closed path. A break in the circuit prevents the steady movement needed for most devices to operate.
Static charge is easier to notice in dry air because dry surfaces and air reduce charge leakage. Humid air leaves a thin water layer on many materials, allowing charge to spread away more easily. A spark occurs when the electric field in air becomes strong enough to pull electrons through the air.
Lightning is the same basic process on a far larger scale. When learning this topic, separate charge from energy and from electric current.
They are connected ideas, but they describe different parts of an electrical situation. Keep track of the chosen system boundary, the direction electrons move, and whether an object is a conductor or an insulator.
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.