01
Foundation
What is Electric Charge?
Electric charge is a fundamental property of matter — just like mass. You can't see it, but you feel its effects every day. When you rub a plastic comb on dry hair, the comb picks up tiny paper bits. Why? Rubbing causes transfer of tiny invisible particles called electrons, giving the comb a charge.
Real life: Balloon rubbed on hair → sticks to wall. Lightning in storms. Static shock when touching metal in winter. All caused by electric charges!
Two types of charge:
🔴 Positive (+) — protons carry this. Example: glass rod rubbed with silk.
🔵 Negative (−) — electrons carry this. Example: plastic rod rubbed with wool.
Golden Rule: Like charges REPEL. Unlike charges ATTRACT.
(Like is boring — they push away. Unlike is exciting — they pull together!)
02
Properties
Properties of Electric Charge
Conservation of Charge:
Charge is never created or destroyed — only transferred. Total charge in the universe is always constant. Before = After.
Quantization of Charge:
Charge comes in fixed, discrete packets. You cannot have half an electron. Every charge is a whole-number multiple of the smallest charge unit:
Think of charge like coins. You can have ₹1, ₹2, ₹5 — but never ₹1.37. Electrons are the "coins" of charge!
03
Force Between Charges
Coulomb's Law
Two charges placed near each other feel a force. Coulomb measured this exactly. The force depends on the size of both charges and the distance between them.
Key observations:
— More charge → more force
— Greater distance → MUCH less force (r² in denominator!)
— Double the distance → force becomes 4× weaker (Inverse Square Law)
— Same sign charges → F is repulsive
— Opposite sign charges → F is attractive
Two magnets: bring them closer → stronger force. Move apart → weaker force. Coulomb's law puts exact numbers on this behavior for electric charges.
Thinking moment: If charge q₁ is doubled AND distance is also doubled, by what factor does the force change? Try calculating before reading on!
04
Influence Zone
Electric Field & Field Lines
A charge creates an invisible region around itself where it can push or pull other charges. This region is the Electric Field. Even if no other charge is present, the field exists!
Electric Field Lines — visualize this:
— Lines shoot OUTWARD from positive charges (they're "escaping")
— Lines point INWARD into negative charges (they're "entering")
— Lines NEVER cross each other
— Crowded lines = STRONG field. Spread-out lines = weak field
— Always drawn with arrows showing direction
Critical difference: Force (F) needs TWO charges interacting. Electric field (E) is created by just ONE charge. F = qE connects them. Don't mix these up in MCQs!
05
Powerful Shortcut
Gauss's Law
Instead of adding forces from every charge one by one, Gauss's law gives us the total electric flux through any closed surface — using just the total enclosed charge. We use an imaginary closed surface called a Gaussian Surface.
Imagine a glowing bulb inside a balloon. The total light escaping through the balloon depends only on the brightness of the bulb — not on how big or what shape the balloon is. Same logic with Gauss's Law!
When to use Gauss's Law: Only when there is clear symmetry — spherical, cylindrical, or planar. For NEET, the most common application is finding field of uniformly charged sphere or infinite plane sheet.
06
Energy Landscape
Electric Potential & Potential Difference
Electric potential at a point is the work done to bring a unit positive charge from infinity to that point — slowly, without acceleration. It's a scalar (no direction!).
Think of potential like altitude on a hill. Water naturally flows downhill (high to low). Positive charges "flow" from high potential to low potential — just like water! This is why current flows in circuits.
Potential Difference: ΔV = V_A − V_B = W/q
This is what we call Voltage in daily life! The 230V at your socket = potential difference between the two terminals. It's what drives current through your devices.
V is a scalar — it has no direction. E is a vector. At the midpoint between two equal and opposite charges, V = 0 but E ≠ 0. This confuses many students in exams!
07
Charge Storage Device
Capacitance & Capacitors
A capacitor stores electric charge (and energy). It has two parallel conducting plates facing each other, separated by a gap or insulating material (dielectric). When connected to a battery, one plate gets +Q and the other −Q.
Capacitor = Rechargeable water tank. Bigger tank (larger A), shallower tank (smaller d) → holds more water (charge) at the same water pressure (voltage). Adding dielectric is like using a special tank material that holds more!
Energy stored in capacitor:
U = ½CV² = ½QV = Q²/2C
Combinations:
Series: 1/C_total = 1/C₁ + 1/C₂ (total C decreases)
Parallel: C_total = C₁ + C₂ (total C increases)
Note: This is OPPOSITE to resistors!