1. Introduction

Electricity and magnetism are two closely related branches of physics that describe the interaction of charged particles and magnetic fields. Together, they form the foundation of electromagnetism, one of the four fundamental forces of nature.

2. Electric Charges

2.1 Properties of Electric Charge

• Two types: positive and negative.
• Like charges repel, unlike charges attract.
• Charge is quantized (smallest unit = electron charge, e = 1.6 × 10⁻¹⁹ C).
• Charge is conserved; it cannot be created or destroyed.

2.2 Coulomb’s Law

The electrostatic force between two point charges is:

F = k (q₁ q₂) / r²

• q₁, q₂ = charges
• r = distance between charges
• k = 9 × 10⁹ Nm²/C²

3. Electric Field

3.1 Definition

The region around a charge where another charge experiences force.

E = F / q

3.2 Properties

• A vector quantity.
• Direction: away from positive charge, toward negative charge.

4. Electric Potential and Potential Difference

4.1 Electric Potential (V)

Work done per unit charge in bringing a charge from infinity to a point:

V = W / q

4.2 Potential Difference

Difference in potential between two points.

V = IR  (Ohm’s Law)

5. Current Electricity

5.1 Electric Current (I)

Flow of electric charge per unit time.

I = Q / t

Unit: Ampere (A)

5.2 Ohm’s Law

V = IR

5.3 Resistance

Opposition to current flow:

R = ρ (L / A)

• ρ = resistivity of material
• L = length of conductor
• A = cross-sectional area

5.4 Electrical Power

P = VI = I²R = V² / R

6. Magnetism

6.1 Magnetic Field (B)

Region around a magnet where magnetic force acts. Field lines go from north pole to south pole.

6.2 Magnetic Force on a Moving Charge

F = q v B sinθ

6.3 Force on a Current-Carrying Conductor

F = B I L sinθ

7. Electromagnetism

7.1 Oersted’s Experiment

Discovered that a current-carrying conductor produces a magnetic field.

7.2 Electromagnetic Induction (Faraday’s Law)

An emf is induced when magnetic flux changes through a circuit:

ε = - dΦB / dt

(Negative sign → Lenz’s Law: induced emf opposes the change)

8. Applications

• Electric motors: electrical → mechanical energy.
• Generators: mechanical → electrical energy.
• Transformers: step up/down AC voltage.
• Electromagnets: used in relays, cranes, MRI machines.
• Power transmission: long-distance transfer via AC.

9. Summary

• Charges interact via Coulomb’s law.
• Electric field = force per unit charge.
• Potential difference drives current in circuits.
• Ohm’s Law: V = IR.
• Magnetic fields interact with moving charges and currents.
• Electromagnetism unites electricity and magnetism.

• Ohm's Law: V = IR (Voltage = Current × Resistance)
• Series Circuits: R_total = R₁ + R₂ + ...
• Parallel Circuits: 1/R_total = 1/R₁ + 1/R₂ + ...
• Magnetic Force: F = BIL sinθ on current-carrying wires
• Electromagnetic Induction: Changing magnetic fields create electric currents