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# CBSE Class 12th Physics Notes for Chapter 1 Electric Charges and Fields

CHAPTER 1 ELECTRIC CHARGES AND FIELDS

• Electric and magnetic forces determine the properties of atoms, molecules and bulk matter.

• There are two types of charges: positive and negative. Charge on a glass rod rubbed with silk is positive and that on a plastic rubbed with fur is negative.

• Like charges repel each other and unlike charges attract each other.

• Conductors allow movement of electric charge through them but insulators do not.

• In metals, the mobile charges are electrons. In electrolytes both positive and negative ions are mobile.

• Electric charge has three basic properties:
• Quantization: Quantization of electric charge means that total charge of a body is always an integral multiple of a basic quantum of charge.
• Additivity: Additiviy of electric charge means that the total charge of a system is the algebraic sum of all individual charges in the system.
• Conservation: Conservation of electric charges means that the total charge of an isolated system remains unchanged with time.

• Coulomb’s Law: This law states that the mutual electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance separating them.

The ratio of electric force and gravitational force between a proton and an electron is

• Superposition principle: The principle based on the property that the forces with which two charges attract or repel each other are not affected by the presence of a third (or more) additional charge(s).

• The electric field E at a point due to charge configuration is the force on a small positive test charge placed at the point divided by the magnitude of the charge.
• An electric field line is a curve drawn in such a way that the tangent at each point on the curve gives the direction of the electric field at that point. Some important properties of field lines are:
• Field lines are continuous curves without any breaks.
• Two field lines cannot cross each other.
• Electrostatic field lines start at positive charges and end at negative charges- they cannot form closed loops.
• The relative closeness of field lines indicates the relative strength of electric field. They crowd near each other in regions of strong electric field and are far apart where the electric field is weak.

• An electric dipole is a pair of equal and opposite charges separated by some distance.

• Gauss’s Law: The flux of electric field through any closed surface S is 1/ε0 times the total charge enclosed by S.