Moving Charges and Magnetism – Class 12 Physics Notes (CBSE | JEE | NEET)
Introduction
This chapter extends electrostatics to situations where charges are in motion. It introduces magnetic effects of electric current, force on moving charges, magnetic fields, and applications such as motors and galvanometers. It is highly important for CBSE, JEE, and NEET.
1. Magnetic Field (Concept)
Definition
Magnetic field is the region around a magnet or current-carrying conductor where a magnetic force can be experienced.
Representation
Magnetic field lines form closed loops.
Diagram: Magnetic Field Lines Around Bar Magnet
N -------- S
| |
( ) → → → ( )
| |
← ← ← ← ← ←
2. Magnetic Force on Moving Charge (Lorentz Force)
Formula
F = q(v × B)
Magnitude:
F = qvB sinθ
Direction
Given by right-hand rule.
Diagram: Force Direction
v →
B ↑
F ⊙ (out of plane)
3. Motion in Magnetic Field
Case 1: v ⟂ B
Circular motion
Radius:
r = mv / qB
Time period:
T = 2πm / qB
Case 2: v at angle
Helical path
4. Biot–Savart Law (Derivation Concept)
Statement
Magnetic field due to current element is proportional to current and inversely proportional to square of distance.
Formula:
dB = (μ₀/4π) (Idl × r̂) / r²
5. Magnetic Field Due to Straight Conductor
Result
B = μ₀I / 2πr
Diagram
I ↑
|
(•) → circular field lines
6. Magnetic Field on Axis of Circular Loop
Formula
B = (μ₀IR²) / (2(R² + x²)^(3/2))
7. Ampere’s Circuital Law
Statement
Line integral of magnetic field around closed loop equals μ₀ times current enclosed.
∮B·dl = μ₀I
8. Force on Current Carrying Conductor
Formula
F = I (L × B)
Magnitude:
F = ILB sinθ
9. Torque on Current Loop
Formula
τ = NIAB sinθ
Where:
- N = turns
- A = area
10. Moving Coil Galvanometer
Principle
Based on torque on current loop.
Deflection
θ ∝ I
11. Conversion of Galvanometer
Into Ammeter
Low resistance in parallel
Into Voltmeter
High resistance in series
12. Magnetic Field of Solenoid
Formula
B = μ₀nI
Uniform inside, zero outside.
13. Cyclotron
Principle
Charged particle accelerated using magnetic field.
Frequency
f = qB / 2πm
Important Formula Sheet
- F = qvB sinθ
- r = mv/qB
- B = μ₀I/2πr
- τ = NIAB sinθ
JEE / NEET Focus
- Circular motion in magnetic field
- Force on conductor
- Ampere law problems
CBSE Board Strategy
- Draw diagrams clearly
- Learn derivations
- Practice numericals
Conclusion
This chapter links electricity and magnetism and forms the basis for electromagnetic theory.