Subtopics - Magnetism (NEET)
Bar magnets, earths field, magnetic materials and hysteresis - from compass needles to domain theory
1) Bar Magnet and Magnetic Dipole
Molecular theory of magnetism (Weber-Ewing), properties of bar magnets, pole strength, magnetic moment, effective vs geometric length, cutting of bar magnets, and the non-existence of magnetic monopoles.
2) Magnetic Field, Force and Earths Magnetism
Magnetic field lines, Coulombs law for magnetism, axial and equatorial fields of a bar magnet, torque and potential energy in uniform field, Gausss law for magnetism, earths magnetic field elements (declination, dip, horizontal component), magnetic maps, neutral points, tangent law, tangent galvanometer and deflection magnetometer.
3) Magnetic Materials and Their Properties
Classification into diamagnetic, paramagnetic and ferromagnetic materials based on susceptibility, permeability and atomic-level explanation. Magnetic permeability, susceptibility, intensity of magnetisation, relation between B, H and I, Curies law and Curie-Weiss law, Curie temperature.
4) Hysteresis and Practical Magnetism
The hysteresis curve for ferromagnetic materials, retentivity and coercivity, comparison of soft iron and steel, energy loss in hysteresis, Bohr magneton, magnetic screening, and key tips for NEET problem solving.
Magnetism Download Notes & Weightage Plan
For each topic in the Magnetism chapter below, you get (2) the exact resources to download and how to use them, and (3) a simple importance & time plan so NEET students know what to do first and what to revise last.
Bar Magnet and Magnetic Dipole
Molecular theory, pole strength, magnetic moment, effective length, cutting rules, monopole non-existence.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Magnetic moment direction (S to N), effective length formula, and cutting rules for pole strength and moment.
- High-risk Area: Students assume cutting a magnet in half gives two monopoles. It does not - each piece is a complete dipole. Also confusing L_e and L_g.
- Best Practice Style: Short conceptual MCQs and numerical cutting problems.
Magnetic Field, Force and Earths Magnetism
Magnetic field formulae for bar magnet, Gausss law, torque and PE, earths field elements, tangent law, instruments.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: B_H = B cos phi, B_V = B sin phi, tan phi = B_V/B_H, and B_axial = 2 times B_equatorial for same distance.
- High-risk Area: Confusing declination (horizontal angle in the horizontal plane) with dip (vertical angle in the magnetic meridian plane). Also forgetting B_axial/B_equatorial = 2, not 1.
- Best Practice Style: Numerical MCQs involving dip angle and field component calculations; conceptual questions on Gausss law.
Magnetic Materials and Their Properties
Classification of dia/para/ferro materials, susceptibility, permeability, intensity of magnetisation, Curies law, Curie temperature, Curie-Weiss law.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Susceptibility sign for each type, mu_r = 1 + chi_m, Curie law chi = C/T, and Curie temperature converting ferro to para.
- High-risk Area: Assigning positive susceptibility to diamagnetic materials (it is negative). Confusing Curies law (paramagnetic, chi = C/T) with Curie-Weiss law (ferromagnetic above T_c, chi = C/(T minus T_c)).
- Best Practice Style: Table-based classification MCQs and match-the-property questions.
Hysteresis and Practical Magnetism
Hysteresis curve interpretation, retentivity, coercivity, soft iron vs steel, energy loss, Bohr magneton, magnetic screening.
1) Download Packs For This Topic (And How To Use Them)
Don't download everything and forget it. Use these like a small "attack kit": read → highlight → test → revise the same sheet again.
2) Importance, Weightage & Time Allocation (Practical)
Use this to avoid over-studying. This topic is usually low effort, quick return if your recall is clean.
- Scoring Focus: Retentivity definition (I when H = 0), coercivity definition (H when I = 0), loop area as energy loss, and material selection logic.
- High-risk Area: Mixing up retentivity and coercivity. Retentivity is the remaining I at H = 0; coercivity is the H needed to make I zero. Students swap these two regularly.
- Best Practice Style: Conceptual MCQs on hysteresis curve identification and material application questions.
Magnetism Chapter NEET Traps & Common Mistakes (Topic-Wise)
Each subtopic below is of the Magnetism chapter and shows what NEET students usually do wrong in NEET examination, a short example of the mistake, and how NEET frames the question to trick you with close options are given below.
Mistake Snapshot (What Students Do Wrong)
- Positive chi for diamagnetic: Students assign positive susceptibility to diamagnetic materials. Diamagnetic chi is always small and negative because induced magnetisation opposes the applied field.
- mu_r greater than 1 for diamagnetic: Since mu_r = 1 + chi_m and chi_m is negative for diamagnetic substances, mu_r is less than 1. Students forget the subtraction and mark mu_r > 1.
Bismuth is diamagnetic with chi approximately equal to minus 1.66 x 10^(minus 4). A question asks whether Bi is attracted or repelled by a strong magnet. Since chi is negative, Bi is repelled. mu_r = 1 + (minus 1.66 x 10^(minus 4)) = 0.9998, which is less than 1.
How NEET Frames The Trap
NEET options typically include both positive and negative chi values for the same material. Always verify the material type first, then assign the sign.
Q. The magnetic susceptibility of a diamagnetic substance is:
A. Small and positive B. Large and positive C. Small and negative D. Zero
Trick: Diamagnetic substances have induced magnetisation opposing the field, so chi is small and negative. Answer is option C.
Mistake Snapshot (What Students Do Wrong)
- Calling coercivity the residual magnetism: Students swap the two: retentivity is the residual I at H = 0, while coercivity is the reverse H needed to bring I to zero. These are different quantities on different axes.
- Assigning high coercivity to soft iron: Soft iron has LOW coercivity (easy to demagnetise). Steel has HIGH coercivity. Students reverse this when choosing materials for permanent magnets.
On a hysteresis loop, the point where the curve crosses the I-axis (H = 0) gives retentivity OC. The point where it crosses the H-axis (I = 0) gives coercivity OD. A student labels OC as coercivity and loses the mark.
How NEET Frames The Trap
NEET gives a labelled hysteresis diagram and asks to identify retentivity and coercivity. Options swap the two axis-intercept meanings.
Q. In a hysteresis loop, the value of magnetic field intensity H required to reduce the residual magnetism to zero is called:
A. Retentivity B. Coercivity C. Susceptibility D. Permeability
Trick: The H needed to reduce I to zero is the coercivity by definition. Retentivity is the I remaining at H = 0. Answer is option B.
Mistake Snapshot (What Students Do Wrong)
- Using sin for horizontal component: B_H = B cos phi (not B sin phi). Students use the wrong trig function, assigning sin to the horizontal component. The dip angle phi is measured from the horizontal, so horizontal projection uses cosine.
- Confusing dip with declination: Declination is a horizontal angle between geographic and magnetic meridians. Dip is a vertical angle in the magnetic meridian plane. Students mix these two independent angular elements.
At a location where dip = 60 degrees and B = 0.5 G: B_H = B cos 60 = 0.5 x 0.5 = 0.25 G. B_V = B sin 60 = 0.5 x 0.866 = 0.433 G. A student who uses sin for B_H gets 0.433 G (wrong) and cos for B_V gets 0.25 G (wrong) - both components are swapped.
How NEET Frames The Trap
NEET provides dip angle and one component, asking for the other. The swapped-trig answer is always present as a distractor option.
Q. At a place where the angle of dip is 30 degrees, the horizontal component of earths field is 0.4 G. The total magnetic field of the earth at that place is:
A. 0.2 G B. 0.8 G C. 0.4/cos 30 G D. 0.4 cos 30 G
Trick: B_H = B cos phi, so B = B_H/cos phi = 0.4/cos 30 = 0.4/0.866 = 0.46 G. Answer is option C. Students who use B = B_H/sin 30 get 0.8 G (option B, wrong).
Mistake Snapshot (What Students Do Wrong)
- B_axial equals B_equatorial: For a short bar magnet at the same distance r, B_axial = (mu_0/4pi)(2M/r3) and B_equatorial = (mu_0/4pi)(M/r3). The axial field is TWICE the equatorial field. Students forget the factor of 2.
- Wrong direction of equatorial field: The equatorial field points antiparallel to M (from N to S externally), while the axial field is parallel to M. Students assume both point in the same direction.
A short bar magnet has M = 0.5 A-m2. At r = 0.1 m on the axis: B_a = (10^(minus 7))(2 x 0.5/0.001) = 10^(minus 4) T. On the equator at same distance: B_e = (10^(minus 7))(0.5/0.001) = 5 x 10^(minus 5) T. Ratio = 2. A student who forgets the 2 in the axial formula gets B_a = B_e.
How NEET Frames The Trap
NEET asks for the ratio of axial to equatorial field at the same distance. The factor of 2 is the expected answer but distractors include 1, 1/2, and 4.
Q. The ratio of magnetic field at a point on the axial line to that on the equatorial line of a short bar magnet, at equal distances, is:
A. 1:1 B. 2:1 C. 1:2 D. 4:1
Trick: B_axial has factor 2M in numerator while B_equatorial has M. Ratio = 2M/M = 2:1. Answer is option B.
Mistake Snapshot (What Students Do Wrong)
- Applying Curie law to ferromagnets: Curie law chi = C/T applies only to paramagnetic materials. For ferromagnets above Curie temperature, the correct law is Curie-Weiss: chi = C/(T minus T_c). Students use C/T for both.
- Below Curie temperature is paramagnetic: Below T_c the material is ferromagnetic (strongly magnetic). Above T_c it becomes paramagnetic. Students reverse this transition direction.
Iron has T_c = 770 degrees C. At 800 degrees C (above T_c): chi = C/(800 minus 770) = C/30 (Curie-Weiss, paramagnetic behaviour). At 500 degrees C (below T_c): iron is ferromagnetic and chi does not follow either simple law.
How NEET Frames The Trap
NEET gives temperature and material, asking for chi behaviour. Options include both C/T and C/(T minus T_c). Choosing the wrong formula gives a completely different answer.
Q. Above the Curie temperature, the magnetic susceptibility of a ferromagnetic material varies as:
A. chi = C/T B. chi = C/(T minus T_c) C. chi = C x T D. chi is constant
Trick: Above T_c, ferromagnets become paramagnetic and follow the Curie-Weiss law chi = C/(T minus T_c), not the simple Curie law. Answer is option B.