Playbook
Magnetic Fields Due to Electric Current
98 q - 2.42/paper - 29% HARD. The field of a conductor is half the chapter at 33%, much of it a figure of wires and arcs.
- Questions in the bank
- 98
- q/paper (2024–25 papers)
- 2.42
- Tagged HARD
- 29%
- Subtopics
- 4
Strand: Long Tail
When you’ll see it
A straight wire, a loop, an arc or a solenoid carrying current; a charge moving through a field; two parallel wires; or a moving-coil galvanometer.
How this chapter is tested
98 q at 2.42 a paper, 29% HARD. Magnetic Field of Current-Carrying Conductor is more than half the chapter (52 q, 33%), and much of it is a figure: two wires crossing, an arc joined to straight leads, a loop inside a loop. The fields must be added with their directions, and a field INTO the page and one OUT of it subtract.
The cheap page is Force on Moving Charge in Magnetic Field (12 q, 0% HARD): F = qvB sin θ, and a charge moving perpendicular to B goes round a circle of radius r = mv/qB. Force between parallel wires (14 q, 43%) is the expensive one — currents in the same direction attract.
The galvanometer page (20 q, 25%) connects to Current Electricity: the current sensitivity nBA/k, and a loop's magnetic moment m = NIA.
The sub-skills
The distinct skills inside the chapter, in the order to learn them.
Standard fields
Long wire μ₀I/2πr; centre of a loop μ₀I/2R; arc of angle θ μ₀Iθ/4πR; solenoid μ₀nI. A straight lead pointing at the centre adds nothing.
Force on a moving charge
F = qvB sin θ; circular path r = mv/qB, period 2πm/qB (independent of speed).
Parallel wires
Force per length μ₀I₁I₂/2πd; like currents attract.
Magnetic moment and galvanometer
m = NIA; torque = mB sin θ; current sensitivity = nBA/k.
Traps to expect
Distractor shapes this chapter reuses. The Traps page covers the patterns that cut across chapters.
Adding fields without their directions
In a figure of wires and arcs, each part's field is into or out of the page. Opposite directions subtract. The all-added option is always printed.
Counting the straight leads
A straight wire whose line passes through the point gives zero field there. Only the arcs and the other wires count.
Learn it before you drill it
This chapter has full teaching notes — foundations, worked examples, self-checks and a per-subtopic mastery checkpoint. Read the notes once, then drill subtopic by subtopic below.
Magnetic Fields Due to Electric Current notesDrill every magnetic fields due to electric current question
98 questions from the bank, scoped to 4 bundled subtopics.
Drill one subtopic at a time
The 4 subtopics this playbook covers, in catalog order.
Related playbooks
Often paired with this one — the technique, the trap or the taxonomy overlaps. Drill these next.