Electric Charges & Fields – JEE Mains 2026 Revision Capsule
Electrostatics begins here. This chapter is not difficult—but it demands clarity. A single idea repeats everywhere: charge creates field, and fields create forces. Once this settles, every tough-looking question melts into geometry and logic.
1. Properties of Electric Charge
Electric charge is strange—neither created nor destroyed, yet the entire universe dances because of it.
- Quantized: \( q = \pm ne \)
- Conserved: total charge remains constant.
- Two types: positive (+) and negative (–).
- Like charges repel, unlike charges attract.
- Induction: charging without contact.
- Conductors: free electrons move.
- Insulators: electrons are locked in place.
2. Coulomb’s Law (The Electrostatic Gravity)
Two point charges push or pull each other with a force:
\( F = k \frac{q_1 q_2}{r^2} \)
Where \( k = 9 \times 10^9 \, \text{Nm}^2\text{C}^{-2} \).
Vector Form:
\( \vec{F}_{12} = k \frac{q_1 q_2}{r^2} \hat{r} \)
The superposition principle makes life easy: calculate individual forces and add vectors.
3. Electric Field – The Agent of All Action
Electric field shows what a charge would feel if placed nearby. It’s the invisible architecture of space.
\( \vec{E} = k \frac{q}{r^2} \hat{r} \)
For multiple charges:
\( \vec{E}_{net} = \sum \vec{E}_i \)
Patterns JEE loves:
- Two like charges → zero field point between them.
- Two opposite charges → no zero point between them.
- Field always points away from +q and towards –q.
4. Fields for Special Charge Distributions
Infinite Line Charge:
\( E = \frac{\lambda}{2\pi \varepsilon_0 r} \)
Infinite Sheet:
\( E = \frac{\sigma}{2 \varepsilon_0} \)
Ring on Axis:
\( E = \frac{kQx}{(x^2 + R^2)^{3/2}} \)
Electric Dipole:
Axial line:
\( E = \frac{1}{4\pi \varepsilon_0} \frac{2p}{r^3} \)
Equatorial line:
\( E = \frac{1}{4\pi \varepsilon_0} \frac{p}{r^3} \)
5. Electric Flux – Counting Field Lines
Flux measures how many field lines pass through a surface:
\( \Phi = \vec{E} \cdot \vec{A} = EA\cos\theta \)
If the angle increases, fewer field lines pass → flux decreases.
6. Gauss’s Law – The Shortcut Machine
\( \oint \vec{E} \cdot d\vec{A} = \frac{Q_{enclosed}}{\varepsilon_0} \)
Best Uses:
- Sphere: field inside = 0, outside behaves like point charge.
- Solid sphere: E ∝ r inside, E ∝ 1/r² outside.
- Infinite line: field decreases as 1/r.
- Infinite sheet: field is constant.
7. PYQ Patterns You Must Know
- Flux through cube surfaces with different orientations.
- Dipole placed at angles—direction of net field.
- Zero field location between charges.
- Conductor + cavity questions (field inside cavity ≠ 0).
- Field due to system of charges using symmetry.
Quick Chapter Summary
This chapter is easy once you visualize charges as tiny kings demanding space and fields as messengers carrying influence. Think in vectors. Trust symmetry. Let formulas guide calculations, not the other way around.
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