Electrostatic Fields (Unit 06) - Study Guide
Coulomb's law, electric field intensity, field lines, and capacitance concepts.
Resource Book Core Concepts
- • Electric Field Lines: Imaginary lines showing the direction a free positive charge would move in an electric field. They always start from the positive (+) terminal and end at the negative (-) terminal.
- • Field Intensity (E): The intensity of a uniform electric field can be found using the equation E = V/d (where V is the potential difference and d is the distance between plates).
Paper Marking Secrets (Exam Traps)
📘 Resource Book Summary: Electrostatic Fields
Key Definitions:
Coulomb's Law: The mutual electrostatic force acting between two point charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them.
Formulas and Equations:
F = (1 / 4πε₀) * (Q₁Q₂ / r²) (ε₀ = Permittivity of free space)
Key Points:
- Like charges (+, + or -, -) repel each other, while unlike charges (+, -) attract each other.
⚠️ Special Notes (Exam Notes/Traps):
This law applies ONLY to point charges. It cannot be applied directly to large charged objects.
Key Definitions:
- Electric Field Intensity (E): The force acting on a unit positive charge placed at a given point in the electric field.
- Gauss's Theorem: The total electric flux through any closed surface is equal to the net charge enclosed by that surface divided by ε₀.
Formulas and Equations:
- Field Intensity: E = F / q = (1 / 4πε₀) * (Q / r²)
- Flux: φ = EA = Q / ε₀
Key Points:
- The electric field intensity at any point inside a conducting sphere is zero (E = 0).
⚠️ Special Notes (Exam Notes/Traps):
When drawing electric field lines, it is compulsory that they never intersect each other and are perpendicular to the conducting surface (with arrows from + to -).
Key Definitions:
Electric Potential (V): The work done in bringing a unit positive charge from infinity to a given point in an electric field.
Formulas and Equations:
- Potential: V = (1 / 4πε₀) * (Q / r)
- Work Done: W = V * q
- Relationship between Intensity and Potential: E = V / d (for parallel plates)
Key Points:
- Potential is a scalar quantity. A positive charge creates a positive potential, while a negative charge creates a negative potential.
⚠️ Special Notes (Exam Notes/Traps):
The potential inside a conducting sphere is constant. It is exactly equal to the potential on the surface of the sphere (V_inside = V_surface).
Key Definitions:
Capacitance (C): The amount of charge that must be given to a conductor to increase its potential by one unit.
Formulas and Equations:
- Capacitance: C = Q / V
- For a parallel plate capacitor: C = ε₀ A / d
- Stored Energy: W = 1/2 * QV = 1/2 * CV²
- Capacitors in series: 1/C_total = 1/C₁ + 1/C₂
- Capacitors in parallel: C_total = C₁ + C₂
Key Points:
- When a dielectric material is placed between the plates, the capacitance increases by k times (C = kε₀ A / d). The dielectric constant k is always greater than 1.
⚠️ Special Notes (Exam Notes/Traps):
If a capacitor is disconnected from the electric source (cell) and a dielectric material is inserted, the charge (Q) remains constant. If it is inserted while still connected to the cell, the potential difference (V) remains constant.
⚡ 3D Electric Field Lines Simulator
Observe live how electric field lines behave as you change the charge configuration.
Exam Question Breakdown (Mini Quiz)
When two charged conducting spheres are touched together and then separated, in what ratio is the charge distributed between them?