What is the unit of measure of magnetic field ?

Question

UnlockIAS · Accepted Answer

Correct — B, Tesla. The tesla, symbol T, is the SI unit of magnetic flux density — the quantity written B and loosely called the magnetic field. It is a derived unit: one tesla is one weber per square metre, and in base units one kilogram per ampere per second squared. The physical definition is the cleanest way to hold it. A particle carrying one coulomb of charge, moving perpendicular to a magnetic field of one tesla at one metre per second, experiences a force of one newton — which is the Lorentz force law read as a definition, and which is also why the tesla can be written as one newton per ampere-metre. The unit was announced at the General Conference on Weights and Measures in 1960 and named for Nikola Tesla, the Serbian-American electrical engineer, on a proposal from the Slovenian engineer France Avčin. A tesla is a very large field. The Earth's own magnetic field at the surface measures between about 25,000 and 65,000 nanotesla — that is, 25 to 65 microtesla, roughly a twenty-thousandth of a tesla — while a clinical MRI scanner runs at 1.5 or 3 tesla, which is where the unit is met in everyday life. In the older CGS system the same quantity is measured in gauss, and one tesla equals ten thousand gauss, so a fridge magnet at a few milliteslas is a few tens of gauss. One distinction is worth making because it explains the third option: physics uses two different 'magnetic field' quantities. The flux density B is measured in tesla; the magnetising field H is measured in amperes per metre, not in amperes. And the total magnetic flux through a surface is measured in webers. A note on this paper's bilingual text: the facing हिन्दी page prints चुम्बकीय बल — magnetic force — where the English column says magnetic field. The answer does not move, because the newton, which would be the unit of a force, is not among the options, and of the four printed only the tesla is a magnetic unit at all.

Why the other options are wrong — (a) The ohm is the unit of electrical resistance — one ohm is one volt per ampere — and it is named after Georg Simon Ohm. It belongs to the same broad topic of electricity and magnetism, which is the whole of its plausibility here; it has nothing to do with fields.  (c) The genuinely tempting wrong answer, because it is close to being right about a different quantity. The ampere is the SI base unit of electric current. The magnetising field H is measured in amperes per metre — but that is per metre, and the bare ampere measures current, not field. A candidate who half-remembers 'ampere per metre' and drops the metre lands here.  (d) Not a unit of anything. Cobalt is a chemical element, atomic number 27, and it is on the list because it is magnetic — one of the three elements that are ferromagnetic at ordinary temperatures, alongside iron and nickel. The option tests whether the candidate is reading the question or matching on the word 'magnetic'.

Concept: Magnetism is described by three different quantities, and most confusion about its units comes from mixing them up. Magnetic flux density B — what a compass needle or a moving charge actually responds to — is measured in tesla. Magnetic flux, the total amount of that field passing through a given surface, is measured in webers, and one tesla is simply one weber spread over one square metre. The magnetising field H, which describes the field a current produces before the medium's own response is accounted for, is measured in amperes per metre. Only the first of these is what an exam means by 'magnetic field'. The SI itself is built from seven base units — metre, kilogram, second, ampere, kelvin, mole and candela — and everything else, including the tesla, the ohm, the newton and the volt, is derived from them. Twenty-two derived units carry special names, and almost all of those names honour a scientist: ampere for Ampère, ohm for Ohm, tesla for Nikola Tesla, weber for Wilhelm Weber, henry for Joseph Henry.

State PCS 2025 Q136 — Science & Technology PYQ with Answer

State PCS 2025 Q136 — Science & Technology PYQ with Answer