Class 12 Physics MCQ Test – Electric charges and fields (Chapter 1) | Full Chapter Test
Class 12 Physics – Chapter 1: Electric Charges and Fields (Detailed Notes)
These notes are designed for CBSE, GSEB, JEE & NEET students. Includes theory, formulas, derivation concepts, and numerical-based understanding.
1. Electric Charge
Electric charge is a fundamental property of matter due to which it experiences force in an electric field.
- Types: Positive (+) and Negative (−)
- SI Unit: Coulomb (C)
- Symbol: \( q \)
Properties of Charge
- Additivity: Total charge is algebraic sum
- Conservation: Charge cannot be created or destroyed
- Quantization: \( q = ne \), where \( e = 1.6 \times 10^{-19} C \)
2. Coulomb’s Law
The electrostatic force between two point charges is directly proportional to product of charges and inversely proportional to square of distance.
\( F = \frac{1}{4\pi \varepsilon_0} \cdot \frac{q_1 q_2}{r^2} \)
- \( \varepsilon_0 \) = Permittivity of free space
- Value: \( \frac{1}{4\pi \varepsilon_0} = 9 \times 10^9 \, Nm^2/C^2 \)
Nature of Force:
- Like charges → Repulsion
- Unlike charges → Attraction
3. Principle of Superposition
Net force on a charge is vector sum of forces due to all other charges.
\( \vec{F}_{net} = \vec{F}_1 + \vec{F}_2 + \vec{F}_3 + ... \)
4. Electric Field
Electric field at a point is the force experienced by unit positive charge.
\( E = \frac{F}{q} \)
For point charge:
\( E = \frac{1}{4\pi \varepsilon_0} \cdot \frac{q}{r^2} \)
- Unit: N/C
- Direction: Along force on positive charge
5. Electric Field Lines
- Originate from positive and end at negative charges
- Never intersect
- Density represents strength
- Closer lines → Strong field
6. Electric Dipole
A pair of equal and opposite charges separated by small distance.
Dipole moment: \( \vec{p} = q \cdot \vec{2a} \)
- Direction: From negative to positive
- Unit: C·m
Electric Field of Dipole
Axial Line:
\( E_{axial} = \frac{1}{4\pi \varepsilon_0} \cdot \frac{2p}{r^3} \)
Equatorial Line:
\( E_{equatorial} = \frac{1}{4\pi \varepsilon_0} \cdot \frac{p}{r^3} \)
7. Electric Flux
Electric flux is measure of electric field passing through a surface.
\( \Phi = \vec{E} \cdot \vec{A} = EA \cos \theta \)
- Unit: \( Nm^2/C \)
8. Gauss’s Law
Total electric flux through closed surface is equal to charge enclosed divided by permittivity.
\( \Phi = \frac{Q_{enc}}{\varepsilon_0} \)
Applications
- Infinite line charge
- Infinite plane sheet
- Spherical shell
9. Electric Field due to Different Charge Distributions
1. Infinite Line Charge
\( E = \frac{\lambda}{2\pi \varepsilon_0 r} \)
2. Infinite Plane Sheet
\( E = \frac{\sigma}{2\varepsilon_0} \)
3. Charged Spherical Shell
- Outside: \( E = \frac{1}{4\pi \varepsilon_0} \frac{Q}{r^2} \)
- Inside: \( E = 0 \)
10. Important Exam Points
- Coulomb’s law is inverse square law
- Field is vector quantity
- Dipole field varies as \( \frac{1}{r^3} \)
- Flux depends on area orientation
- Gauss law depends only on enclosed charge
⬇️ Attempt the test given below after revising these notes
VEDANT CLASSES
Class 12 Physics MCQ Test – Electric Charges & Fields (Full Chapter Test)
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