Magnetism
Magnetism questions test magnetic poles, flux, flux density, electromagnets, and forces on current-carrying conductors. You must distinguish flux from flux density and apply the motor-effect relationship correctly.
Core rules
- Like magnetic poles repel, unlike poles attract; field lines run from north to south outside a magnet.
- Magnetic flux Φ is measured in webers (Wb); flux density B is flux per unit area, measured in tesla (T), where 1 T = 1 Wb/m².
- Permeability describes a material's ability to support magnetic flux; ferromagnetic materials have high permeability.
- An electromagnet is a coil with current; adding turns or current strengthens the field.
- Force on a conductor: F = BIL sinθ, maximum when the conductor is perpendicular to the field.
- Fleming's left-hand rule predicts force direction for a current in a magnetic field.
Worked example
A 0.40 m conductor carrying 10 A sits perpendicular to a 0.50 T field. Find the force. F = BIL = 0.50 × 10 × 0.40 = 2.0 N. Reasoning: perpendicular means sinθ = 1. If the conductor were parallel to the field, the force would be zero.
Traps
- Mixing webers and teslas; webers measure flux, teslas measure flux density.
- Forgetting the sinθ factor when the conductor is not perpendicular.
- Assuming magnetic poles can be isolated as single poles.
- Confusing conventional current direction with electron flow in the left-hand rule.