Electric Charges and Fields for NEET UG
Electric charges push and pull on each other. Coulomb's law, the electric field and Gauss's law let you work out those forces for any arrangement of charges.
PhysicsClass 1263 MCQs3 lessons12 flashcards3 memory tricks
Practise Electric Charges and Fields free
What you'll learn
- Charge and Coulomb's law
Use Coulomb's law to find the force between point charges, including vector addition. - Electric field and dipoles
Find the electric field of point charges and of a dipole, and the torque on a dipole. - Electric flux and Gauss's law
Use Gauss's law to find fields for symmetric charge distributions.
Key formulas
| Coulomb's law | F = kq₁q₂/r², k = 1/(4πε₀) = 9 × 10⁹ N m² C⁻² |
|---|---|
| In a medium of dielectric constant K | F = kq₁q₂/(Kr²) |
| Field of a point charge | E = kq/r² |
| Dipole moment | p = q × 2a (points from − to +) |
| Dipole field on its axis (far away) | E = 2kp/r³ |
| Dipole field on the equator (far away) | E = kp/r³ |
| Torque in a uniform field | τ = pE sin θ |
| Electric flux | Φ = E · A = EA cos θ |
Memory tricks
Lines start on positive and end on negative charges. They never cross, because the field has only one direction at each point. Closer lines mean a stronger field.
Total flux through a closed surface = q_enclosed/ε₀. It doesn't depend on the surface's shape or on charges outside it.
Charge on a conductor sits on its outer surface, and the electric field inside a closed conducting shell is zero.
Try 3 NEET-style questions
Q1. The law of conservation of charge states that
- charge can be created by friction
- the total charge of an isolated system remains constant
- the charge on every body is always zero
- positive charge can turn into negative charge
Show answer
Answer: (2) the total charge of an isolated system remains constant
Charge can neither be created nor destroyed; it is only transferred. The total charge of an isolated system stays constant. Rubbing transfers electrons from one body to another.
Q2. The electric field due to an infinite plane sheet with surface charge density σ = 8.85 × 10⁻¹² C/m² is (ε₀ = 8.85 × 10⁻¹² C²/N m²)
- 0.5 N/C
- 1 N/C
- 2 N/C
- 0.25 N/C
Show answer
Answer: (1) 0.5 N/C
E = σ/2ε₀ = 1/2 = 0.5 N/C.
Q3. Electric field lines
- intersect each other
- never intersect each other
- form closed loops around a positive charge
- are always parallel
Show answer
Answer: (2) never intersect each other
Field lines never cross, because at any point the field has only one direction. They start at positive charges and end at negative charges.
Practise all 63 questions Free · explanations for every option · see where you go wrong