Magnetic Effects of Current and Magnetism
What this domain tests
Magnetic Effects of Current and Magnetism covers the Biot–Savart law, Ampere's law, force on moving charges and conductors, torque on current loops, galvanometers and magnetic dipole behaviour.
Core rules
- Straight wire: B = μ0I/(2πr). Centre of a circular loop: B = μ0I/(2R). Solenoid: B = μ0nI.
- Force on a charge: F = qvB sinθ; on a wire: F = BIL sinθ. Direction by Fleming's left-hand rule.
- Parallel wires: F/L = μ0I1I2/(2πd), attractive for currents in the same direction.
- Torque on a loop: τ = NIAB sinθ; magnetic moment m = NIA.
- Moving-coil galvanometer: deflection proportional to current; a shunt makes an ammeter, a series multiplier makes a voltmeter.
Worked example
Two long parallel wires 0.10 m apart carry 5 A and 10 A in the same direction. Force per unit length = μ0I1I2/(2πd) = (4π×10⁻⁷ × 5 × 10)/(2π × 0.10) = 1.0×10⁻⁴ N m⁻¹. The force is attractive because the currents are parallel.
Traps
- Using sinθ = 1 when the charge moves parallel to B; the force is then zero.
- Getting the direction wrong for a negative charge by forgetting to reverse the rule.
- Confusing magnetic moment with torque.
- Forgetting that a galvanometer shunt is a small resistance connected in parallel.