Charge Conservation: Total charge in the universe is constant; charges cannot be created or destroyed, only transferred.
Charge Quantization: Charge exists in discrete units, specifically integer multiples of the charge of an electron (1.6 x 10^-19 Coulombs).
Additivity Property: Net charge (Q_net) is the algebraic sum of individual charges, considering their signs.
Charge Transfer Mechanisms
Charges can shift through:
Friction: E.g., rubbing a glass rod with silk cloth transfers electrons from the rod to the cloth, charging the rod positively and the cloth negatively.
Contact: Touching a charged body allows charge to move to a neutral body.
Induction: Bringing a charged object close to a neutral conductor induces a charge distribution without direct contact.
Earthing: Grounding allows excess charge to flow away.
Electroscope
Definition: An instrument used to measure charge.
How it Works: When a charged body contacts the metal rod of the electroscope, charge is transferred to gold leaves causing them to diverge.
Divergence Angle: Proportional to the magnitude of the charge.
Coulomb's Law
Describes the force between two point charges:
Formula: F = (1/(4πε₀)) * (Q₁ * Q₂) / r²
Where ε₀ = permittivity of free space (≈ 8.85 x 10^-12 C²/(N·m²)).
Force Direction:
Attractive for unlike charges.
Repulsive for like charges.
Modification in Medium: In a medium, the formula becomes:
F = (1/(4πε₀K)) * (Q₁ * Q₂) / r² (where K is the dielectric constant).
Vector Form of Coulomb's Law
In vector form, the force between charges can be calculated using position vectors and will incorporate unit vectors (i hat, j hat).
Examples and Problem Solving
Square Charge Configuration: For charges at the corners of a square, determining the charge needed at the center for equilibrium involves balancing forces.
Force Analysis: Using Coulomb’s law, analyze forces acting on different charges in various configurations to achieve equilibrium.
Key Questions and Concepts
Equilibrium of Charges: Understanding where to place a charge relative to others for balance.
Surface Tension and Electrostatics: Exploring forces due to surface tension in conjunction with electrostatic forces.
Summary of Key Takeaways
Electrostatics involves fundamental principles regarding charge behavior and interactions.
Understanding how to measure and calculate forces between charges is critical, including the applications of Coulomb’s law.
Charge conservation, quantization, and the methods of charge transfer are foundational concepts.
Electroscopes provide practical means to visualize and measure electric charge.