Chapter 1
Electric Charges & Fields
Class 12 Physics | Complete Visual Guide
Electric Charge
Charge is a fundamental property of matter that causes electrostatic forces. It comes in two types:
- Positive charge: Carried by protons
- Negative charge: Carried by electrons
Key Properties:
- SI Unit: Coulomb (C)
- Quantization: q = ne (n = ±1, 2, 3,...; e = 1.6 × 10⁻¹⁹ C)
- Conserved in isolated systems
Coulomb's Law
The electrostatic force between two point charges is:
F = k (q₁q₂)/r²
where:
k = 9 × 10⁹ Nm²/C² (Coulomb's constant)
q₁, q₂ = magnitudes of charges
r = distance between charges
where:
k = 9 × 10⁹ Nm²/C² (Coulomb's constant)
q₁, q₂ = magnitudes of charges
r = distance between charges
Practical Example:
Calculate force between two charges of 2 μC and 3 μC placed 1 m apart.
Solution:
F = 9 × 10⁹ × (2 × 10⁻⁶)(3 × 10⁻⁶)/1² = 0.054 N
Calculate force between two charges of 2 μC and 3 μC placed 1 m apart.
Solution:
F = 9 × 10⁹ × (2 × 10⁻⁶)(3 × 10⁻⁶)/1² = 0.054 N
Visual Representation
Field lines show the direction of force on a positive test charge. They never intersect!
Electric Field
The electric field (E) at a point is the force experienced per unit positive charge:
E = F/q₀
Unit: N/C or V/m
Unit: N/C or V/m
Key Characteristics
- Vector quantity (has magnitude and direction)
- Field lines start from positive charges, end at negative charges
- Density of lines indicates field strength
Interactive: Electric Field Simulator
Click buttons to see different charge configurations
Gauss's Theorem
The total electric flux through any closed surface is equal to the net charge enclosed divided by ε₀:
Φ = ∮E·dA = qenclosed/ε₀
Important Applications
- Infinite plane sheet: E = σ/2ε₀
- Charged conductor surface: E = σ/ε₀
- Spherical shell: E = 0 (inside), E = kq/r² (outside)
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Frequently Asked Questions
What is the SI unit of electric flux?
The SI unit of electric flux is volt-meter (V·m) or N·m²/C.
Why do electric field lines never cross each other?
Electric field lines never cross because the electric field at any point has a unique direction. If lines crossed, it would imply two different directions for the field at the crossing point, which is physically impossible.
How is Coulomb's law different from Newton's law of gravitation?
While both are inverse-square laws, Coulomb's law deals with electric charges (can be attractive or repulsive) and depends on the medium's permittivity. Gravitational force is always attractive, much weaker, and depends on masses rather than charges.