Physics Lab
h = 6.626E-34 J.s λp T = 2.90E-3 m•K c = λ f E=hf KEmax = h f – Wo Wo = h fo KE = ½ m v2 1 eV = 1.6E-19 J λ = h / p = h / m v m λ = d sinϴ h f = E u – El En = (−13.6 eV)Z2/n2 1/λ = (13.6 eV/h c) × Z2(1/n2 – 1/n’2) https://phet.colorado.edu/en/simulation/blackbody-spectrum Open the link. Click on Graph Values, Labels, and Intensity. We have a Black Body curve, for the sun. This is very similar to the T = 6,000 K curve (green) in the lecture. Here it is red, centered in the 400-700 nm visible range. The “thermometer” indicates its temperature as 5800 K. 1.a. Its peak wavelength is _____ nm (convert µm to nm). 1.b. Using Wien’s law, its temperature is ______ K. Does this check with the upper-right indicated temperature? Yes/No. 1.c. Pull the thermometer temperature down to 4000 K (above a light bulb). Using Wien’s law, its peak wavelength is ______ nm. Does this check with the simulation’s indicated λp? Yes/No. Is its intensity less, at lower temperature? Yes/No. https://phet.colorado.edu/en/simulation/legacy/photoelectric Open the link. Pull the Intensity slider to 100 %. λ = 400 nm (violet) photons are hitting the sodium metal’s surface on the left. It is randomly emitting electrons, moving to the right. We are seeing that the photon’s energy “h f” exceeds the “work function” Wo for the metal (per Einstein’s thinking). Longer wavelength photons would have less “f” and thus less energy “h f”. We can move the wavelength slider to a longer wavelength, and the photoelectric effect will stop. 2.a. Try doing this, it occurs at about λ = 540 nm. Record where you see no more emitted electrons, λ = _____ nm. 2.b. This means no more electrons having KE > 0, so this is the threshold where their KE = 0. Calculate the work function for this wavelength, in eV, Wo = _____ eV. The lecture (Example 27-5) gives Wo = 2.28 eV for sodium. Is your calculation close? Yes/No. 2.c. Move the photon’s wavelength back to 400 nm (exactly, with the slider). As in Example 27-5 from the lecture, calculate the moving electron’s KEmax, and then their speed. Their mass is 9.1E-31 kg. v = ______E5 m/s (use your value of Wo from 2b). https://phet.colorado.edu/en/simulation/legacy/quantum-wave-interference Open the link Turn on the laser with its red button, however we want electrons to show their behavior as waves. So click on the pull-down Photons (to the right of this), and click on Electrons. Then to the right, click on Double Slits. Interference occurs, and we see the intensity build up on the upper screen. Click on Ruler, to make 3 measurements: 3.a. Slit separation (center to center), verify d =1.2 nm, Yes/No. 3.b. On the screen, central max to 1st order max, verify 1.0 nm, Yes/No. 3.c. Slit to screen distance, rotate ruler, verify 1.9 nm, Yes/No. 3.d. Calculate interference angle, ϴ = tan-1(1.0/1.9) = _____o. 3.e. Use m λ = d sinϴ to calculate λ = _____ nm. 3.f. Use λ = h / m v with mass, m = 9.1E-31, to calculate v = _____E6 m/s. Does this check with the lower velocity slider indicating about 1100 km/s? Yes/No. https://phet.colorado.edu/en/simulation/legacy/discharge-lamps Open the link Click on the upper tab Multiple Atoms. Click on “Continuous” under Electron Production. Under Options (lower right) click on Spectrometer and Squiggles. The discharge tube contains hydrogen gas with electrons (from the voltage on the tube’s electrodes) going through the tube, getting absorbed by the atoms, and then emitting photons. The squiggles on the right show the electrons jumping through energy levels as the atoms get excited, and then emitting photons as the excited electrons drop down to lower levels. The ground state is n = 1, and other levels up to n = 6 are shown. The squiggles show electrons jumping between energy levels as atoms get excited, and then emitting photons as the excited electrons drop down to lower levels. The ground state is n = 1, and levels up to n = 6 are shown. The spectrometer shows the build up of photons, at spectral line locations coming from Bohr’s equation. 4.a. Example 27-14 calculated the location of the violet Balmer line at λ = 411.1 nm. This was for n = 6 to n’ = 2. Calculate the red Balmer line, n = 3 to n’ = 2, λ = _____ nm. 4.b. Sodium: click on this under the Atom Type pull-down. Sodium has 11 protons, Z= 11. We see a prominent yellow line, and its wavelength λ = 589 nm. The squiggles in the energy levels show this occurring for the n’ = 2 and n= 1 states. Use sodium’s Z, n’ and n, to calculate the predicted wavelength for the Bohr model, λ = _____ nm. Is this close to 589 nm? Yes/No. 4.c. Conclusion: the Bohr model works well for hydrogen: Yes/No. The Bohr model works well for sodium: Yes/No. Next page Photos: Nothing to turn in, photos of discharge tubes and a diffraction grating “held in front” to show spectral lines. The hydrogen discharge tube has an overall pinkish light, and several lines none of which are pink. The mercury tube has a blue overall appearance, and predominant blue and green spectral lines. The sodium tube has a yellow visual appearance, and yellow spectral line. Yellowish streetlamps are sodium vapor, as shown. The neon tube has a bright orange appearance, and a “fan” of bright orange spectral lines. Neon is unmistakable. It’s interesting that hydrogen’s pink appearance is due to our eyes combining purple, blue, turquoise and red spectral lines. We might never have guessed this without a spectrometer, using a diffraction grating. Bohr “stepped up” to this challenge, explaining it with his electron energy levels, relating to photons of light. Other atoms might not be so surprising, yet Bohr’s model could not predict the observed wavelengths. Our next Chapter 28, Quantum Mechanics, builds on Bohr’s model to provide the complete picture that we yet use today.
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