Tangent Law and Magnetometers – Complete Notes, Revision, Important Questions & Downloads
Tangent Law and Magnetometers combines Tangent Law, Tangent Galvanometer, Deflection Magnetometer, and Vibration Magnetometer into one experimental block where the earth's horizontal field BH acts as the reference field. NEET usually tests this topic through the equilibrium result B = BH tan theta, the tangent-galvanometer current relation i = K tan theta, the Tan A versus Tan B formulas used in deflection magnetometer, and the vibration relation T = 2pi root(I/MBH). The trap is operational: each instrument uses the same earth-field idea, but the measurable quantity changes from deflection angle to time period, so formula-swapping is common unless the working principle is fixed first.
NEET Weightage & Exam Pattern
Magnetism| NEET Year | Questions from this Topic | Bar | Marks |
|---|---|---|---|
| 2024 | 1 | 4 | |
| 2023 | 1 | 4 | |
| 2022 | 1 | 4 | |
| 2021 | 2 | 8 | |
| 2020 | 1 | 4 | |
| Topic Weightage | 6 | 24 |
Vibration magnetometer changes the observable from angle to time period, which is why T squared relations are the fastest way to compare fields or moments.
Reduction factor K, Tan A position, Tan B position, and sum-and-difference method are favorite terms because each labels a specific measurement setup rather than a generic formula.
Preparation Strategy
Lock the Common Spine First Start from B = BH tan theta before revising any instrument. Once that equilibrium is fixed, tangent galvanometer and deflection magnetometer become two different ways of generating B rather than two unrelated formulas.
Tag Each Instrument by Measured Quantity Tangent galvanometer measures current, deflection magnetometer compares magnetic moments from angular deflection or null distance, and vibration magnetometer uses time period. This one-label habit prevents the most common formula substitution error.
Separate Tan A from Tan B by Field Geometry Tan A uses the axial field of the magnet, while Tan B uses the equatorial field. The right side changes by a factor of 2 in the short-magnet form, so the geometry must be read before the formula is written.
Use T Squared Relations Without Re-deriving Everything From T = 2pi root(I/MBH), infer T squared proportional to 1/BH or 1/M whenever the other quantities stay fixed. This turns comparison questions into ratio questions quickly.
Download Topic Notes
PDF · Cheat Sheet · MCQ Set · PYQTopic Coverage
2-Column Table| Column A | Column B |
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Quick Revision
Concept → Trap → Example1) Tangent Law
EquilibriumWhen a small magnet is acted on by two uniform mutually perpendicular fields B and BH, equilibrium gives B = BH tan theta. This is the central relation behind several magnetic measurement instruments in this topic.
- The angle theta is measured with respect to the earth's horizontal field BH.
- The law is valid because the torque balance involves perpendicular fields acting on the same magnetic needle.
- Trap: using tangent law when the two fields are not mutually perpendicular or when the angle is read from the wrong reference direction.
2) Tangent Galvanometer
Current MeasurementIn a tangent galvanometer, the field due to the circular coil is B = mu0Ni/2r, and tangent law gives i = (2rBH/mu0N) tan theta = K tan theta. The constant K is called the reduction factor.
- The coil must be kept in the magnetic meridian so that the field due to current is perpendicular to BH.
- For fixed BH, radius, and number of turns, the deflection angle directly controls the measured current.
- Trap: forgetting that sensitivity is highest near 45 degree because tan theta changes rapidly but not too steeply there.
3) Deflection Magnetometer
Moment ComparisonA deflection magnetometer works on tangent law and compares magnetic moments using the field of a bar magnet against BH. Tan A position uses axial field, while Tan B position uses equatorial field.
- In Tan A position, BH tan theta = (mu0/4pi)(2M/r^3) for a short magnet.
- In Tan B position, BH tan theta = (mu0/4pi)(M/r^3) for a short magnet.
- Trap: treating Tan A and Tan B as mere labels instead of reading the associated axial or equatorial geometry.
4) Vibration Magnetometer
Time PeriodA vibration magnetometer uses oscillation of a freely suspended magnet in earth's field, with T = 2pi root(I/MBH). It is therefore ideal for comparing magnetic moments or horizontal magnetic fields through squared-time ratios.
- For fixed I and M, T squared is inversely proportional to BH.
- For fixed I and BH, magnetic moment M is inversely proportional to T squared.
- Trap: confusing the deflection-based instrument with the vibration-based instrument and using tan theta relations in a time-period question.
US Curriculum Gaps
Note for NRI/OCI students studying abroad.Instrument Names Carry Formula Commitments In NEET
Many curricula use magnetic instruments as historical context, while NEET treats each one as a formula-tagged device with its own measured quantity and geometry condition.
- angle versus time period
- Tan A versus Tan B
Earth's Horizontal Field Is Used Operationally
Instead of only defining BH, this topic makes it the reference field in current measurement, moment comparison, and time-period analysis.
- BH is the reference field
- ratio questions dominate
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Frequently Asked Questions
Notes · Downloads · Revision · Important QuestionsWhy is tangent law so important in this topic?
What does a tangent galvanometer measure?
What is the reduction factor?
What is the difference between Tan A and Tan B positions?
What does a deflection magnetometer compare?
Why does vibration magnetometer use time period?
What is the most common mistake in this topic?
How does NEET usually frame questions from this block?
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