Class Twelve Physics: Electric Charges and Fields

Introduction

• Class began with a welcome by Ganesh
• Class 12 Physics, first chapter: Electric Charges and Fields

Topic: Electric Charge

• A property of matter
• Charged objects experience force when near each other
• Associated with electrons, protons, neutrons

Charging Methods

Rubbing

• Suitable for insulators
• Example: Balloon and wall, plastic comb, and hair

Contact

• Transfer of electrons when in contact
• Example: Spoon and paper pieces

Induction

• Not brought into contact but taken close
• Opposite charges are induced

Charge Properties

• Additivity: Total charge = sum of all individual charges
• Conservation: Total charge remains constant even at the microscopic level
• Quantization
• Electric charge is an integral multiple of the fundamental unit of charge (e)

Coulomb's Law

• Force between charge and distance
• Force is proportional to the product of charges and inversely proportional to the square of the distance

Superposition

• Vector sum of total force due to different charges
• Calculation of final force from the vector sum

Electric Field

• Creation of a field around a charge to attract or repel another charge
• E = F/q
• Unit: Newton/Coulomb
• Vector quantity
• Outward from positive charge, inward towards negative charge

Field Line Properties

• Never intersect
• Do not form closed loops
• Proportional to the amount of charge
• Uniform field has the same field strength everywhere

Dipole

• Combination of two equal and opposite charges
• Axial point: On the dipole line
• Equatorial point: On the perpendicular bisector
• Torque: P X E
• Dipole moment: Q X 2a

Gauss's Law

• Flux through a surface enclosed by charge = charge/ε₀
• Easy calculation for symmetric surfaces
• Useful for uniformly charged sphere, long wire

Electric Flux

• Total number of field lines crossing an area
• Unit: Nm²/C
• Closed surface: flux = ∮E·dA
• Intersecting area normals

Numerical Examples

• Calculation of the field in various situations
• Long wire and sphere-shell case

Tips

• Keep the basic concepts clear
• Practice difficult numerical problems

End

• Special videos soon to clarify numerical problems further