Electromagnetism in NET covers the magnetic force on moving charges and wires, the field of a wire and a solenoid, Faraday's and Lenz's laws, inductance, the transformer, and turning a galvanometer into an ammeter or voltmeter. The maths is light. The marks go to students who get the angle in sinθ right and remember that 1 gauss is 10⁻⁴ T.
Key facts
- Physics is 30% of NET-Engineering, a derived 60 of the 200 MCQs.
- Our syllabus data estimates about 7 questions from this chapter.
- Syllabus: magnetic force on charges and conductors, Faraday's and Lenz's laws, inductance, the transformer, and galvanometer-based instruments.
Source: NUST NET-Engineering composition and the SailToNUST physics syllabus, checked October 2026
What the chapter covers
The chapter has two halves. The first half is about magnetic fields pushing on things: a charge moving through a field, a current-carrying wire, a coil that turns in a motor or galvanometer. The second half runs the other way: a changing field producing an emf, which gives you induction, inductors, generators and transformers.
Formulas you need
Force on a moving charge F = qvB sinθ N
Force on a wire F = BIL sinθ N
Unit of B tesla, T = N A⁻¹ m⁻¹ = Wb m⁻²
1 gauss = 10⁻⁴ T
Charge in a uniform field (v perpendicular to B)
radius r = mv / (qB)
period T = 2πm / (qB) (does not depend on v)
μ₀ = 4π × 10⁻⁷ T m A⁻¹
Long straight wire B = μ₀I / (2πr)
Solenoid B = μ₀nI (n = turns per metre)
Torque on a coil τ = NBIA cosα (α = angle between coil plane and B)
Magnetic flux Φ = BA cosθ (θ from the normal) Wb
Faraday's law ε = −N ΔΦ/Δt
Motional emf ε = BLv
Self-inductance ε = −L ΔI/Δt; energy = ½LI² (L in henry, H)
Inductors: series L₁ + L₂, parallel like resistors
Ideal transformer Vs/Vp = Ns/Np = Ip/Is
Galvanometer (resistance Rg, full-scale current Ig)
to ammeter of range I shunt Rs = IgRg / (I − Ig) in parallel
to voltmeter of range V R = V/Ig − Rg in series
Worked MCQs
1. Force on a wire in gauss. A 40 cm wire carries 3 A in a field of 500 gauss, at 30° to the field. What force acts on it?
(a) 3 × 10⁻⁴ N (b) 3 × 10⁻² N (c) 3 × 10⁴ N (d) 3 × 10² N
B = 500 × 10⁻⁴ = 0.05 T
F = BIL sinθ = 0.05 × 3 × 0.4 × 0.5 = 0.03 N
Answer (b). Without the gauss conversion you get 300 N, option (d).
2. Proton in a magnetic field. A proton moves at 2 × 10⁶ m/s at right angles to a 0.5 T field. What is the radius of its path? (mₚ = 1.67 × 10⁻²⁷ kg, e = 1.6 × 10⁻¹⁹ C)
(a) 4.2 mm (b) 4.2 cm (c) 42 cm (d) 4.2 m
r = mv / (qB) = (1.67 × 10⁻²⁷ × 2 × 10⁶) / (1.6 × 10⁻¹⁹ × 0.5)
= 3.34 × 10⁻²¹ / 8 × 10⁻²⁰ ≈ 0.042 m
Answer (b), about 4.2 cm. Double the speed and the radius doubles, but the time for one circle does not change.
3. Rod moving through a field. A 0.5 m rod moves at 4 m/s at right angles to a 0.2 T field. What emf appears across its ends?
(a) 0.1 V (b) 0.4 V (c) 2.5 V (d) 4 V
ε = BLv = 0.2 × 0.5 × 4 = 0.4 V
Answer (b).
4. Step-down transformer. An ideal transformer has 1000 primary turns on 220 V and 50 secondary turns. The secondary supplies 2 A. What are the secondary voltage and the primary current?
Vs = Vp × Ns/Np = 220 × 50/1000 = 11 V
Ip = Is × Ns/Np = 2 × 50/1000 = 0.1 A
Check: VpIp = 22 W = VsIs
A step-down transformer steps current up. Power in equals power out for an ideal transformer, which is a fast way to check your answer.
5. Galvanometer to ammeter. A galvanometer of resistance 50 Ω gives full-scale deflection at 1 mA. What shunt turns it into an ammeter reading up to 1 A?
(a) 0.05 Ω in parallel (b) 0.05 Ω in series (c) 950 Ω in series (d) 9950 Ω in series
Rs = IgRg / (I − Ig) = (0.001 × 50) / 0.999 ≈ 0.050 Ω, in parallel
Answer (a). To make a 0 to 10 V voltmeter from the same galvanometer instead, add 10/0.001 − 50 = 9950 Ω in series, which is option (d) and the most common wrong pick here.
Traps that cost marks
- Parallel to the field means no force. A charge moving along B has θ = 0 and feels nothing. It keeps going in a straight line.
- The magnetic force does no work. It is always perpendicular to velocity, so it bends the path but never changes the speed or kinetic energy.
- Which angle? In F = qvB sinθ, θ is between v and B. In τ = NBIA cosα, α is between the plane of the coil and B. If the question gives the angle from the normal, use sin instead. Torque is maximum when the coil plane is parallel to the field.
- Lenz's law and the hanging ring. Push a magnet toward a suspended metal ring and the ring moves away. The induced current opposes the change that caused it. Pull the magnet away and the ring follows.
- Transformers need AC. A steady DC current gives no changing flux, so no emf in the secondary.
- Two equal inductors in parallel give L/2, the same as resistors. Capacitors are the odd one out.
Speed tips for the exam
Convert units first, in the margin: gauss to tesla, cm to m, mA to A. The arithmetic after that is usually two multiplications.
For direction questions, pick one hand rule and use it every time. Fleming's left-hand rule gives the force on a current. The right-hand grip rule gives the field around a wire. Mixing them mid-paper is a classic slip.
For transformers, write Ns/Np once and use it for both voltage (multiply) and current (divide). That one ratio answers both parts.
This chapter builds directly on the circuits in current electricity. The NET physics guide covers how much time to give each chapter.
FAQ
What is the difference between Faraday's law and Lenz's law?
Faraday's law gives the size of the induced emf: rate of change of flux linkage. Lenz's law gives its direction: it always opposes the change. The minus sign in ε = −NΔΦ/Δt is Lenz's law.
How do you convert a galvanometer into an ammeter?
Connect a small shunt resistance in parallel with it, so most of the current bypasses the coil. For a voltmeter, connect a large resistance in series instead.
Is electromagnetism hard for NET?
The formulas are simple. Most mistakes come from units and angles, so a set of 30 timed questions usually fixes the problem faster than rereading the chapter.
What is the SI unit of magnetic field?
The tesla, equal to one weber per square metre or one newton per ampere metre. Its dimensions are M T⁻² A⁻¹.
