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CBSE Class 12 · Physics

Moving Charges and Magnetism

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Moving Charges and Magnetism” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 9 concepts

1Magnetic Force

Magnetic force on a moving charge: F = q v B sinθ as taught, perpendicular to v and B. A rest charge feels no magnetic force. Force is a sideways push, not a speed-up along v (that was electric).

Using F=qE as this heading is a steal.

Figure. When v is across B, F stands at a right angle to both. It can turn the charge; it cannot change the speed.

How it works

  1. Need q, v, B, and the angleThe inputs.
  2. Form q v B sinθ, direction as taught (right-hand)The force.
  3. Keep v=0 as F=0The caution.

2Motion in a Magnetic Field

Motion in B: a perpendicular B bends the path to a circle as taught, r = m v / (q B). Helix if a parallel part sits. Motion is a bend, not a stop.

A straight path with only perpendicular B is a miss.

Figure. F always points at the centre, so the path is a circle of radius mv/qB. Speed stays; only the direction turns. The circle itself is not drawn.

How it works

  1. Split v into parallel and perpendicularThe parts.
  2. Write r=mv/(qB) for the bendThe circle.
  3. Keep a helix if both parts sitHonest.

3Magnetic Field due to a Current Element, Biot-Savart Law

Biot–Savart: dB from a current-element as taught, 1/r^2 with a sine. Biot–Savart is the pair-law for currents, like Coulomb was for charges. One element-arrow is enough to name it.

Using Coulomb’s 1/r^2 on charges as this heading is a steal.

Figure. Biot–Savart: this bit of wire, the distance to P, and sin of the 45° opening set dB at P. Far along the wire the angle collapses and that bit almost stops contributing.

How it works

  1. Name a current-element and the pointThe given.
  2. Use the taught dB writeBiot–Savart.
  3. Keep it a current-source of BThe idea.

4Magnetic Field on the Axis of a Circular Current Loop

Axis of a loop: B = μ0 I R^2 / 2(R^2+x^2)^{3/2} as taught, or the centre μ0 I / (2R). A loop is a circle-current. Centre is the school first number.

Using a straight-wire formula on a loop is a miss.

Figure. Seen edge-on, the loop is a cut. On its axis, B points along that axis. Move P farther out and B falls; at the centre, x is zero.

How it works

  1. Name I, R, and the axis-pointThe given.
  2. Use the loop write (centre or axis)B.
  3. Keep the centre as μ0 I / (2R) if askedThe simple case.

5Ampere’s Circuital Law

Ampere’s law: around a closed loop, Σ B·dl = μ0 I_enclosed as taught. Ampere is a symmetry-shortcut (a long wire, a solenoid). Enclosed is the current through the loop’s face.

Using every current on the page as I_encl is a miss.

Figure. Ampère’s law on a long straight wire gives B falling as 1/r. Near the wire the field is large; far away it has already dropped.

How it works

  1. Pick a symmetric loopThe Amperian.
  2. Write B ℓ = μ0 I_in as taughtThe law.
  3. Keep outside currents off I_inThe caution.

6The Solenoid

The solenoid: a long coil’s inside B is μ0 n I as taught, nearly uniform; outside is weak. Solenoid is a tunnel-of-B. n is turns per length.

Using μ0 I / (2π r) as the solenoid inside is a wire-steal.

Figure. Side view of a long solenoid. The three interior arrows are the same length because B is uniform inside. Outside, the turns cancel.

How it works

  1. Name n and IThe given.
  2. Write B=μ0 n I insideThe solenoid.
  3. Keep outside as the weak lookHonest.

7Force between Two Parallel Currents, the Ampere

Force between parallel currents: F/ℓ = μ0 I1 I2 / (2π d) as taught. Same direction pulls; opposite pushes. The ampere as a definition-look if named. Parallel is a two-wire story.

A perpendicular pair as this write is a miss.

Figure. Two parallel wires carrying current the same way pull together. Each sits in the other’s field. Reverse one current and both force arrows flip outward.

How it works

  1. Copy I1, I2, dThe given.
  2. Form the taught F/ℓThe force.
  3. Read same-way as pullThe direction.

8Torque on Current Loop, Magnetic Dipole

Torque on a current loop: τ = I A B sinθ as taught (or m×B with m=I A). A loop is a magnetic dipole. Torque tries to align the area-arrow with B.

A net force in a uniform B as the first claim is a miss — torque first.

Figure. A rectangular coil in a uniform B. Equal opposite forces on the two sides make a couple. The moment is I times the area; torque is m B sinθ.

How it works

  1. Name I, A, B, and the angleThe inputs.
  2. Form I A B sinθThe torque.
  3. Keep align as the jobm with B.

9The Moving Coil Galvanometer

Moving-coil galvanometer: a coil in a radial B, deflection ∝ current as taught. A spring or the taught restore sits against the torque. Galvanometer is a current-meter, not a voltmeter until you add series R.

Calling it a solenoid because it has a coil is a miss.

Figure. In a radial field the deflecting torque is NIAB, the spring restores with kφ, so the needle angle tracks the current. That straight line is the instrument.

How it works

  1. Name coil + radial B + restoreThe parts.
  2. Read deflection as a current-lookThe job.
  3. Keep series-R as a later convert if taughtHonest extra.
A rest charge in B feels
  1. No magnetic force as taught
  2. q v B always
  3. q E

Need v.

Notes

  • Mapped to the official NCERT chapter “Moving Charges and Magnetism”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • F=q v B sinθ
  • r=mv/(qB)
  • B_solenoid=μ0 n I
  • F/ℓ=μ0 I1 I2 /(2π d)
  • τ=I A B sinθ

Recap

Hold these pegs from the official chapter “Moving Charges and Magnetism”. The wording is ExamMaster’s teaching, not a textbook recap.

Magnetic Force
Magnetic force on a moving charge: F = q v B sinθ as taught, perpendicular to v and B.
Motion in a Magnetic Field
Motion in B: a perpendicular B bends the path to a circle as taught, r = m v / (q B).
Magnetic Field due to a Current Element, Biot-Savart Law
Biot–Savart: dB from a current-element as taught, 1/r^2 with a sine.
Magnetic Field on the Axis of a Circular Current Loop
Axis of a loop: B = μ0 I R^2 / 2(R^2+x^2)^{3/2} as taught, or the centre μ0 I / (2R).
Ampere’s Circuital Law
Ampere’s law: around a closed loop, Σ B·dl = μ0 I_enclosed as taught.
The Solenoid
The solenoid: a long coil’s inside B is μ0 n I as taught, nearly uniform; outside is weak.

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