Electric Charges Overview

Overview

This lecture covers the nature of electric charges, distinguishes between positive and negative charges, explains the law of conservation of charge, quantization of charge, and addresses related questions and calculations.

Types of Electric Charges

• There are two types of electric charges: positive (protons) and negative (electrons).
• Positive charges are due to excess protons; negative charges are due to excess electrons.
• A neutral object has equal numbers of protons and electrons, so its charges cancel each other.

Charging and Charge Transfer

• An object is charged if it has more of one type of charge than the other.
• If an object gains electrons, it becomes negatively charged.
• If an object loses electrons, it becomes positively charged.
• Electric charge is measured in coulombs (C).

Law of Conservation of Charge

• The law states that the total electric charge in a closed (isolated) system remains constant.
• Electric charge cannot be created or destroyed, only transferred from one material to another.
• In an isolated system, the total number of positive and negative charges does not change.

Quantization of Charge

• The magnitude of charge is denoted by 'e' (elementary charge).
• Charge is quantized: it occurs as integer multiples of 'e' (q = n·e, where n is an integer).
• Non-integer or fractional values of charge (e.g., 2.5e) are not possible in normal matter.

Example Calculations & Q&A

• If a conductor has a certain charge, use Q = n·e to find the number of excess or deficit electrons.
• Q&A covered definitions of positive/negative charge, law of conservation of charge, quantization, and properties of electric charge.

Key Terms & Definitions

• Positive Charge — excess of protons compared to electrons.
• Negative Charge — excess of electrons compared to protons.
• Neutral Object — equal numbers of protons and electrons.
• Coulomb (C) — SI unit of electric charge.
• Law of Conservation of Charge — total charge in a closed system is constant.
• Quantization of Charge — charge exists only as integer multiples of the elementary charge.
• Elementary Charge (e) — smallest unit of charge, 1.6 × 10⁻¹⁹ C.

Action Items / Next Steps

• Complete Exercise 4.1 (definitions and properties of charge).
• Complete Exercise 4.2 (calculations involving number of electrons and charge).
• Review section 4.2 on methods of charging a body.