* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Photoelectric Effect
Survey
Document related concepts
Quantum electrodynamics wikipedia , lookup
Electromagnetism wikipedia , lookup
Woodward effect wikipedia , lookup
Old quantum theory wikipedia , lookup
Faster-than-light wikipedia , lookup
Aharonov–Bohm effect wikipedia , lookup
Thomas Young (scientist) wikipedia , lookup
History of physics wikipedia , lookup
Electrical resistance and conductance wikipedia , lookup
Time in physics wikipedia , lookup
Theoretical and experimental justification for the Schrödinger equation wikipedia , lookup
Transcript
Date : Photoelectric effect 1. Photoelectric effect Background Photoelectric effect Planck’s constant Work function Photocell Aim of the experiment To determine Planck‟s quantum of action from the photoelectric voltages measured at different wavelengths. Apparatus required Photocell Interference filter set Spectral lamp Hg Power supply for spectral lamps Mounting plate Electrometer amplifier Digital multimeter Connecting cords Theory Inside of the photocell there is a metal coated cathode. The annular anode is opposite the cathode. When a photon of frequency strikes the cathode, then an electron can be ejected from the metal (external photoelectric effect) provided the photon has sufficient energy. Some of these ejected electrons reach the (unilluminated) anode so that a potential difference is set up between anode and cathode, which reaches the limiting value V after a short (charging) time. The electrons flow against the electric field set up by the voltage V. The kinetic energy of the electrons, determined by the light frequency, , h W 1 m v 2 (Einstein equation) 2 where, h = Planck’s constant, W = work function of the cathode surface, v = electron velocity and m = rest mass of the electron. Electrons will thus only reach the anode as long as their energy in the electric field is equal to the kinetic energy: eV 1 mv2 2 with, e = electron charge. 15 Photoelectric effect An additional contact potential occurs, because the surfaces of the anode and cathode are different: eV 1 mv2 2 If we assume that W and „ ‟ are independent of frequency, then there will be a linear relationship between the voltage V (to be measured at high impedance) and the light frequency , V W h e e Next, by measuring the voltages at different wavelength (i.e., at different frequencies) one plots the voltage vs frequency graph. A straight line is drawn through the experimental points (i.e. V = a + b ). One can determine the value of Planck‟s constant from the slope of the straight line, the literature value of which is h 6.63 10 34 Js. 16 Photoelectric effect G Procedure Supply Power (12 V~) Electrometer Amplifier Multimeter V Lamp Photocell C G Fig. 1 Experimental set up for determining Planck‟s quantum of action 1.Make the electrical connections as per diagram 1. Keep the lamp and filter in the same line. Connect the ground of the photocell to the ground of the electrometer amplifier. Note the ground terminal of the amplifier and connect correctly (you may connect it to the base plate). 2. Set the digital voltmeter range to 2V DC. Measure the voltage without any filter. Hold the terminal G in hand while taking the readings, if the voltage fluctuates too much in the multimeter. 3. Close the photocell slit. Place one filter in front of the photocell and discharge the electrometer by short-circuiting the ground terminal with the positive electrometer amplifier entrance before measuring any voltage. Open the photocell slit and measure the voltage. Wait until the voltage reading is steady. 4. Measure voltages for various filters by repeating the procedure 3. 5. Repeat the experiment for different lamp – filter separations. 6. Plot frequency vs. voltage and find h from the slope of the graph. 17 Photoelectric effect Observations Voltage for closed slit………..V Voltage without any filter (open slit) …………… V Separation between lamp and filter Table 1 Determination of h Wavelength Frequency () ( = c/) (nm) (sec-1) 366 405 436 546 578 366 405 436 546 578 366 405 436 546 578 Results and Calculations Value of h from the plot …………………………J. sec. 18 Voltage (V) (volt) Photoelectric effect Error Calculation: If the slope is calculated using voltages V1 and V2 from the graph, then h V1 V2 h V1 V2 Discussion i. ii. The view that the light propagates as a series of little packets of energy (photons) is directly opposed to the wave theory of light. According to the wave theory, which provides the sole mean of explaining the optical effects like interference and diffraction, the energy carried out by the light is distributed continuously through out the wave pattern. According to the quantum theory, which is strikingly successful in explaining photoelectric effect, light spreads out from the source as a series of localized concentration of energy. In a specific event light exhibits either a wave or a particle nature, never both simultaneously. The wave theory of light and the quantum theory of light are complement to each other. Questions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. What is the value of Planck‟s constant? What are the sources of error in this experiment? What is photoelectric effect? Define “work-function”. What is the time lag between the arrival of light at a metal surface and the emission of photoelectron? What do you mean by stopping potential/extinction voltage/cut off voltage? What type of material should be chosen for photoelectron emission? What is photoelectric cell? What do you know about the structure of photovoltaic cell? Can you name a recent method of a very accurate determination of Planck‟s constant? Interpret thermionic emission in light of photoelectric effect. In which phenomenon do you see the inverse photoelectric effect? References 1. PHYWE, LEP 5.1.04 Planck‟s “quantum of action” from photoelectric effect 2. Prospective of Modern Physics by A. Beiser 539 BEI/P N69 3. The Feynman Lectures on Physics (Vol III) by R.P. Feynman 530 FEY/L 19 Photoelectric effect Graph : Photoelectric effect 20 Photoelectric effect Detail of the Electrometer amplifier Charges that have resulted from static electricity can be determined by transferring the charge to a capacitor of known capacitance and measuring the electrical potential of this. It is difficult to measure this potential with customary measuring instruments, as a current flows through the measuring instrument and this leads to decay of the applied charge. The smaller the internal resistance of the measuring instrument, the quicker the potential reduced. Customary measuring instruments have an internal resistance of about 10 M ohms. The electrometer amplifier has a voltage input with a very high internal resistance (>1013 ohm) and can be used to measure such charges. Functional and operating elements 1. Amplifier input 2. Auxiliary input. 3. Amplifier output 4. Trimmer for offset voltage 5. LED for display of operating voltage (12 V~) 6. For alternating voltage supply (12 V~) 7. Connecting socket for plug-in power supply unit with hollow plug. 8. For external connection 9. Reference potential (earth) 21