Instructions
Total Questions: 20 | Marks: 4 each | No Negative Marking
Magnetism & Matter – IIT JEE Notes (Set 11)
Introduction to Magnetism
Basic Concept
Magnetism is a physical phenomenon produced by moving electric charges and intrinsic magnetic moments of particles. It results in attractive or repulsive forces between objects. In classical physics, magnetism is closely related to electricity, forming the basis of electromagnetism.
Magnetic Field
A magnetic field is the region around a magnet or current-carrying conductor where a magnetic force can be experienced. It is represented by magnetic field lines and denoted by B. The SI unit of magnetic field is Tesla (T).
Magnetic Field Lines
Properties
Magnetic field lines are imaginary lines used to represent the direction and strength of a magnetic field. They always form closed loops, emerging from the north pole and entering the south pole outside the magnet, and continuing inside the magnet from south to north.
Important Insight
The density of field lines indicates the strength of the magnetic field. Closer lines indicate a stronger field, while widely spaced lines indicate a weaker field.
Magnetic Dipole and Dipole Moment
Magnetic Dipole
A magnetic dipole consists of two equal and opposite magnetic poles separated by a small distance. A bar magnet is a classic example of a magnetic dipole.
Magnetic Dipole Moment
The magnetic dipole moment (m) is a vector quantity defined as the product of pole strength and separation distance. For a current loop, it is given by m = IA, where I is current and A is area.
Torque on a Magnetic Dipole
Formula
τ = mB sinθ
Explanation
When a magnetic dipole is placed in a uniform magnetic field, it experiences a torque that tends to align it with the field. The torque is maximum when the dipole is perpendicular to the field.
Magnetic Field Due to Current
Straight Current-Carrying Wire
The magnetic field at a distance r from a long straight conductor carrying current I is given by B = μ₀I / 2πr. This shows that the field decreases with increasing distance from the wire.
Circular Current Loop
The magnetic field at the center of a circular loop is B = μ₀I / 2R, where R is the radius of the loop. This field is stronger than that of a straight wire at the same distance.
Solenoid
A solenoid is a long coil of wire. The magnetic field inside a long solenoid is uniform and given by B = μ₀nI, where n is the number of turns per unit length. Outside the solenoid, the field is nearly zero.
Earth’s Magnetism
Concept
The Earth behaves like a giant bar magnet with its magnetic south pole near the geographic north pole and vice versa. This allows a compass needle to align along the north-south direction.
Magnetic Elements
Magnetic declination is the angle between geographic north and magnetic north. Magnetic inclination (or dip) is the angle made by the Earth’s magnetic field with the horizontal plane.
Magnetic Properties of Materials
Diamagnetic Substances
Diamagnetic materials have a small negative magnetic susceptibility and are weakly repelled by magnetic fields. Examples include bismuth and copper.
Paramagnetic Substances
Paramagnetic materials have a small positive susceptibility and are weakly attracted by magnetic fields. Examples include aluminum and platinum.
Ferromagnetic Substances
Ferromagnetic materials have very large positive susceptibility and are strongly attracted by magnetic fields. They can retain magnetism even after the external field is removed. Examples include iron, cobalt, and nickel.
Magnetic Permeability and Susceptibility
Magnetic Permeability
Magnetic permeability (μ) measures how easily a material can support the formation of a magnetic field within itself.
Magnetic Susceptibility
It indicates how much a material will become magnetized in an external magnetic field. It is positive for paramagnetic and ferromagnetic materials and negative for diamagnetic materials.
Hysteresis Loop
Concept
When a ferromagnetic material is magnetized and demagnetized, the magnetic field (B) does not follow the same path with magnetizing field (H). This lag is called hysteresis.
Energy Loss
The area of the hysteresis loop represents energy loss per cycle due to magnetic reversal. This is important in transformer cores and electrical machines.
Retentivity and Coercivity
Retentivity
It is the ability of a material to retain magnetism after the external magnetic field is removed. Materials with high retentivity are used for permanent magnets.
Coercivity
It is the reverse magnetic field required to reduce the magnetization of a material to zero. Materials with high coercivity are used for making permanent magnets.
Magnetic Field Strength and Flux
Magnetic Flux
Magnetic flux (Φ) is defined as the total number of magnetic field lines passing through a surface. It is given by Φ = BA cosθ.
Unit
The SI unit of magnetic flux is Weber (Wb).
Applications of Magnetism
Electric Motors
Electric motors work on the principle that a current-carrying conductor placed in a magnetic field experiences a force.
Generators
Generators convert mechanical energy into electrical energy using electromagnetic induction.
Transformers
Transformers use magnetic properties of materials and mutual induction to step up or step down voltage.
Important Relationships
Force on Moving Charge
F = qvB sinθ
Force on Current-Carrying Conductor
F = BIL sinθ
Right-Hand Rule
Used to determine the direction of magnetic field around a current-carrying conductor.
Conceptual Insights
Key Understanding
Magnetic fields do not do work because the force is always perpendicular to velocity. However, they can change the direction of motion of charged particles.
Common Mistakes
Students often confuse magnetic field direction and force direction. Use right-hand rules carefully.
Important Exam Concepts
Conceptual Traps
Magnetic field lines never intersect. Inside a solenoid, the field is uniform. Magnetic force does not change speed, only direction.
JEE Strategy
Focus on formulas, diagrams, and conceptual clarity. Practice numerical problems involving magnetic fields due to currents and torque on dipoles. Understanding field line patterns and material properties is crucial for scoring well.