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Magnetic Field Lines - Conceptual Points: - Strong Field Weak Field

The document outlines key concepts in magnetism, including magnetic field lines, Oersted's experiment, Biot-Savart Law, and Ampere's Circuital Law. It discusses the behavior of magnetic fields in solenoids and toroids, the Lorentz force on moving charges, and principles like Fleming’s Left-Hand Rule and the superposition principle. Additionally, it compares electric and magnetic fields, introduces cyclotron frequency, and highlights important applications such as the force between current-carrying wires and magnetic levitation.

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33 views3 pages

Magnetic Field Lines - Conceptual Points: - Strong Field Weak Field

The document outlines key concepts in magnetism, including magnetic field lines, Oersted's experiment, Biot-Savart Law, and Ampere's Circuital Law. It discusses the behavior of magnetic fields in solenoids and toroids, the Lorentz force on moving charges, and principles like Fleming’s Left-Hand Rule and the superposition principle. Additionally, it compares electric and magnetic fields, introduces cyclotron frequency, and highlights important applications such as the force between current-carrying wires and magnetic levitation.

Uploaded by

prema141277
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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1.

Magnetic Field Lines – Conceptual Points

• Magnetic field lines are closed continuous loops (No start or end).
• Outside magnet: From North pole to South pole.
• Inside magnet: From South pole to North pole.
• Field lines never cross each other (would imply two directions of B at one point —
impossible).
• Crowded lines → Strong field; Sparse lines → Weak field.

2. Oersted’s Experiment & Discovery

• Current-carrying conductor produces magnetic field around it.


• Deflection reverses when current direction is reversed.
• First experimental evidence of relation between Electricity & Magnetism.

3. Biot-Savart Law – Direction Concept

• Magnetic field vector dB is perpendicular to the plane of current element (Idl)


and position vector (r).
• B is stronger near the wire and reduces as 1/r² (inverse square law).
• Used for calculating B of finite wires, arcs, loops etc.

4. Ampere's Circuital Law – When to Apply

• Best for high symmetry cases:


o Infinite long straight wire.
o Solenoid (long coil).
o Toroid (doughnut shaped coil).
• Path chosen (Amperian loop) should simplify B · dl integral.
• Ampere’s Law → No effect from external currents (outside the loop).

5. Magnetic Field of Solenoid & Toroid – Conceptual Insights

• Long solenoid: B is uniform inside, negligible outside.


• Toroid: B is confined inside the core. No B outside the toroid.
• Direction of B in solenoid/toroid: Right-hand curl rule (curl fingers along current →
thumb gives B-direction).

6. Force on Moving Charge in Magnetic Field (Lorentz Force)

• Magnetic force never changes speed, only direction.


• Work done by magnetic force = 0.
• Charge moving parallel or antiparallel to B → No force.
• Charge moving perpendicular to B → Circular path.
• Charge moving at an angle θ to B → Helical path.
• Pitch of Helix: Distance moved along B in one revolution = v_parallel * T.

7. Fleming’s Left-Hand Rule

• Predicts direction of force on a current-carrying conductor in magnetic field.


o Thumb → Force (motion)
o First Finger → Field (B)
o Second Finger → Current (I)

8. Superposition Principle in Magnetism

• Net magnetic field at a point is vector sum of individual fields.


• Direction and magnitude are both important.
• Common in cases with multiple wires, loops, or combinations (like square loops,
adjacent coils).

9. Helmholtz Coil Arrangement

• Two identical coaxial circular coils separated by distance equal to their radius.
• Used to produce a highly uniform magnetic field.
• Field is uniform at mid-point between the coils.

10. Comparison between Electric and Magnetic Fields


Electric Field (E) Magnetic Field (B)

Acts on stationary & moving charges Acts only on moving charges


Electric Field (E) Magnetic Field (B)

Can do work on charge Cannot do work (force ⊥ displacement)

Field lines begin and end on charges Field lines are always closed loops

11. Cyclotron Frequency & Radius (Quick Concept)

• Cyclotron Frequency: Frequency of revolution of charge in magnetic field.


o f = qB / (2πm)
• Radius of Path: r = mv / (qB)
o r increases with particle velocity.

12. Important Application Scenarios

• Force between two parallel current-carrying wires: Attraction if currents are in the
same direction; repulsion if opposite.
• Galvanometer → Ammeter/Voltmeter Conversion: Involves magnetic torque on
current loop.
• Magnetic levitation (Maglev trains): Magnetic forces counteract gravitational force.

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