A7LAM CSCA ACADEMYCSCA Academy / Physics / Electromagnetism
CSCA Physics · Lesson 9 of 16Magnetic fields
Champ magnétique · المجال المغناطيسي
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Magnetic fields push on moving charges and on wires that carry current. The direction always comes from a hand rule.
Key ideas
- Magnetic flux density B is measured in tesla (T).
- Force on a current (Ampère force): F = BIL when the wire is perpendicular to B.
- Force on a moving charge (Lorentz force): F = qvB, perpendicular to v.
- A charge moving perpendicular to B goes round a circle of radius r = mv/(qB).
Formulas to know
- F = BIL sin θ
- F = qvB sin θ
- Flux: Φ = BS
Worked example
A 0.3 m wire carries 2 A at right angles to a 0.5 T field. Force on the wire?
- Formula: F = BIL
- Replace: F = 0.5 × 2 × 0.3
- Compute: F = 0.3 N
Answer: F = 0.3 N
Practice questions
1. How much work does the Lorentz force do on a moving charge?
qvB · Zero ✓ · ½mv² · It depends on B
The force is always perpendicular to the velocity.
2. A proton (1.6 × 10⁻¹⁹ C) moves at 10⁶ m/s perpendicular to B = 0.5 T. Force?
8 × 10⁻¹⁴ N ✓ · 3.2 × 10⁻¹³ N · 8 × 10⁻¹³ N · 1.6 × 10⁻¹³ N
1.6×10⁻¹⁹ × 10⁶ × 0.5 = 8 × 10⁻¹⁴ N.
3. A wire parallel to B carries a current. The force on it is:
BIL · maximum · zero ✓ · BIL/2
sin 0° = 0.