If the maximum kinetic energy of electrons emitted in a photocell is 5ev

Solution Let V 0 be the stopping potential . Given, K . E . max = 5.5 e V = 5.5 × 1.6 × 10 − 19 We know that K . E . max = e V 0 where e = 1.6 × 10 − 19 C ∴ 5.5 × 1.6 × 10 − 19 = 1.6 × 10 − 19 × V 0 Note : If the maximum kinetic energy of emitted electron is given in electron volts , it is numerically equal to the stopping potential in volts.

Discussion dilemma Under the right circumstances light can be used to push electrons, freeing them from the surface of a solid. This process is called the photoelectric effect (or photoelectric emission or photoemission), a material that can exhibit this phenomenon is said to be photoemissive, and the ejected electrons are called photoelectrons; but there is nothing that would distinguish them from other electrons. All electrons are identical to one another in mass, charge, spin, and magnetic moment. The photoelectric effect was first observed in 1887 by Heinrich Hertz during experiments with a spark gap generator (the earliest device that could be called a radio). In these experiments, sparks generated between two small metal spheres in a transmitter induce sparks that jump between between two different metal spheres in a receiver. Compared to later radio devices, the spark gap generator was notoriously difficult to work with. The air gap would often have to be smaller than a millimeter for a the receiver to reliably reproduce the spark of the transmitter. Hertz found that he could increase the sensitivity of his spark gap device by illuminating it with visible or ultraviolet light. Later studies by J.J. Thomson showed that this increased sensitivity was the result of light pushing on electrons — a particle that he discovered in 1897. While this is interesting, it is hardly amazing. All forms of electromagnetic radiation transport energy and it is quite easy to imagine th...

Lecture 28, Nov. 13, 2000 Recall from last lecture: • c = f l for electromagnetic radiation • Peak of blackbody spectrum: l maxT = 0.2898×10 -2m.K • Electromagnetic radiation quantized in packets called photons. • Planck's constant h = 6.63×10 -34Js • The energy of a photon of frequency f is E ph = hf. 27.2 The Photoelectric Effect Although Planck's resolution of the "ultraviolet catastrophe" for the blackbody radiation spectrum assumed that electromagnetic radiation is quantized in packets of energy we call photons, it was not understood if this was reality, or simply a coincidence of nature. The photoelectric effect was the first clear demonstration that photons are real, not just a fudge. The photoelectric effect occurs when light (visible or ultraviolet) shines on a metal. Energy from the light can cause electrons to be ejected from the surface of the metal. Let's review the reasons why the photoelectric effect supports the postulate of quantization of electromagnetic energy in photons. The photoelectric effect experiments use a device called a photocell. A photocell is made of two "electrodes" (pieces of metal) inside of an evacuated glass bulb. (drawing like Figure 27.4) One of the electrodes is prepared for the photoelectric effect, and is called the photocathode (a cathode is a source of electrons). The second electrode doesn't participate in the photoelectric effect, but is intended to collect the ejected electrons, and is called the anode. When light shines on th...