Moving Charges & Magnetism And Magnetism & Matter: Complete NEET 2026 Revision Guide

Moving Charges and Magnetism and Magnetism and Matter are high-weightage chapters for NEET 2026. Together, they carry at least two direct questions (8 marks) in the examination. Many students lose marks because of confusion in right-hand rules, vector cross products, and formula signs.

This chapter revision breaks down all core concepts and standard formulas into short, simple sentences. Use this for quick revision before test series and exam day.

Moving Charges And Magnetism And Magnetism And Matter 

These chapters are important for NEET 2026 because at least one question is expected from each. The main challenges are choosing the correct direction, applying vector rules, and remembering standard formulas. Careful use of the right-hand rule, Ampere’s law, magnetic-force relations, and material properties is essential for accurate problem solving.

Biot–Savart Law

For a current-carrying wire, the magnetic field is calculated by considering a small current element.

Vector form:

dB = (mu0 / 4pi) (I dl cross r-unit) / r^2

Scalar form:

dB = (mu0 / 4pi) (I dl sin theta) / r^2

Here, theta is the angle between the current element and the line joining it to the observation point.

The vector form may be asked directly in examinations.

Magnetic Field Due To Straight Wires

For a finite straight wire:

B = (mu0 I / 4pi r) (sin theta1 + sin theta2)

Here, r is the perpendicular distance from the point to the wire. If both angles have the same orientation, the corresponding terms are subtracted:

B = (mu0 I / 4pi r) (sin theta1 - sin theta2)

The infinite-wire result is one of the most frequently used formulas in this topic.

The magnetic field direction is found using the right-hand thumb rule:

• Point the right thumb in the direction of current.

• The curled fingers show the magnetic-field direction.

Magnetic field lines around a straight wire are closed circular loops.

At any point lying along the axis of a straight current-carrying wire:

B = 0

Magnetic Field Due To Circular Loops And Arcs

At the centre of a circular loop:

B = mu0 I / 2r

For a coil with n turns:

B = mu0 nI / 2r

Unlike the straight-wire result, this expression contains no pi.

For a circular loop, curl the fingers of the right hand in the direction of current. The thumb gives the field direction

For an arc subtending angle theta:

B = (mu0 I / 2r) (theta / 360 degrees)

Thus:

• Semicircle: B = mu0 I / 4r

• Quarter circle: B = mu0 I / 8r

• 270-degree arc: B = 3mu0 I / 8r

For a loop on its axis:

B = mu0 I r^2 / [2(r^2 + x^2)^(3/2)]

The field is maximum at the centre and decreases symmetrically on both sides.

If the same wire is rewound into n turns:

r-new = r / n

B-new = n^2 B

In symmetric loops, equal and opposite contributions cancel:

Net field at centre = 0

For composite wires, divide the conductor into sections, calculate each contribution, assign directions, and add the fields algebraically.

Ampere’s Circuital Law

Ampere’s law is:

Closed integral of B dot dl = mu0 I-enclosed

Only currents passing through the selected Amperian loop are included. Their signs are assigned using the right-hand rule and added algebraically.

For an ideal solenoid:

B = mu0 nI

Here, n is the number of turns per unit length. At the end:

B-end = mu0 nI / 2

Immediately outside an ideal solenoid, the field is taken as zero.

A toroid is a solenoid bent into a circular shape:

B = mu0 NI / 2pi r-mean

r-mean = (r-inner + r-outer) / 2

Magnetic Field Inside Cylindrical Wires

For a hollow cylindrical wire:

• Inside: B = 0

• Outside: B = mu0 I / 2pi r

For a solid cylindrical wire of radius R:

Inside:

B = mu0 I r / 2pi R^2

Outside:

B = mu0 I / 2pi r

Thus, inside a solid wire, the field increases directly with r; outside, it decreases as 1/r.

Magnetic Force On Moving Charges

The magnetic force on a charge is:

F = q v B sin theta

It is perpendicular to both velocity and magnetic field. Therefore, magnetic force changes the direction of motion but not speed, kinetic energy, or momentum magnitude.

• Parallel or antiparallel motion: force is zero.

• Perpendicular motion: force is maximum, F = qvB.

For perpendicular motion, the path is circular:

r = mv / qB

T = 2pi m / qB

For oblique motion, the particle follows a helix:

v-perpendicular = v sin theta

v-parallel = v cos theta

r = m v-perpendicular / qB

Pitch = v-parallel T

Lorentz Force And Velocity Selector

The Lorentz force is:

F = q(E + v cross B)

In a velocity selector, electric and magnetic forces cancel when:

qE = qvB

Therefore:

v = E / B

At this speed, the particle passes undeflected.

Force On A Current-Carrying Wire

The force is:

F = I L B sin theta

It is zero when the wire is parallel to the field and maximum when perpendicular. For a bent wire, use the straight-line displacement between its ends as the effective length.

Magnetic Moment And Galvanometer

For a current loop:

Magnetic moment = nIA

For a circular loop:

m = I pi r^2

Torque:

Torque = mB sin theta

Potential energy:

U = -mB cos theta

Stable equilibrium occurs when the magnetic moment and the field are parallel. Unstable equilibrium occurs when they are antiparallel.

A moving-coil galvanometer has radial magnetic fields. Its deflection is:

theta = nIAB / C

Current sensitivity:

theta / I = nAB / C

Voltage sensitivity:

theta / V = nAB / CR

Doubling turns doubles current sensitivity, while voltage sensitivity remains unchanged if resistance also doubles.

Magnetism And Matter

The magnetic moment of a bar magnet is:

M = pole strength x effective length

Its direction is from the south pole to the north pole. For a bent magnet, use the direct distance between the poles, not the curved length.

A magnetic monopole does not exist. Therefore, the net pole strength through a closed surface is zero.

Torque and energy of a magnetic dipole are:

Torque = MB sin theta

U = -MB cos theta

Work external = MB(cos theta1 - cos theta2)

Magnetic Materials

Magnetisation is magnetic moment per unit volume:

I = magnetic moment/volume

Magnetic susceptibility is:

I = chi H

Relative permeability is:

mu-r = 1 + chi

Diamagnetic materials have paired electrons and move toward weaker fields. Paramagnetic materials have unpaired electrons that align weakly with an external field. Ferromagnetic materials contain domains that align strongly.

Temperature And Hysteresis

Diamagnetic behaviour is nearly independent of temperature. Paramagnetic magnetism decreases as temperature increases:

Magnetisation is proportional to 1/T

Above the Curie temperature, a ferromagnetic material becomes paramagnetic.

In a hysteresis loop:

• Retentivity is the magnetism remaining when the external field becomes zero.

• Coercivity is the reverse field required to remove magnetism.

• The loop area represents energy loss per unit volume.

Soft iron is used in electromagnets because it has low retentivity, low coercivity, high permeability, and a small hysteresis-loop area.

Key Tips to Solve Moving Charges & Magnetism and Magnetism & Matter Important Questions

Use these tips to solve direction and formula-based questions faster and more accurately.

• Keep both right-hand rules separate. For a straight wire, the thumb shows current, and the curled fingers show the field. For a loop, the curled fingers show current, and the thumb shows the field.

• Learn the standard wire results. Infinite wire: B = μ₀I / 2πr. Semi-infinite wire: B = μ₀I / 4πr.

• Use the fraction method for arcs. B = (μ₀I / 2r) × (θ / 360°), which gives μ₀I / 4r for a semicircle and μ₀I / 8r for a quarter circle.

• Remember that magnetic force only changes direction. Speed, kinetic energy, and momentum magnitude stay constant, and the circular path has r = mv / qB.

• Connect hysteresis to practical use. The loop area is the energy loss per unit volume, which is why soft iron is used in electromagnets.

Revising Moving Charges and Magnetism and Magnetism and Matter requires a clear understanding of magnetic field formulas, force, circular motion, magnetic materials, and hysteresis. Practise NEET-level MCQs and revise key formulas to strengthen your Physics preparation. For dedicated guidance, concept revision, and regular practice, explore PW’s NEET 2026 preparation batches 

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