Magnetism – Physics Study Notes
BNYS (Naturopathy & Yoga) – Program Module
1. Simple Explanation
Magnetism is a force that arises from moving electric charges.
- Moving charges (such as electrons in a current) create a magnetic field (B).
- The magnetic field exerts a force on other moving charges or on magnetic materials.
- The same phenomenon that makes a compass needle point north is a result of the Earth’s magnetic field.
Key idea: The world of magnetism is governed by the Lorentz force – the force that a magnetic field exerts on a moving charge.
2. Key Points
| Concept | Symbol | Typical Unit | What It Means |
|---|---|---|---|
| Magnetic field | B | Tesla (T) | Vector field that describes the magnetic influence at a point. |
| Magnetic flux density | B | T | Strength and direction of the field. |
| Magnetic force on a charge | F | Newton (N) | F = q(𝐯 × 𝐁) |
| Magnetic force on a current‑carrying wire | F | N | F = I (𝐿 × 𝐁) |
| Lorentz force magnitude | F = qvB sinθ | ||
| Magnetic dipole moment | µ | A·m² | µ = I·A (current × area of loop) |
| Torque on a dipole | τ | N·m | τ = µ × B |
| Types of magnetic materials | Diamagnetic, Paramagnetic, Ferromagnetic, Antiferromagnetic | ||
| Right‑hand rule | A mnemonic for determining direction of F, B, or I in cross products. |
2.1. Producing Magnetic Fields
| Source | Description | Example |
|---|---|---|
| Moving charges | Current in a wire | Electric circuit |
| Permanent magnets | Alignment of atomic magnetic moments | Bar magnet |
| Electromagnets | Current‑carrying coil | MRI machine |
2.2. Interaction of Fields
| Interaction | Formula | Result |
|---|---|---|
| Force on a moving charge | F = q(𝐯 × 𝐁) | Charge moves in a circular path (cyclotron) |
| Force on a wire | F = I (𝐿 × 𝐁) | Wire experiences a sideways push (used in electric motors) |
| Torque on a dipole | τ = µ × B | Dipole aligns with the field |
2.3. Magnetic Materials
| Material | Response to B | Typical Example |
|---|---|---|
| Diamagnetic | Weakly repelled | Graphite, bismuth |
| Paramagnetic | Weakly attracted | Al₂O₃, Fe₂O₃ |
| Ferromagnetic | Strongly attracted, remanence | Iron, nickel |
| Antiferromagnetic | Cancelled internal moments | MnO |
3. Worked Example
Problem:
A straight copper wire 2 m long carries a current of 5 A. It lies in a uniform magnetic field B = 0.30 T, oriented perpendicular to the wire. What is the magnitude of the magnetic force on the wire?
Solution Steps
-
Identify the relevant formula
For a straight wire:
[ F = I,L,B,\sin\theta ] where (\theta) is the angle between the wire and the magnetic field.
Since the field is perpendicular to the wire, (\theta = 90^\circ) and (\sin\theta = 1). -
Insert the numbers
[ I = 5 \text{ A},\quad L = 2 \text{ m},\quad B = 0.30 \text{ T} ] -
Calculate
[ F = (5 \text{ A})(2 \text{ m})(0.30 \text{ T})(1) = 3 \text{ N} ]
Answer: The wire experiences a magnetic force of 3 N perpendicular to both the current direction and the magnetic field.
4. Quick Revision Summary
- Magnetic field (B) is a vector field created by moving charges.
- Lorentz force: ( \mathbf{F} = q(\mathbf{v}\times\