But what is energy? And what is matter?

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1 Announcements Astronomy 101: Oct. 7, 2008 Take-home exams due now. Please hand your answer sheet to Alex. Exam grades will be posted on OWL. Next OWL homework will be up on Thursday afternoon or Friday morning. If you had to describe the Universe in the simplest possible way, what would you say? The Universe is a mixture of energy and matter. But what is energy? And what is matter? Energy & Matter: spirits in a material world Basic concepts about energy Basic concepts about matter The relationship between energy and matter Energy and Matter Matter is stuff, e.g., the chairs you re sitting on, PRS transmitters, Hasbrouck, annoying professors... Energy is what makes matter move (Reading: Unit 20.1) 1

2 Three Basic Types of Energy kinetic energy energy of motion potential energy stored energy radiative energy energy transported by light IMPORTANT!!!!!!!! Energy is conserved! Energy cannot be destroyed; it can only be converted from one form to another. one form of energy Another form In science, energy is often measured in Joules A calorie is some defined amount of energy. Likewise, a Joule (J) is an amount of energy. Power is the rate of energy usage: Power = Energy Time Power is measured in Watts (W) and is the number of Joules used in one second. 1 Watt = 1 Joule/second How much energy is in a Joule? 4184 Joules = 1 calorie. A typical human needs calories per day, so each day a human uses: 10,000,000 Joules (1 x 10 7 Joules) For comparison, every second the Sun pumps out this much energy in the form of light (radiative energy): 400,000,000,000,000,000,000,000,000 Joules (4.0 x Joules) 2

3 Kinetic Energy Kinetic energy is always due to some kind of motion. Objects at rest have no kinetic energy. You can calculate an object s kinetic energy from its mass and velocity. Large mass, small velocity KE = 1/2 mv 2 small mass, large velocity KE = 1/2 mv 2 mass Velocity squared If mass is in kg and velocity is in m/s, then the kinetic energy is measured in Joules A 10 gram bullet moving at 700m/s and a 500kg rock rolling at 3m/s have roughly the same amount of kinetic energy! PRS Question. You want to launch a rocket into space, and you want to maximize its kinetic energy. Suppose that you can increase the rocket s mass or it s launch velocity by about the same amount, but you can t increase both. Which would you choose? 1. I d make the rocket more massive 2. I d make the rocket faster Watch out for fast things! Damage in a car crash is linearly proportional to mass but is proportional to v 2. Velocity is more important than mass. A hurricane with 120 m.p.h. winds packs four times the punch of a tropical storm with 60 m.p.h. winds. 3

4 Potential Energy gravitational potential energy is the energy which an object stores due to its ability to fall It depends on: the object s mass (m) the strength of gravity (g) the distance which it falls (d) Increasing m, g, and/or d increases the gravitational potential energy stored in an object m Gravitational potential energy = m x d x g d g If we race two equal-mass balls down these two tracks, which ball will win the race? Another delightful interlude from the good folks at PRS 1. The ball on the flat track 2. The ball on the V-shaped track 3. Neither! Both balls will reach the finish line at the same time What happens if I place a ball at the top of the V in the lower track and then let it go? The ball starts at rest with maximum gravitational potential energy (GPE) and zero kinetic energy (KE) because v = 0 As gravity pull the ball toward the ground, the GPE is converted to kinetic energy, and the ball rolls faster At the bottom of the V, it attains its minimum GPE and maximum KE. This is where it rolls the fastest As it climbs back up the track, the KE is converted back into GPE and the ball slows down New energy is never created! All forms of energy on Earth and in the Universe can be traced back to an earlier incarnation of energy. Food contains chemical potential energy Question: Why do sweet fruits generally grow in the south? 4

5 Mind vs. Matter Energy vs. Matter Bart Simpson: Dad, what is the mind? Is it just a system of impulses, or is it something tangible? Homer Simpson: Relax... What is mind? No matter. What is matter? Never mind! Bart Simpson: Ummm... thanks, Dad... There is a profound connection between energy and matter. Matter itself is a form of potential energy. Hiroshima: 1 gram It s not easy to do, but... Matter can be converted to energy! energy is stored in matter itself this mass-energy is what would be released if an amount of mass, m, were converted into energy E = mc2 [ c = 3 x 108 m/s is the speed of light; m is in kg, then E is in joules] Conversion of 0.1 kg of mass (about 3 oz.) to energy produces a a 1 megaton bomb. That is, one magaton of dynamite... Two types of nuclear reactions that release mass-energy Fission Fusion Less mass is present after than before the reaction... Human nuclear power plants use fission (e.g., of uranium or plutonium) E = mc2 Cosmic power plants use fusion (e.g., two hydrogen atoms fused to form one helium atom) 5

6 A nice cosmic power plant: our Sun! E = mc 2 A Sun-like star puts out 9 x Joules every second. The speed of light is 3 x 10 8 m/s. How much mass does the star convert to energy every second? Hurray! PRS Question kg kg kg kg kg A Sun-like star puts out 9 x Joules every second. The speed of light is 3 x 10 8 m/s. How much mass does the star convert to energy every second? Hurray! PRS Question E = mc kg kg kg kg kg Cosmic power plant mass-energy = E/c 2 c 2 = (3 x 10 8 m/s) 2 = 9 x m 2 /s 2 E = 9 x Joules Therefore, mass = kg E = mc 2 6

7 But what about HEAT? Heat, or thermal energy is a form of kinetic energy. Thermal energy is the collective kinetic energy of the many microscopic motions of particles within a substance. lower T Temperature vs. Heat higher T Temperature is the average kinetic energy. Heat (thermal energy) is the total kinetic energy. less heat more heat same T Temperature Scales Absolute zero: coldest possible temperature. Fahrenheit scale zero at freezing saltwater 100 degrees at body temperature Kelvin scale zero at zero energy step size same as Celcius energy proportional to temperature! SUMMARY Energy comes in different forms. Kinetic Energy Bulk motion Thermal Energy Potential Energy Gravitational potential energy Chemical potential energy Electrical potential energy Nuclear potential energy Mechanical potential energy Radiative Energy We ll spend at least one entire class on this one. 7

8 The Phases and Properties of Matter Matter comes in different forms: Solid Liquid Gas (and plasma ) The particular form we see depends on the temperature and the kind of matter we re looking at. Phases of Matter Matter can take many forms depending mostly on its temperature and internal energy. As temperature increases, i.e., as the internal kinetic energy is increased, the strength of the bonding between particles decreases. Greek ideas about matter All matter is composed of four elements: fire, water, earth, and air Democritus: hypothesized fundamental matter particle called the atom (Greek for indivisible) Electrical charge: a fundamental property of matter Oppositely charged particles attract each other Similarly charged particles repel each other 8