Class 12 Physics Electric Charges and Fields Notes

1.1 Introduction

Electricity plays an important role in our daily life. Phenomena such as lightning, electric current, and attraction between charged objects are explained using the concept of electric charge. This chapter introduces the basic ideas of electric charges, forces between them, and electric fields.

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1.2 Electric Charge

Electric charge is a fundamental property of matter responsible for electrical interactions.

• There are two types of charges:
  • Positive charge
  • Negative charge
• Like charges repel each other.
• Unlike charges attract each other.
• The SI unit of electric charge is the coulomb (C).
• Charge on an electron = −1.6 × 10⁻¹⁹ C
• Charge on a proton = +1.6 × 10⁻¹⁹ C

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1.3 Conductors and Insulators

Materials are classified based on their ability to allow electric charges to move.

• Conductors: Allow free movement of charges
  • Examples: Copper, aluminium, silver
• Insulators: Do not allow free movement of charges
  • Examples: Rubber, glass, plastic

In conductors, electrons are loosely bound, whereas in insulators they are tightly bound to atoms.

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1.4 Basic Properties of Electric Charge

Electric charge has the following important properties:

1. Additivity of Charge

Total charge is the algebraic sum of individual charges.

2. Conservation of Charge

Charge can neither be created nor destroyed, only transferred.

3. Quantisation of Charge

Charge exists in discrete amounts.$$q = ne$$q=ne

where
n = integer
e = charge of one electron

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1.5 Coulomb’s Law

Coulomb’s law describes the force between two point charges.

Statement:
The force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.$$F = k \frac{q_1 q_2}{r^2}$$F=kr2q1​q2​​

where
k = 1 / (4πε₀)
ε₀ = permittivity of free space

• The force acts along the line joining the charges.
• It is attractive for unlike charges and repulsive for like charges.

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1.6 Forces between Multiple Charges

When more than two charges are present, the principle of superposition is used.

Principle of Superposition:
The net force on a charge is the vector sum of forces due to all other charges acting independently.$$\vec{F}_{net} = \vec{F}_1 + \vec{F}_2 + \vec{F}_3 + …$$Fnet​=F1​+F2​+F3​+…

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1.7 Electric Field

Electric field is a region around a charged object where it exerts force on another charge.

Electric field intensity (E):$$E = \frac{F}{q}$$E=qF​

• SI unit: N/C
• Direction of electric field is the direction of force on a positive test charge.

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1.8 Electric Field Lines

Electric field lines are imaginary lines used to represent electric fields.

Properties of Electric Field Lines

• Originate from positive charges and end on negative charges.
• Never intersect each other.
• Closer lines indicate stronger electric field.
• They are perpendicular to the surface of a conductor.

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1.9 Electric Flux

Electric flux measures the number of electric field lines passing through a surface.$$\Phi_E = \vec{E} \cdot \vec{A}$$ΦE​=E⋅A

• SI unit: N m²/C
• Flux depends on electric field, area, and orientation of the surface.

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1.10 Electric Dipole

An electric dipole consists of two equal and opposite charges separated by a small distance.

Dipole Moment

$$\vec{p} = q \vec{d}$$p​=qd

• Direction: From negative to positive charge
• SI unit: C·m

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1.11 Dipole in a Uniform External Field

When an electric dipole is placed in a uniform electric field:

• It experiences a torque but no net force.
• Torque tends to align the dipole with the field.

$$\tau = pE \sin\theta$$τ=pEsinθ

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1.12 Continuous Charge Distribution

When charge is distributed continuously over a body, it is described using:

• Linear charge density (λ)
• Surface charge density (σ)
• Volume charge density (ρ)

These help in calculating electric fields of extended objects.

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1.13 Gauss’s Law

Gauss’s law relates electric flux through a closed surface to the charge enclosed.$$\oint \vec{E} \cdot d\vec{A} = \frac{Q_{enclosed}}{\varepsilon_0}$$∮E⋅dA=ε0​Qenclosed​​

It is especially useful for calculating electric fields in symmetrical situations.

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1.14 Applications of Gauss’s Law

Gauss’s law is applied to find electric fields of:

• Infinitely long straight charged wire
• Uniformly charged spherical shell
• Uniformly charged solid sphere

It simplifies calculations when symmetry is present.

Questions with One/Two-Word Answers

1. Fundamental Concepts

1. Which particle carries negative charge? – Electron
2. Which particle carries positive charge? – Proton
3. Charge of a neutron? – Zero
4. SI unit of charge? – Coulomb
5. Law stating charge cannot be created or destroyed? – Conservation
6. Smallest unit of charge? – Electron charge
7. Material that allows charge to flow? – Conductor
8. Material that resists flow of charge? – Insulator
9. Material intermediate between conductor and insulator? – Semiconductor
10. Type of charges that repel each other? – Like charges
11. Type of charges that attract each other? – Unlike charges

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2. Coulomb’s Law & Force

12. Law giving force between two charges? – Coulomb law
13. Force between charges is directly proportional to? – Product of charges
14. Force between charges is inversely proportional to? – Square of distance
15. Principle used to calculate net force on a charge? – Superposition principle
16. Unit of force between charges? – Newton

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3. Electric Field

17. Region around a charge where force is experienced? – Electric field
18. Electric field due to a point charge formula? – E = kQ/r²
19. SI unit of electric field? – Newton per Coulomb
20. Electric field inside a conductor in electrostatics? – Zero
21. Electric field lines emerge from which charge? – Positive charge
22. Electric field lines terminate on which charge? – Negative charge

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4. Electric Flux & Gauss’s Law

23. Flux through a closed surface is proportional to? – Enclosed charge
24. Law connecting flux and enclosed charge? – Gauss law
25. Shape of surface commonly used in Gauss’s law? – Sphere / Cylinder / Plane
26. Symbol for electric flux? – ΦE
27. SI unit of electric flux? – Nm²/C

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5. Electric Potential

28. Work done per unit charge? – Potential
29. Surface on which potential is constant? – Equipotential
30. Potential due to point charge formula? – V = kQ/r
31. Potential energy per unit charge? – Potential
32. Quantity whose gradient gives electric field? – Potential
33. Potential inside a conductor in electrostatics? – Constant

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6. Dipoles

34. System of equal and opposite charges? – Dipole
35. Distance between charges in a dipole? – Separation
36. Dipole moment formula? – p = q × 2a
37. Torque on dipole in uniform field? – pE sinθ
38. Potential at axial line of dipole? – Axial potential
39. Potential at equatorial line of dipole? – Equatorial potential

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7. Charging Methods

40. Charging by touching? – Conduction
41. Charging without touching? – Induction
42. Redistribution of charge on a conductor? – Polarization
43. Device to store electric charge? – Capacitor
44. Energy stored in electric field? – Electrostatic energy

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8. Miscellaneous

45. Constant in Coulomb law? – k
46. Relative permittivity of medium? – Dielectric constant
47. Shielding from external electric field? – Faraday cage
48. Type of quantity for potential? – Scalar
49. Type of quantity for electric field? – Vector
50. Example of natural electric dipole? – Water molecule