Physics Tutor in Laxmi Nagar Nagpur – Dielectric in Capacitor With Battery and Without Battery

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If you are searching for Physics Tutor in Laxmi Nagar Nagpur, then Kumar Sir can help you understand difficult Class 12 Physics concepts like capacitor, dielectric, electric field, potential difference, charge and energy in a very simple way. Many students memorize formulas, but in NEET, JEE and board exams, the question is usually conceptual: what changes when dielectric is inserted in a capacitor?

This topic is very important for IIT Physics Tutor, JEE Physics Tutor, NEET Physics Tutor, IB Physics Tutor, IGCSE Physics Tutor, A-Level Physics Tutor, AP Physics Tutor and CBSE Physics Tutor.

Basic Concept of Capacitor and Dielectric

A capacitor stores charge and energy in the form of electric field. For a parallel plate capacitor:

C = ε0 A / d

When dielectric of dielectric constant K is inserted fully between the plates:

C’ = K C

So, the capacitance increases K times.

Here:

C = original capacitance

C’ = new capacitance after dielectric

K = dielectric constant

A = area of plates

d = separation between plates

ε0 = permittivity of free space

Now the main question is: what happens to charge, potential difference, electric field and energy?

There are two cases:

1. Capacitor connected with battery while dielectric is inserted

2. Capacitor charged first, battery removed, then dielectric inserted

Case 1: Battery Connected, Then Dielectric Inserted

In this case, capacitor remains connected to the battery. Battery maintains constant potential difference.

So:

Potential difference remains constant

Formula:

V’ = V

Since capacitance increases:

C’ = K C

Charge on capacitor is:

Q = C V

After dielectric:

Q’ = C’ V

Q’ = K C V

Q’ = K Q

So charge increases K times.

Electric field between plates:

E = V / d

Since V and d remain constant:

E’ = E

So electric field remains same because battery maintains voltage.

Energy stored:

U = 1/2 C V²

After dielectric:

U’ = 1/2 C’ V²

U’ = 1/2 K C V²

U’ = K U

So energy increases K times.

Table: Battery Connected Case

Simple Kumar Sir style logic:

When battery is connected, battery keeps voltage fixed. Dielectric increases capacitance. Since Q = CV, charge increases. Since U = 1/2 CV², energy also increases.

Case 2: Battery Removed, Then Dielectric Inserted

In this case, capacitor is first charged by battery. Then battery is removed. After that dielectric is inserted.

Now capacitor is isolated. No charge can enter or leave.

So:

Charge remains constant

Formula:

Q’ = Q

Capacitance increases:

C’ = K C

Potential difference:

V = Q / C

After dielectric:

V’ = Q / C’

V’ = Q / KC

V’ = V / K

So potential difference decreases K times.

Electric field:

E = V / d

Since potential difference decreases:

E’ = E / K

Energy stored:

U = Q² / 2C

After dielectric:

U’ = Q² / 2C’

U’ = Q² / 2KC

U’ = U / K

So energy decreases K times.

Table: Battery Removed Case

Simple Kumar Sir style logic:

When battery is removed, charge is trapped on plates. Dielectric increases capacitance. Since V = Q/C, voltage decreases. Since U = Q²/2C, energy also decreases.

Combined Comparison Table

Why This Concept Is Important

This concept is asked again and again in NEET, JEE Main, JEE Advanced and CBSE Class 12 Physics. Students often make mistakes because they do not identify whether battery is connected or removed.

Kumar Sir teaches one golden rule:

If battery is connected, voltage is constant.

If battery is removed, charge is constant.

After this, every result comes automatically.

Corona Charge and Need of Earthing

Corona charge is produced when electric field near a sharp conductor becomes very strong. In high voltage systems, sharp points, edges or wires can ionize nearby air. Due to this ionization, air becomes conducting and charge starts leaking into the surrounding air. This effect is called corona discharge.

In simple words, when charge density becomes very high at sharp points, electric field becomes strong. If electric field is greater than the breakdown strength of air, air molecules get ionized and charge starts escaping.

Formula idea:

E = V / d

If electric field becomes very high, air breakdown can happen.

Why Earthing is Needed?

Earthing means connecting a conductor to the ground. Earth is considered a very large conductor and can absorb or supply large amount of charge without changing its potential much.

Earthing is needed because it gives extra charge a safe path to go into the ground. If a body or building gets excess charge due to lightning or leakage current, earthing sends that charge safely to earth.

Lightning Arrester in Buildings

Tall buildings are more likely to be struck by lightning. Lightning carries huge charge and very high current. To protect buildings, we use a lightning arrester.

A lightning arrester is a metal rod placed at the top of the building and connected to the ground by a thick conducting wire. When lightning strikes, charge flows through the metal rod and wire directly into the earth.

So the building is protected because charge does not pass through walls, people or electrical devices.

Important point:

Lightning arrester provides a low resistance path to ground.

Kumar Sir Style Concept

Sharp points collect more charge density, so electric field becomes strong near them. That is why lightning arresters are pointed at the top. They attract or receive lightning charge and safely transfer it to earth.

In short:

High charge → strong electric field → air ionization → corona discharge

Lightning arrester → safe path → charge goes to ground

Earthing protects building, people and electrical appliances.

Physics Tutor in Laxmi Nagar Nagpur for Nearby Students

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How Kumar Sir Teaches This Topic

Kumar Sir does not teach Physics only by formulas. He first builds the concept:

• What is capacitance?

• Why dielectric increases capacitance?

• Why battery keeps voltage constant?

• Why isolated capacitor keeps charge constant?

• Why energy increases in one case and decreases in another?

• How to solve NEET and JEE numerical questions quickly?

Kumar Sir has 30 years teaching experience and teaches one-to-one online Physics with proper concept clarity, doubt solving, numerical practice and test-based preparation.

Final Words

If you live in Laxmi Nagar Nagpur and Physics feels difficult, especially capacitor, dielectric, electrostatics and energy stored in capacitor, then learn from Kumar Sir.

For serious preparation, choose Kumar Sir for Physics Tutor in Laxmi Nagar Nagpur.

Call / WhatsApp: +91-9958461445
Website: Kumar Physics Classes

1. What is Capacitance?

Capacitance means the charge storing ability of a capacitor. If a capacitor can store more charge at the same potential difference, its capacitance is more.

Formula:

C = Q / V

Here:

C = capacitance
Q = charge stored
V = potential difference

For a parallel plate capacitor:

C = ε0 A / d

So capacitance depends on:

• Area of plates

• Distance between plates

• Medium between plates

Larger plate area means more capacitance. Smaller distance between plates means more capacitance.

2. Why Dielectric Increases Capacitance?

When dielectric is inserted between capacitor plates, dielectric molecules get polarized. Positive and negative charge centres shift slightly. Due to this polarization, an internal electric field is produced opposite to the original electric field.

Because of this, net electric field decreases. When electric field decreases, potential difference also decreases.

Formula idea:

V = E d

If E decreases, V decreases.

But capacitance is:

C = Q / V

For same charge, if V decreases, C increases.

With dielectric:

C’ = K C

Here K is dielectric constant.

3. Why Battery Keeps Voltage Constant?

A battery is a constant potential difference source. Its job is to maintain fixed voltage between its terminals.

If capacitor is connected to battery and dielectric is inserted, capacitance increases. Now capacitor can store more charge at the same voltage. So battery supplies extra charge to the capacitor.

That is why in battery connected case:

V remains constant

C increases

Q increases

Because:

Q = C V

4. Why Isolated Capacitor Keeps Charge Constant?

When capacitor is charged and then battery is removed, the capacitor becomes isolated. There is no external path for charge to enter or leave.

So total charge on plates remains fixed.

That is why in isolated capacitor case:

Q remains constant

If dielectric is inserted:

C increases

But:

V = Q / C

Since Q is constant and C increases, V decreases.

5. Why Energy Increases in One Case and Decreases in Another?

Energy stored in capacitor:

U = 1/2 C V²

Battery Connected Case

Battery keeps voltage constant.

Dielectric increases capacitance:

C’ = K C

So energy becomes:

U’ = K U

Energy increases because battery supplies extra charge and extra energy.

Battery Removed Case

Charge remains constant.

Energy formula:

U = Q² / 2C

Dielectric increases capacitance:

C’ = K C

So energy becomes:

U’ = U / K

Energy decreases. The lost energy is used in pulling the dielectric inside the capacitor or appears as mechanical work/heat.

6. How to Solve NEET and JEE Numerical Questions Quickly?

Use this shortcut:

Step 1: Identify battery condition

If battery is connected:

V = constant

If battery is removed:

Q = constant

Step 2: Write capacitance change

With dielectric:

C’ = K C

Step 3: Use correct formula

If V constant, use:

Q = C V

U = 1/2 C V²

If Q constant, use:

V = Q / C

U = Q² / 2C

Final Shortcut Table