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Monday, March 29, 2010

KE max of photoelectrons


One cannot see the photoelectrons, hence it has to be measured indirectly by measuring the stopping potential. There are two types accelerating potential or stopping potential. V acc will increase KE of electron but Vs will decrease the KE of electron to zero , hence electron cannot touch the collector. I cannot see the electron stopping just before the collector but I can see the ammeter reducing to zero value (no electron collected) as I increase the magnitude of stopping potential. Hence stopping potential correspond the the KE of the fastest electron (KE max). Formula : qVs = KE max.
Analogy: If you can jump very high , I deduce that you can run very fast . This is a known fact in our everyday life.The equation : KE = mgh. Compare will KE = qVs. Do you see similarity in this two worlds micro and macro world. Hence measuring how high you can jump indirectly gives me your KE. Do the same to photoelectrons , as V stop can be measured using voltmeter hence KE max can be found. Electrons are not visible so it is impossible to measure its velocity directly.