Lecture on JEE Revision: Moving Charges and Magnetism
Introduction
- Welcome by Kadi Ma'am, Master Teacher for Physics.
- Emphasis on the importance of revisions and regular attendance.
- Discussion on veltech university admission queries.
- Focus of the class: Moving Charges and Magnetism.
Key Topics Covered
Importance of the Topic
- Moving charges and magnetism have high weightage in JEE exams.
- Typically, 1-2 questions are asked from this topic in JEE.
Basic Concepts
Interaction of Charges with Magnetic Field
- Charged particles in magnetic fields experience a force.
- Force Formula: **
F = q(ar{v} imes ar{B})
**where q = charge, v = velocity, and B = magnetic field.
- Vector form: **
F = q(ar{v} imes ar{B})
- Force is zero when the angle θ (between velocity vector and magnetic field) is 0° or 180°.
- Maximum force occurs at θ = 90°.
Movement in Magnetic Field
- When a charged particle moves perpendicular to the magnetic field, it takes a circular path.
- Circle radius formula: **
r = rac{mv}{qB}
- Centripetal force for circular motion provided by magnetic force.
- Relations using momentum and kinetic energy: **
r = rac{p}{qB} and r = rac{ oot{2mk}}{qB}
Helical Path
- If the velocity of a charged particle is not perpendicular (θ ≠ 90°), it follows a helical path.
- Components of velocity: parallel **
(v ext{cos} θ) ** and perpendicular **.
``(v ext{sin} θ
**
- Radius of circular path (helix): **
r = rac{mv ext{sin} θ}{qB}
- Pitch of helical path: **
pitch = ext{velocity along axis} * ext{time period} = v ext{cos} θ * T where **
T = rac{2πm}{qB}
Biot-Savart Law
- Magnetic field due to a small segment of current-carrying wire Formula:
B = rac{μ₀}{4π} * rac{Idl ext{sin} θ}{r²}
- Application to circular loops, solenoids, etc.
- Magnetic field at a point on the axis of a circular loop, and center calculations:
- For center:
B = μ₀nI / (2r)
B = (μ₀nIr²) / (2(r² + x²)^(3/2))
where n = number of turns, r = radius, x = distance along the axis.
Straight Conductor and Current Loop
- Magnetic field around a straight current-carrying conductor and semicircular loop.
- Infinite wire magnetic field due to a straight conductor: **
B = μ₀I / (2πr)
- Semicircular loop magnetic field:
B = rac{μ₀IΦ}{4πR}
- Magnetic force between two parallel conductors carrying currents I1 and I2: **
F/L = (μ₀I1I2) / (2πd)
Practical Applications and Formulas
- Torque on current-carrying loop: **
τ = nBIA sin θ
- Potential Energy of magnetic dipole: **
U = -μB cos θ
- Moving coil galvanometer working principle essentials
- Current and Voltage sensitivity in galvanometers
- Conversion from galvanometer to ammeter and voltmeter.
Problem Solving
-
**Homework Problems Blood
-
Repeated JEE questions: Focus topics and high-yield concepts such as equilibrium torque, axis field calculations, and Biot-Savart law.
Conclusion
- Encouragement to practice, revise, and understand the basic concepts thoroughly.
- Regular class times and resources shared for further practice and study.
- Importance of Physics in competitive exams reiterated and motivations for continuous study.