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Electrostatics – Chapter 2 (IIT–JEE)

Page 1: Introduction to Capacitors

1️⃣ Why Do We Need Capacitors?

In electrostatics, we learned that charges can be stored on conductors. But storing large amount of charge safely requires a special arrangement.

👉 A capacitor is a device used to store electric charge and electrical energy.

Capacitors are used in:

  • Mobile phones & computers
  • Power supplies
  • Camera flash units
  • Signal filtering circuits

2️⃣ What Is a Capacitor?

A capacitor consists of:

  • Two conductors (plates)
  • Separated by an insulator (dielectric)

✔ The plates carry equal and opposite charges ✔ Net charge of the capacitor is zero

3️⃣ Simplest Capacitor – Two Isolated Plates

Consider two conducting plates placed near each other:

  • +Q charge on one plate
  • −Q charge on the other plate

A potential difference develops between them due to separation of charges.

⚠️ Important: Charge resides only on the surfaces, not inside conductors.

4️⃣ Capacitance – Definition

The ability of a system to store charge is called capacitance.

C = Q / V

  • C → Capacitance
  • Q → Charge on one plate
  • V → Potential difference between plates

5️⃣ SI Unit of Capacitance

The SI unit of capacitance is Farad (F).

1 Farad = Capacitance when 1 coulomb charge produces 1 volt potential difference

In practice:

  • μF (microfarad)
  • nF (nanofarad)
  • pF (picofarad)

6️⃣ Key Observations (JEE Concept)

  • Capacitance depends on geometry, not charge
  • Capacitance does NOT depend on Q or V
  • Adding charge increases voltage proportionally

⚠️ JEE Trap: Changing charge does not change capacitance.

7️⃣ Energy Aspect (Preview)

A charged capacitor stores energy in the electric field between plates.

This energy concept will be studied in later pages.

🔑 Page-1 Summary

✔ Capacitor stores charge and energy ✔ Capacitance = Q / V ✔ Depends only on geometry and medium ✔ Foundation for all capacitor problems

📌 Strong basics here = easy Advanced problems later.

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