MHT-CET Physics · Teaching notes
Magnetic Fields Due to Electric Current — MHT-CET Physics
Magnetic Fields Due to Electric Current has 98 past-year questions in the MHT-CET bank, and 28 of them — more than a quarter — are HARD. It runs in two directions: a magnetic field pushes on a moving charge, a current-carrying wire and a current loop; and a current makes a magnetic field — around a straight wire, at the centre of a coil or an arc, inside a solenoid. Most of the HARD questions are the second kind: a wire bent into arcs and straight pieces, and fields from several parts added as vectors. Every PYQ is tagged.
Every subtopic, worked example, formula and trap in one printable document — answers shown, ready to share.
Subtopic notes
Force on a Moving Charge, and Circular Motion in a Field
12 PYQsA charge q moving with velocity v through a magnetic field B feels F = q(v × B), perpendicular to both, so it does no work and only bends the path; moving across a uniform field the charge goes round a circle of radius mv/(qB), and adding an electric field gives the full Lorentz force qE + q(v × B).
Force on a Current-Carrying Wire, and Between Parallel Wires
14 PYQsA wire of length L carrying current I in a field B feels F = I(L × B); two long parallel wires each sit in the other's field, so they attract when their currents run the same way and repel when opposite, with a force per unit length μ₀I₁I₂/(2πd).
Magnetic Moment, Torque on a Loop, and the Galvanometer
20 PYQsA current loop behaves like a small magnet of moment m = NIA, feeling a torque m × B in a field; for a fixed length of wire the torque is largest when the loop is a circle, an orbiting electron is a tiny loop whose moment is e/(2m) times its angular momentum, and a galvanometer becomes an ammeter when a small shunt takes most of the current.
Magnetic Field of Wires, Coils, Arcs, Solenoids and Toroids
52 PYQsEvery piece of current makes a field by the Biot–Savart law; added up, a long straight wire gives μ₀I/(2πd) in circles around it, a circular coil gives μ₀NI/(2R) at its centre, an arc gives the same times its fraction of a circle, a half-infinite wire gives half the long-wire value, and a long solenoid or a toroid gives μ₀nI inside.
Formula & revision sheet
10 formulas · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formula & revision sheet
10 formulas · 12 gotchas across all subtopics — the exam-eve cheat-sheet
Formulas (2)
Watch out for (2)
- Thinking a magnetic field can change a charge's speed→ The Magnetic Force on a Moving Charge
- Scaling the radius with the energy→ Circular Motion in a Magnetic Field
Formulas (2)
Watch out for (2)
- Expecting the near and far sides to cancel→ Force on a Straight Conductor
- Getting attraction and repulsion backwards→ Parallel Wires: Attraction, Repulsion and Balance
Formulas (2)
Watch out for (2)
- Keeping the turns fixed when a coil is rewound→ Magnetic Moment and the Torque on a Loop
- Using the total current over the galvanometer current→ Converting a Galvanometer: the Shunt
Formulas (4)
Watch out for (6)
- Adding the fields of like currents at the midpoint→ Field of Straight Wires
- Adding fields of perpendicular coils directly→ Field of a Circular Coil: Centre, Axis, and Rotating Charges
- Keeping the radius when a wire is rewound→ Field of a Circular Coil: Centre, Axis, and Rotating Charges
- Counting a straight piece that points at the centre→ Arcs and Bent Wires
- Adding every piece without checking its direction→ Arcs and Bent Wires
- Confusing B and H in a solenoid→ Solenoids, Toroids and Displacement Current
PYQ weightage by concept
10 concepts · 98 PYQs — where the marks actually sit, so you know what to drill first
PYQ weightage by concept
10 concepts · 98 PYQs — where the marks actually sit, so you know what to drill first
| Concept | PYQs | Share |
|---|---|---|
| Circular Motion in a Magnetic Field | 7 | 7% |
| The Magnetic Force on a Moving Charge | 5 | 5% |
| Concept | PYQs | Share |
|---|---|---|
| Parallel Wires: Attraction, Repulsion and Balance | 11 | 11% |
| Force on a Straight Conductor | 3 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Magnetic Moment and the Torque on a Loop | 17 | 17% |
| Converting a Galvanometer: the Shunt | 3 | 3% |
| Concept | PYQs | Share |
|---|---|---|
| Field of a Circular Coil: Centre, Axis, and Rotating Charges | 20 | 20% |
| Arcs and Bent Wires | 13 | 13% |
| Field of Straight Wires | 12 | 12% |
| Solenoids, Toroids and Displacement Current | 7 | 7% |
Test yourself on Magnetic Fields Due to Electric Current
20 past MHT-CET questions from this chapter, timed at 18 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.