- Define and explain Magnetic Fields in your own words
- Use key terms such as field accurately
- Apply what you have learned to new examples and questions
- Avoid the common mistakes learners make with this topic
This lesson focuses on Magnetic Fields: finding field patterns and the force F = BIl on current-carrying wires.
Finding field patterns and the force F = BIl on current-carrying wires.
Key ideas
Moving charges feel magnetic forces
A current-carrying wire in a magnetic field feels F = BIl when perpendicular to the field. Fleming's left-hand rule gives the direction: First finger field, seCond finger current, thuMb motion. Charged particles curve in magnetic fields — the principle behind cyclotrons, mass spectrometers and the aurora.
Fields are mapped with lines
Field lines show direction — the way a test mass or positive charge would be pushed — and spacing shows strength: closer lines mean a stronger field. Gravitational and electric field lines radiate from masses and charges; magnetic field lines form closed loops, emerging from north poles and entering south poles.
Key term — field: A region where an object experiences a non-contact force, mapped by field lines.
Use Fleming's left-hand rule: magnetic field into the page, current to the right. Which way is the force?
Upwards — with the first finger pointing into the page and the second finger to the right, the thumb points up.
Answer: Upwards — with the first finger pointing into the page and the second finger to the right, the thumb points up.
- Drawing field lines crossing Field lines never cross — a crossing would mean two field directions at one point, which is impossible.
- Confusing electric potential with field strength Potential (V) is energy per unit charge; field strength (V/m) is its gradient — E = −ΔV/Δx.
Practice
0.5 × 2 × 0.3 = 0.3 N.
Radial lines pointing straight outwards from the charge, getting further apart with distance as the field weakens.
(6.67 × 10⁻¹¹ × 6.0 × 10²⁴) ÷ (6.4 × 10⁶)² = 4.0 × 10¹⁴ ÷ 4.096 × 10¹³ ≈ 9.8 N/kg.
Potential is constant along an equipotential, so ΔV = 0 and W = QΔV = 0.
Quick check
Which of these best defines "field"?
A satellite orbits at twice Earth's radius from the centre. How does g there compare to the surface value?
- Magnetic Fields: finding field patterns and the force F = BIl on current-carrying wires.
- Moving charges feel magnetic forces: A current-carrying wire in a magnetic field feels F = BIl when perpendicular to the field.
- field strength: Force per unit mass (N/kg) or per unit charge (N/C) at a point in the field.
- Watch out for: drawing field lines crossing