Electromagnetism: MDCAT Notes and MCQs
Magnetic flux density B
- Defined from force on a current-carrying conductor: F = ILB sin θ → B = F/(IL) when ⊥.
- SI unit tesla (T) = N A−1 m−1 = Wb m−2. 1 T = 104 gauss. Earth's field ≈ 5 × 10−5 T.
- 1 tesla: field in which a 1 m conductor carrying 1 A at right angles experiences 1 N force.
Magnetic flux Φ
- Φ = B · A = BA cos θ (θ between B and the area vector/normal). Scalar; unit weber (Wb) = T m² = N m A−1.
- Maximum (BA) when B is ⊥ to the surface (parallel to the area vector); zero when B is parallel to the surface.
Force on a moving charge
- F = q(v × B), magnitude F = qvB sin θ. Zero if the charge is at rest or moves parallel/antiparallel to B; maximum when ⊥.
- Force is always ⊥ to velocity → does no work, changes direction but not speed or KE.
- Particle entering ⊥ to a uniform field moves in a circle: qvB = mv²/r → r = mv/(qB); frequency f = qB/2πm, period T = 2πm/(qB) — independent of speed (cyclotron principle).
- e/m of electron: e/m = v/(Br) = 2V/(B²r²) (J.J. Thomson). If v is at an angle, path is a helix.
- Direction: Fleming's left-hand rule / right-hand rule (reverse for negative charges).
PMDC learning outcomes for this unit
- Magnetic field & force: 10.1 Define magnetic flux density and its units.
- Magnetic field & force: 10.2 Describe magnetic flux Φ as scalar product of B and A: Φ = B⊥A = B·A.
- Magnetic field & force: 10.3 Describe quantitatively the path of a charged particle shot into a magnetic field perpendicular to the field.
- Magnetic field & force: 10.4 Explain that a force may act on a charged particle in a uniform magnetic field.