Class-12th Ch-3 (Current Electricity)

📘 Class 12 Physics – Chapter 3: Current Electricity (Notes)

1. Electric Current (I)

• Definition: Flow of electric charge per unit time.

• Formula: I = q / t

• Unit: Ampere (A)

• Current direction: From positive to negative (conventional current)

2. Drift Velocity (v_d)

• Definition: Average velocity of free electrons under electric field.

• Formula: v_d = (e × E × τ) / m

• Also, I = n × A × e × v_d
  Where:

• n = number of electrons per unit volume

• A = area of cross-section

• e = electron charge

• τ = relaxation time

• m = mass of electron

3. Ohm’s Law

• Statement: Current is directly proportional to voltage (if temperature is constant).

• Formula: V = I × R

• R = Resistance (in ohms Ω)

4. Resistance and Resistivity

• Formula: R = ρ × (l / A)
  Where:

• R = resistance

• ρ = resistivity

• l = length of conductor

• A = cross-sectional area

• Resistivity is material-specific. Unit: ohm meter (Ω·m)

5. Effect of Temperature on Resistance

• For metals: Resistance increases with temperature

• For semiconductors: Resistance decreases with temperature

6. Resistors in Series and Parallel

• Series: R_eq = R₁ + R₂ + R₃ + ...

• Parallel: 1 / R_eq = 1 / R₁ + 1 / R₂ + ...

7. EMF and Internal Resistance of a Cell

• EMF (E): Maximum voltage of a cell (no current condition)

• Terminal voltage: V = E - I × r
  Where:

• r = internal resistance

• I = current in circuit

8. Kirchhoff’s Laws

• KCL (Current Law): Sum of currents entering a junction = sum leaving
  ΣI_in = ΣI_out

• KVL (Voltage Law): Sum of voltages in a closed loop = 0
  ΣV = 0

9. Wheatstone Bridge

• Used to find unknown resistance.

• Balanced bridge condition:
  P / Q = R / S
  (If bridge is balanced, no current flows through galvanometer)

10. Meter Bridge

• Based on Wheatstone Bridge principle

• Used to calculate unknown resistance using balancing length

11. Potentiometer

• Measures EMF of cells or compares EMFs

• More accurate than voltmeter (no current drawn from cell)

• Principle: Potential difference ∝ length of wire
  V ∝ L

✅ Key Concepts Summary:

• Current = charge/time

• Resistance depends on material, length, area, temperature

• Ohm’s law applies to ohmic conductors

• Kirchhoff’s laws help solve complex circuits

• Potentiometer and bridge methods are precise tools for measurement