Download Understanding Mass Defects and Nuclear Energy from Fusion: Light Energy from the Sun and more Study notes Physics in PDF only on Docsity! 1 PHY 101 Lecture #18 Light Energy from the Sun 1 PHY 101 Lecture #18: Light Energy from the Sun Prof. Peter R. Saulson saulson@physics.syr.edu http://physics.syr.edu/courses/PHY101/ Off. Hrs: Tue 9:30 –11:00, Physics 263-4 Prof. Schwarz’s Problem Sessions: Mon and Tues, 5:15 – 6:15, Physics 202/204 PHY 101 Lecture #18 Light Energy from the Sun 2 Special lab this week Studios this week meet at Rm. 306 Women’s Building, in the Exercise Science lab of Prof. Jill Kanaley Please go there directly, for the lab exercise on human bioenergetics. If you have volunteered to be a human subject, please remember your gym clothes/shoes! PHY 101 Lecture #18 Light Energy from the Sun 3 Outline 1. Why aren’t atomic masses given by integers? 2. Mass defects and E = mc2 3. Nuclear energy from fusion 4. The sun 5. Earth’s solar energy budget 6. Light PHY 101 Lecture #18 Light Energy from the Sun 4 Isotopes Most elements come in different “flavors” called isotopes. All isotopes of element share same Z, but have different N. Chemical properties determined by Z. Isotopes have different number of neutrons in nucleus. PHY 101 Lecture #18 Light Energy from the Sun 5 Isotopes and atomic weights Atomic weight of each isotope almost precisely an integer But admixture can make average atomic weight not an integer. Example: Lithium, Z = 3, has two isotopes. Li6 has 3 protons, 3 neutrons. Li7 has 3 protons, 4 neutrons. 92.6 % of lithium atoms are Li7 , rest are Li6. ==> atomic mass of Li is 6.94. PHY 101 Lecture #18 Light Energy from the Sun 6 Mass defects of pure isotopes Purify isotopes, masses still aren’t perfect integers. C12 has mass 99.22% of 12 hydrogen atoms. Fe56 has mass 99.11% of 56 hydrogen atoms. Shortage of mass is called mass defect. 2 PHY 101 Lecture #18 Light Energy from the Sun 7 Mass defect in α decay Decay of Po210 by α emission to Pb206. Masses: Pb206 205.97447 amu He4 4.00260 final mass after decay 209.97707 initial mass of Po210 209.98228 loss of mass 0.00581 amu In decay, 0.00581 amu of mass is lost, but 5.4 MeV of kinetic energy is created. PHY 101 Lecture #18 Light Energy from the Sun 8 Ratio of KE to mass loss Convert numbers in α decay to SI units. Mass loss = 9.65 * 10-30 kg. KE appearing = 8.65 * 10-13 J. ( ) . m/sec10*99.2 sec/m10*96.8 loss mass 2 28 2216 c KE = = = PHY 101 Lecture #18 Light Energy from the Sun 9 E = mc2 When mass is lost in α decay, energy is created. KE created = c2 * decrease in mass, or This is also explanation for mass defects. Nuclei gave up energy to come together from separated protons and neutrons, so mass is lower. Mass is just another form of energy. Transformation into other forms is possible. .2mcE = PHY 101 Lecture #18 Light Energy from the Sun 10 Fusing hydrogen into helium Fusion refers to the creation of a nucleus of He4 out of 4 protons (4 nuclei of H1.) Masses: He4 4.002603 amu 4 @ H1 4.031301 amu loss of mass 0.028698 amu In fusion of hydrogen into helium, mass of 0.029 amu (= 4.77 * 10-29 kg) is lost. PHY 101 Lecture #18 Light Energy from the Sun 11 Energy from fusion Mass loss in H ==> He fusion is huge. 0.72% percent of original mass disappears. Remember E=mc2. Loss of 4.77 * 10-29 kg means that energy of 4.29 * 10-12 J is created. This is lots of energy if many H atoms fuse. Many H atoms fuse in: hydrogen bomb fusion reactor sun PHY 101 Lecture #18 Light Energy from the Sun 12 The sun Sun’s mass is 2 * 1030 kg. Most of this is hydrogen. H doesn’t fuse under ordinary conditions. We made hydrogen gas safely in electrolysis. Strong electrical repulsion between protons prevents fusion in most conditions. Sun’s core is very dense, hot: 15 million deg C. Fusion proceeds rapidly in these conditions.
