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P. 4182. A pointlike discharge lamp of power 5 W emits monochromatic light of wavelength 425 nm. The cathode of a photocell is placed 1 m from the light source, perpendicularly to the light rays. The radius of the cathode is 0.5 cm and the work function of its material is 2.10-19 J. A capacitor of capacitance 2 nF is connected to the anode and the cathode of the photocell.

a) What is the energy of one photon emitted by the discharge lamp? How many photons hits the cathode of the photocell in each second?

b) What is the maximum of the speed of the electrons which leaves the cathode?

c) What will the voltage of the capacitor be?

(5 points)

Deadline expired on 12 October 2009.

Google Translation (Sorry, the solution is published in Hungarian only.)

Megoldás. $\displaystyle a)$ $\displaystyle 3{,}1\cdot 10^{-5}~\rm J$

és $\displaystyle 6\cdot 10^{13}$ db.

$\displaystyle b)$ $\displaystyle v_{\text{max}}=7{,}7 \cdot 10^5~\frac{\rm m}{\rm s}.$

$\displaystyle c)$ $\displaystyle U=1{,}67~\rm V.$

Statistics on problem P. 4182.
 56 students sent a solution. 5 points: Benyó Krisztián, Béres Bertold, Bodonyi Ádám, Bolgár Dániel, Budai Ádám, Csopor Dávid, Ercsey Tamás, Farkas Martin, Fonai Dániel, Galzó Ákos Ferenc, Garaguly Gergő, Hartstein Máté, Illés Dorottya, Janosov Milán, Kiss 620 Gábor, Knódel Máté, Kovács 616 Philip, Kovács Márk, Krämer Zsolt, Laczkó Zoltán Balázs, Lájer Márton, Mayer Martin János, Pálovics Péter, Patartics Bálint, Pázmán Koppány, Rácz Viktor, Sápi András, Szabó 928 Attila, Tamási Mátyás, Trényi Róbert, Varju 105 Tamás, Várnai Péter, Vuchetich Bálint. 4 points: 19 students. 3 points: 3 students. 2 points: 1 student.

• Problems in Physics of KöMaL, September 2009

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