6 · IGCSE Physics
Electromagnet strength
Change the current, the number of turns and the core material, and watch how the magnetic field and the number of paper clips lifted respond.
B = μ₀ μᵣ N I / L
μ₀ = 4π × 10⁻⁷ T m A⁻¹, L = 5.00 cm
iron: μᵣ ≈ 100, saturates near 1.6 T
air: μᵣ = 1
clips lifted ∝ B²
Field in core: –
N × I: –
Clips lifted: –
Top end: –
Try this
- With the iron core and N = 100, find the current that lifts 5 clips. Now set N = 200: what current lifts 5 clips?
- Set I = 5.0 A and N = 200, then remove the core. Compare the field strength and the clips lifted.
- At N = 200, raise the current in 1 A steps. Does each step add the same number of clips? Why not near the top?
Exam tip: when asked how to make an electromagnet stronger, give all three ways – more current, more turns, a soft-iron core – and remember that reversing the current swaps the poles but the clips are still attracted.