NEB Class 12 Physics Magnetic field Notes in PDF Complete Handwritten. Physics Notes 2081: All Chapters | New Curriculum | Class 12 Physics Notes download.
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NEB Class 12 Physics Magnetic field Note Handwritten in PDF
Magnetic Field Lines and Flux
Magnetic field lines:
- Start from N pole, end at S pole (outside the magnet)
- Form closed loops inside the magnet (N to S internally)
- Never cross each other
- Closer lines = stronger field
Magnetic flux (ฮฆ): ฮฆ = BA cosฮธ (for uniform field B, area A, angle ฮธ between B and normal to area)
- Unit: Weber (Wb) = Tยทmยฒ
Oersted’s Experiment
In 1820, Hans Oersted discovered that a current-carrying wire deflects a nearby compass needle โ proving that electric current creates a magnetic field.
Key outcomes:
- Current creates magnetic field in circles around the wire
- Right-hand thumb rule: thumb points in direction of current โ fingers curl in direction of B
- Reversing current reverses field direction
- Established the link between electricity and magnetism (electromagnetism)
Force on a Moving Charge (Lorentz Force)
A charge q moving with velocity v in magnetic field B experiences:
F = qv ร B = qvB sinฮธ
- Maximum when ฮธ = 90ยฐ (velocity โฅ field)
- Zero when ฮธ = 0ยฐ or 180ยฐ (velocity โฅ field)
- Direction: given by right-hand rule (or left-hand rule for electrons)
Circular motion in magnetic field: When v โฅ B, force is always perpendicular to v โ circular motion
- Radius: r = mv/qB
- Time period: T = 2ฯm/qB (independent of speed! This is the cyclotron principle)
Force on Current-Carrying Conductor
F = BIL sinฮธ (for length L, current I, field B, angle ฮธ)
Direction: Fleming’s Left-Hand Rule (used for motors)
- Index finger = field B direction
- Middle finger = current I direction
- Thumb = force F direction
Torque on Rectangular Coil and Moving Coil Galvanometer
For a rectangular coil (N turns, area A) carrying current I in uniform field B:
Torque: ฯ = NBIA sinฮธ
- Maximum when plane of coil is parallel to field (ฮธ = 90ยฐ)
- Zero when plane is perpendicular to field (coil is in equilibrium position)
Moving Coil Galvanometer:
- Coil placed in radial magnetic field (using curved poles + soft iron cylinder)
- Radial field ensures torque is always = NBIA (sinฮธ = 1 always)
- Restoring spring torque = kฮธ
- At equilibrium: NBIA = kฮธ โ ฮธ โ I (deflection proportional to current)
- Current sensitivity = NBA/k
Hall Effect
When a current-carrying conductor is placed in a perpendicular magnetic field, a voltage appears across the conductor (perpendicular to both current and field).
Hall voltage: V_H = BI/ntq
Where:
- B = magnetic field, I = current
- n = charge carrier density, t = thickness, q = charge of carrier
Uses:
- Measure magnetic field strength (Hall probe)
- Determine type of charge carrier (positive or negative)
- Measure current without breaking circuit
Biot-Savart Law
The magnetic field due to a small current element:
dB = (ฮผโ/4ฯ) ร (I dl sinฮธ / rยฒ)
Applications:
- At center of circular coil (N turns, radius r): B = ฮผโNI/2r
- Due to long straight wire at distance r: B = ฮผโI/2ฯr
- Inside a long solenoid: B = ฮผโnI (n = turns per unit length)
Ampere’s Circuital Law
โฎBยทdl = ฮผโI_enclosed
The line integral of B around any closed path = ฮผโ ร total current enclosed.
Same applications as Biot-Savart but simpler for symmetric cases:
- Long straight wire: B = ฮผโI/2ฯr
- Solenoid: B = ฮผโnI inside, B = 0 outside
- Toroid: B = ฮผโnI inside the toroid
Force Between Parallel Current-Carrying Conductors
Two parallel wires carrying currents Iโ and Iโ, separated by distance d:
Force per unit length: F/L = ฮผโIโIโ/2ฯd
- Same direction currents โ Attractive force
- Opposite direction currents โ Repulsive force
Definition of Ampere: 1 Ampere is defined as the current that, when flowing in each of two infinitely long parallel wires 1 metre apart in vacuum, produces a force per unit length of 2 ร 10โปโท N/m between them.
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Frequently Asked Questions
What is Biot-Savart law in Class 12 Physics?
Biot-Savart law gives magnetic field from a current element: dB = (ฮผโ/4ฯ)(I dl sinฮธ/rยฒ). Applications: field at centre of circular coil B = ฮผโNI/2r, due to long straight wire B = ฮผโI/2ฯr, inside solenoid B = ฮผโnI. These three applications are the most tested derivations from this chapter in NEB exams.
What is Ampere’s circuital law Class 12?
Ampere’s law: โฎBยทdl = ฮผโI_enclosed. Line integral of B around any closed path equals ฮผโ times total enclosed current. Applications: long straight wire B = ฮผโI/2ฯr, inside solenoid B = ฮผโnI (zero outside), inside toroid B = ฮผโnI (zero outside toroid). Simpler than Biot-Savart for symmetric field configurations.
What is Hall effect in Class 12 Physics?
When current-carrying conductor is placed in perpendicular magnetic field, a voltage develops perpendicular to both current and field this is Hall voltage. Formula: V_H = BI/ntq where n is charge carrier density, t is thickness, q is charge. Used to measure magnetic field strength and determine charge carrier type.
What is the working principle of moving coil galvanometer?
Current-carrying coil in radial magnetic field experiences torque ฯ = NBIA. Restoring spring torque = kฮธ. At equilibrium NBIA = kฮธ, giving ฮธ = (NBA/k)I. Deflection is proportional to current. Radial field (using curved poles and soft iron cylinder) ensures sinฮธ = 1 always giving linear scale deflection throughout.
What is the force between two parallel current carrying conductors?
Force per unit length between two parallel conductors: F/L = ฮผโIโIโ/2ฯd. Same direction currents attract each other. Opposite direction currents repel each other. Definition of ampere: 1 A is current that produces force of 2ร10โปโท N per metre between two infinite parallel wires 1 metre apart in vacuum.
What is Lorentz force in Class 12 Physics?
Lorentz force on moving charge in magnetic field: F = qv ร B = qvB sinฮธ. Maximum when velocity perpendicular to field (ฮธ=90ยฐ). Zero when parallel (ฮธ=0ยฐ). When F is always perpendicular to velocity, charge moves in a circle: radius r = mv/qB. This is the principle of cyclotrons and mass spectrometers.
What is the difference between Biot-Savart law and Ampere’s law?
Both calculate magnetic field from currents but differ in application. Biot-Savart works for any current distribution but integration is complex. Ampere’s law is simpler but only works easily for highly symmetric configurations (straight wire, solenoid, toroid). For circular loop use Biot-Savart; for solenoid use Ampere’s law.
Which magnetic field topics are most important for Exam
Most important: Biot-Savart law applications โ circular coil and solenoid (4 marks each), Ampere’s law applications (4 marks), moving coil galvanometer working (4 marks), force between parallel conductors and definition of ampere (2 marks). These derivations appear in every NEB Class 12 Physics board paper.
