Electric charge
Section: Electricity & Magnetism | Syllabus: Cambridge AS Level Physics 9702
Types of Electric Charge There are two types of electric charge: positive (+) and negative (−) . Law of Electric Charges Like charges repel (+ and +, or − and −) Unlike charges attract (+ and −) The force between charges decreases as their separation increases.
Coulomb (C) The SI unit of electric charge. The charge on one electron e = 1.6 × 10^-19 C. Charging by Friction When two insulators are rubbed together, electrons transfer from one material to the other: Material that loses electrons → becomes positively charged Material that gains electrons → becomes negatively charged Example: Charging Polythene When polythene is rubbed with a cloth: Electrons transfer from the cloth to the polythene Polythene gains electrons → becomes negative Cloth loses electrons → becomes positive Important Only electrons move during charging.
Protons remain fixed in the nucleus. Charging involves transfer of negative charge (electrons), not creation of charge. Detecting Electrostatic Charge A gold leaf electroscope can be used to detect electric charge and determine if an object is charged.
Structure Metal cap on top connected to a metal rod Thin gold leaf attached to the bottom of the rod Enclosed in a glass case to prevent air currents FIG 4.2.4: Gold leaf electroscope Diagram showing: metal cap at top, metal rod passing through insulating plug, thin gold leaf hanging from rod inside glass case.
When uncharged, leaf hangs straight down. When charged (positively or negatively), leaf rises as it repels from the rod. How it Works Touch a charged object to the metal cap Charge spreads to the rod and gold leaf Both rod and leaf gain the same type of charge Like charges repel → gold leaf rises Greater charge = larger deflection angle Testing for Charge Uncharged object: No deflection of gold leaf.
Charged object: Gold leaf deflects (rises) away from rod. Conductors and Insulators Electrical conductor A material with free electrons that can move from atom to atom, allowing charge to flow easily.
Electrical insulator A material where all electrons are tightly bound to their atoms and cannot move freely. Good Conductors Good Insulators All metals (copper, aluminium, silver) Plastics (polythene, PVC, Perspex) Carbon (graphite) Rubber - Glass, dry air Experiment: Testing Conductors and Insulators Set up a simple circuit with a cell, lamp, and gap for test material Place material across the gap If lamp lights → conductor; if lamp stays off → insulator Electric Fields Electric field A region in which an electric charge experiences a force.
Direction of electric field The direction of the force on a positive charge placed at that point. Electric Field Patterns FIG 4.2.7: Electric field patterns (a) Positive point charge: field lines radiate outward in all directions with arrows pointing away from the charge.
(b) Positively charged conducting sphere: field lines emerge perpendicular to the surface, radiating outward. (c) Parallel plates: positive plate on left, negative on right. Uniform field shown by evenly-spaced parallel lines with arrows pointing from + to −.
Key Points Field lines point away from positive charges Field lines point towards negative charges Field lines are perpendicular to conducting surfaces Between parallel plates, the field is uniform (evenly spaced parallel lines)
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