Class 12 Physics Chapter 2 Electrostatic Potential and Capacitance

Physics time! And yes, Electrostatic Potential and Capacitance can sound like a lot of formulas and abstract stuff. But honestly? Once you get the basics, it’s not that scary, and scoring here is actually super doable. Whether it’s understanding potential, capacitance, or dielectrics, this blog has got you covered with exam-ready notes, simple explanations, and formulas you’ll actually remember.

These notes are short, sorted, and student-friendly - perfect if you’re revising on the go or last-minute before boards.

Electrostatic Potential and Capacitance Class 12 Notes

Reading the full physics chapter again and again? Not the move. So we’ve made these Electrostatic Potential and Capacitance Class 12 notes to save your time and sanity. All the formulas, key definitions, NCERT highlights or typical exam questions are  all packed in here.

And if you like to revise on the go, we’ve also added Electrostatic Potential and Capacitance Class 12 PDF notes for a quick scroll-and-learn. Clean, clear, and perfect for last-minute pre-board revisions.

Electrostatic Potential - Class 12 Physics Notes Simplified

Electrostatic potential at a point in an electric field is the work done to bring a unit positive charge from infinity to that point without acceleration. It indicates the potential energy per unit charge at that point in the field.

Formula:  V = W / q

Where:

• V = Electrostatic potential (Volts)
• W = Work done (Joules)
• q = Test charge (Coulombs)

Points to Remember:

• Electrostatic potential is a scalar quantity, meaning it has magnitude but no direction.
• The potential is positive for positive charges and negative for negative charges.
• The potential decreases as the distance from the charge increases.
• For a system of multiple charges, the total potential at a point is the algebraic sum of individual potentials.

Potential Due to a Point Charge

The electrostatic potential at a distance r from a point charge Q is given by:

Formula: V = k × Q / r

Where:

• V = Electrostatic potential (Volts)
• k = Coulomb’s constant, 9 × 10⁹ N·m²/C²
• Q = Charge producing the electric field (Coulombs)
• r = Distance from the charge (meters)

Key Points:

• The potential decreases as the distance from the charge increases.
• A positive charge produces positive potential, while a negative charge produces negative potential.
• The potential is scalar, so it does not depend on direction.

Example: If a +5 μC charge is placed in space, the potential at a point 2 m away can be calculated by substituting the values into V = kQ/r.

Potential Due to Multiple Charges

When there is a system of multiple point charges, the total electrostatic potential at a point is the algebraic sum of potentials due to each individual charge:

Formula:  V_total = V₁ + V₂ + … + Vₙ

Where V₁, V₂, …, Vₙ are the potentials produced by charges Q₁, Q₂, …, Qₙ at that point.

• Since potential is scalar, you just add them normally - no fancy vector math here.
• An electric field, on the other hand, is a vector, so that one needs direction-based addition.
• Basically, when finding potential, think “just add it up”; when finding a field, think directions matter.

Equipotential Surfaces

Imagine a surface where every point has exactly the same electrostatic potential. Sounds fancy, but the cool thing is: if you move a charge along this surface, you don’t need to do any work. Zero. Nada.

Key Things to Remember:

1. The electric field is perpendicular to an equipotential surface. That means the field lines always cross these surfaces at a 90° angle.

2. Potential stays the same along the surface, but it decreases as you move away from the charge. So, closer to a positive charge = higher potential; farther away = lower.

3. Common examples you’ll see in exams:

• Spherical surfaces around a point charge - think of imaginary “shells” around a single charge.
• Metallic surfaces in electrostatic equilibrium - the whole surface has the same potential, which is why metals are used for shielding.

Why it matters: Equipotential surfaces make life simpler when you’re calculating work or visualizing electric fields. Instead of worrying about tiny movements, you just know: along the surface → no work, across → work is done.

Electric Field & Potential

Electric Field 

The electric field is a measure of the force experienced by a charge in a region of space. The electric potential tells us how much work is needed to bring a unit positive charge to a point in the field.

Key Points:

• Electric field points in the direction where the potential decreases the fastest.
• In regions of constant potential, the electric field is zero.
• Formula linking them: E = - dV / dr

Conductors in Electrostatics

When charges are at rest (called electrostatic equilibrium), conductors behave in a few interesting ways:

1. Electric Field Inside a Conductor is Zero: Free electrons inside the conductor rearrange themselves to cancel any internal electric field. So, no matter what, the inside stays field-free.

2. Excess Charge Stays on the Surface: Any extra charge doesn’t remain inside. It always spreads out on the outer surface of the conductor.

3. Surface is an Equipotential: The entire surface of a conductor has the same potential. Moving a charge along this surface requires no work.

4. Electric Field is Perpendicular to the Surface: The field lines always meet the surface at a right angle. If they weren’t perpendicular, charges would move, breaking equilibrium.

5. Electrostatic Shielding: Conductors can protect the inside from external electric fields. A classic example is a Faraday cage, which shields its contents from lightning or other external influences.

Dielectrics and Polarisation - Chapter 2 Physics Class 12 Notes

A dielectric is a material in which electric charges cannot move freely. However, when a dielectric is placed in an external electric field, its molecules respond to the field. This response is called polarisation.

During polarisation, the positive and negative centres of molecules shift slightly in opposite directions. This creates tiny electric dipoles inside the material. As a result, the effective electric field inside the dielectric reduces.

Types of Dielectrics

• Polar molecules: Molecules that already have a permanent dipole moment.
  Example: Water (H2O)
• Non-polar molecules: Molecules that do not have a permanent dipole moment but become polarised only in an electric field.
  Example: Carbon dioxide (CO2)

Important point: Polarisation increases the charge-storing capacity of a capacitor.

Capacitance - Electrostatic Potential and Capacitance Notes Class 12

Capacitance is the ability of a system to store electric charge for a given potential difference. It is defined as the ratio of charge stored to the potential difference applied. Capacitance equals charge divided by potential difference.

SI Unit

The SI unit of capacitance is farad. One farad means one coulomb of charge stored per volt of potential difference.

Parallel Plate Capacitor

For a parallel plate capacitor:

• Capacitance depends on the area of the plates
• It is inversely proportional to the distance between the plates

If a dielectric is inserted between the plates, the capacitance increases, allowing the capacitor to store more charge.

Energy Stored in a Capacitor 

When a capacitor is charged, electrical energy is stored in it. The energy stored depends on:

• The capacitance of the capacitor
• The potential difference applied
• The amount of charge stored

Energy stored increases when either the voltage or the stored charge increases.

Practical Uses of Stored Energy

• Camera flash units
• Defibrillators
• Electronic circuits and power backup systems

Quick Exam Tips for Electrostatic Potential and Capacitance Class 12 Notes

Electrostatic Potential and Capacitance is one of those Class 12 Physics chapters where formulas look easy, but marks are lost due to small conceptual mistakes. These quick tips will help you revise smartly before exams without overloading your brain.

1. Always understand the meaning before using formulas

Electrostatic potential is about the work done per unit charge, not force. If the question talks about effort or energy, think potential, not electric field.

2. Remember potential is a scalar quantity

This is important for numericals. Potentials from multiple charges are added normally, unlike electric fields which need direction.

3. Distance matters a lot in potential questions

Potential decreases as you move away from a charge. Many one-mark questions test this basic idea directly.

4. For capacitance, focus on what increases or decreases it

Larger plate area increases capacitance. Smaller distance between plates increases capacitance. Adding a dielectric also increases capacitance. These points are frequently asked in reasoning questions.

5. Do not mix up charge, voltage, and capacitance

Capacitance depends on geometry and medium, not on charge or voltage. This is a very common MCQ trap.

6. Energy stored questions are formula-based but conceptual

Energy stored increases if voltage increases or if more charge is stored. Always check what is kept constant in the question.

7. Diagrams can fetch easy marks

Simple diagrams like parallel plate capacitor, equipotential surfaces, or field lines are scoring and should be neat and labelled.

8. Revise NCERT examples and definitions once

Many direct questions in boards come straight from NCERT lines, especially definitions and applications.

Most Important Terms & Definitions

This chapter is formula-based but very scoring if you remember the basics clearly. These quick tips will help you avoid silly mistakes in numericals and theory questions.

• Electrostatic Potential: Electrostatic potential is the amount of work done to bring a unit positive charge from infinity to a point in an electric field. It tells us how much energy a charge has at that point.
• Potential Difference: Potential difference is the difference in electric potential between two points. It decides how easily charges will move from one point to another.
• Electric Field: An electric field is the region around a charge where another charge experiences a force. It always points in the direction of decreasing potential.
• Equipotential Surface: An equipotential surface is a surface where all points have the same electric potential. No work is done when a charge moves along this surface.
• Capacitance: Capacitance is the ability of a system to store electric charge for a given voltage. Higher capacitance means more charge can be stored.‍
• Dielectric: A dielectric is an insulating material that does not allow charges to move freely but reduces the electric field when placed between capacitor plates.

FAQs

Q1. What is electric potential and how is it different from electric field?

Ans. Electric potential is basically how much energy a unit positive charge has at a point. The electric field, on the other hand, tells you the force that would act on a charge at that point. Think of potential as energy, field as push.

Q2. What is capacitance and what does it depend on?

Ans. Capacitance tells us how much charge a capacitor can store for a given voltage. It depends on plate size, distance between plates, and the material (dielectric) used. Bigger plates or closer plates - more capacitance.

Q3. What happens when a dielectric is placed between capacitor plates?

Ans. The dielectric makes the capacitor store more charge by reducing the effective electric field between plates. So, your capacitor basically becomes stronger at holding charge.

Q4. Why is the electric field zero inside a conductor?

Ans. Inside a conductor at rest, free electrons move around to cancel any internal field. So, if you go inside a metal, the electric field is zero - that’s why conductors shield you from external fields.

Q5. Why don’t we do any work moving a charge on an equipotential surface?

Ans. All points on an equipotential surface have the same potential. Since there’s no change in energy, you don’t need to do any work to move a charge along it.