Introduction to electromagnetism: Difference between revisions

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Electric charge has several important properties:

* it is ''quantised'': this means that it can only take integer multiple values of the [[elementary charge]] ''e'' of an electron or proton (i.e. it can only take values of ''q'' = 0, ±''e'', ±2''e'', ±3''e'' , ...).<ref name=":1">{{Cite book|last=Serway|first=Raymond A.|title=Physics for Scientists and Engineers, Technology Update|publisher=Cengage Learning|year=2015|isbn=9781305465398|edition=9th|pages=692}}</ref> Although it is only a matter of definition, by convention the electron is said to have a negative charge −''e'' and the proton is said to have a positive charge +''e'' .<ref name=":0" /><ref name=":1" /> The first measurement of and experimental confirmation of the quantisation of charge was [[Robert Andrews Millikan|Robert Millikan's]] [[oil drop experiment]] in which the electric force on the particle is set to exactly counter the gravitational force that pulls it down, and the [[terminal velocity]] of this particle can be used to calculate its charge.<ref>{{Cite web|title=UChicago Breakthroughs: 1910s|url=https://www.uchicago.edu/breakthroughs/1910s/|access-date=2020-11-26|website=The University of Chicago|language=en}}</ref><ref>{{Cite web|title=Robert Millikan|url=http://www.aps.org/programs/outreach/history/historicsites/millikan.cfm|access-date=2020-11-26|website=APS physics|language=en}}</ref> This experiment is still one of the best confirmations of the quantisation of charge; one large experiment concluding in 2015 used over 100 million oil drops finding no evidence for charges that were not integer multiple values of ''e.<ref>{{cite web|date=January 2007|title=SLAC – Fractional Charge Search – Results|url=http://www.slac.stanford.edu/exp/mps/FCS/FCS_rslt.htm|access-date=26 November 2020|publisher=Stanford Linear Accelerator Center}}</ref>''

* it is ''conserved'': according to the [[Charge conservation|law of charge conservation]], the overall charge of a [[closed system]] (where no charge can leave or enter) cannot change. Quantum theory tells us that charges can be created but only in the [[pair production]] of oppositely charged [[Particle|particles]] and [[Antiparticle|antiparticles]] whose charges exactly cancel out so that charge is always conserved overall.<ref name=":0" /> Research suggests that the overall charge in the universe is neutral so we know that all the positive charges and negative charges in the universe will always cancel out in total.<ref>S. Orito; M. Yoshimura (1985). "Can the Universe be Charged?". ''Physical Review Letters''. '''54''' (22): 2457–60. {{Bibcode|1985PhRvL..54.2457O}}. {{doi|10.1103/PhysRevLett.54.2457}}. {{PMID|10031347}}.</ref><ref>E. Masso; F. Rota (2002). "Primordial helium production in a charged universe". ''Physics Letters B''. '''545''' (3–4): 221–25. {{arXiv|astro-ph/0201248}}. {{Bibcode|2002PhLB..545..221M}}. {{doi|10.1016/S0370-2693(02)02636-9}}.</ref>

* it produces [[Electric field|electric fields]]: by convention, electric [[Field line|field lines]] start at positive charges and end at negative charges, pointing in the direction of the electric force on a positive charge in the field (and in the opposite direction to the direction of the force on negative charges).<ref name=":2">{{Cite web|last=Pumplin|first=Jon|date=2000|title=Electric field lines|url=https://web.pa.msu.edu/courses/2000fall/phy232/lectures/efields/efieldlines.html|access-date=18 October 2018|website=Michigan State University Physics}}</ref><ref name=":3">{{Cite web|last=Nave|first=R|title=Electric Field|url=http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elefie.html|access-date=16 October 2018|website=Georgia State University Hyperphysics}}</ref> Electric field lines are drawn more densely the stronger the electric field to visualise the strength of the electric force on charged particles in the field.<ref name=":2" /> The electric field is defined as the force on a charge per unit charge so that Coulomb's law can be rewritten in terms of the electric field as shown:<ref name=":3" /><ref>{{Cite book|last=Purcell, Edward M.|url=https://www.worldcat.org/oclc/805015622|title=Electricity and magnetism|date=21 January 2013|isbn=978-1-107-01402-2|edition=Third|___location=Cambridge|pages=7|oclc=805015622}}</ref>