PHY183-Lecture17 - E n e rg y, W o rk , P o w e r ! What we...

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Unformatted text preview: E n e rg y, W o rk , P o w e r ! What we will learn this week: • Kinetic energy is the energy contained in the motion of an object. • Work is defined as energy transferred to an object or transferred from an object due to the action of a force. Positive work is defined as a transfer of energy to the object, and negative work is a transfer of energy from the object. Positive work is work done on the object, while negative work is work done by the object. • Work is the scalar product of the force vector and the displacement vector. • The change in kinetic energy due to a force is equal to the work done by the force. • Power is the rate at which work is done 1 September 30, 2006 Physics for Scientists&Engineers 1 2 Physics for Scientists & Engineers 1 F a ll S e m e s t e r 2 0 0 6 Lectur e 1 7 September 30, 2006 Physics for Scientists&Engineers 1 E n e r g y in o u r E c o n o m y ! ! ! ! ! Energy production Energy consumption Energy efficiency Energy resources National and global energy policy • Needs informed citizens to join discussion E n e r g y , th e B IG P IC T U R E September 30, 2006 Physics for Scientists&Engineers 1 3 September 30, 2006 Physics for Scientists&Engineers 1 4 E n e r g y fr o m F o s s il F u e ls ! F o s s i l f u e ls • Coal • O il • Gas ! B asic ch e m ical co m po sit io n : H yd r ocar b on s ! B u r n in g h yd r o car b o n s w i t h o x y g e n r e le a s e s e n e r g y -> u s e f u l f o r p o w e r p r o d u c t io n o n a v e r y la r g e s c a le ! B asis f o r m o st o f o u r ch e m ical i n d u s t r y a s w e ll ! P r o b le m s : • N o n -re n e w a b le re s o u rc e • P o llu tio n : c a r b o n d io x id e e m is s io n : a c id r a in , g lo b a l w a r m in g September 30, 2006 Physics for Scientists&Engineers 1 5 S o la r P o w e r ! ! ! ! Radiation from Sun contains tremendous amounts of energy Guaranteed to be provided for next couple of billion years No pollution Use: • Primary: solar cells Solar farm in CA http://kjcsolar.com/aerial1.jpg • Secondary: wind, hydro September 30, 2006 Physics for Scientists&Engineers 1 6 “Renewable” Energy Sources ! ! ! ! Wind Hydroelectric Geo-thermal Biomass (plants, animals) E n e r g y fr o m N u c le a r R e a c t io n s ! R e a c t i o n s o f a t o m i c n u c le i c a n li b e r a t e t r e m e n d o u s p o w e r • S p littin g h e a v y n u c le i ( u r a n iu m , p lu to n iu m ) • F u s in g h y d r o g e n in to h e liu m ! A d van tages: • N o g re e n h o u s e g a s e m is s io n • P r a c tic a lly in fin ite s u p p ly S c h e m a tic d r a w in g o f IT E R ! P r o b le m s : • V e r y d a n g e r o u s r a d io a c tiv e w a s te n e e d s v e r y l o n g te r m s to r a g e • P o te n tia l fo r u s e in w e a p o n s o f m a s s d e s tr u c tio n September 30, 2006 Physics for Scientists&Engineers 1 7 September 30, 2006 Physics for Scientists&Engineers 1 8 H yd ro g e n E c o n o m y ! Basic Idea: • Produce hydrogen from renewable (or at least non-fossil) energy sources • Burn hydrogen with oxygen into water ! Advantages • Reduced pollution • Reduced dependence on fossil fuels ! Challenges/problems • Transport and storage • Sufficient production of hydrogen (may need to invest more into nuclear power sources) www.emagazine.com/january-february_2003/0103feat1.htm September 30, 2006 Physics for Scientists&Engineers 1 9 September 30, 2006 E n e rg y a n d Y O U Physics for Scientists&Engineers 1 10 E n e r g y , C a lo r ie s , E x e r c is e ! Organisms need regular energy supply for survival ! Caloric intake adds energy ! Physical activity “burns” energy ! Excess energy supply is stored as fat ! Weight loss only through reduction in caloric intake or increase in physical activity L e t ’ s S t a r t a t t h e B e g in n in g : K in e t ic E n e r g y ! Energy contained in the motion of an object ! Definition 2 1 K = 2 mv ! Unit of kinetic energy: ! This energy unit has received its own name, Joule (J), named after British physicist James Joule (1818-1889) ! Energy unit ! Useful conversion: 1 J = 1 N m [ K ] = [ m ] ! [ v]2 = kg m 2 / s 2 1 J = 1 kg m 2 / s 2 11 September 30, 2006 Physics for Scientists&Engineers 1 12 September 30, 2006 Physics for Scientists&Engineers 1 K in e t ic E n e r g y E x a m p le s ! Car of mass 1,310 kg driving 55 mph (=24.6 m/s) O t h e r E n e r g y U n it s ! Atomic and nuclear physics introduced the electron-Volt K car = 1 mv 2 = 1 (1310 kg)(24.6 m/s)2 = 4.0 ! 10 5 J 2 2 ! Mass of the Earth is 6·1024 kg, and it orbits the Sun with a speed of 30,000 m/s K sun = 1 mv 2 = 1 (6.0 ! 10 24 kg)( 3.0 ! 10 4 m/s)2 = 2.7 ! 10 33 J 2 2 ! Baseball (mass “5 ounces avoirdupois” = 0.142 kg) thrown at 80 mph (= 35.8 m/s) has kinetic energy K baseball = 1 mv 2 = 1 (0.142 kg)( 35.8 m/s)2 = 91 J 2 2 ! Electron (me = 9.1·10-31 kg) moving with a speed of 1.3·106 m/s (= 0.4% of the speed of light): K e = 1 mv 2 = 1 (9.1 ! 10 "31 kg)(1.3 ! 10 6 m/s)2 = 7.7 ! 10 "19 J 2 2 September 30, 2006 Physics for Scientists&Engineers 1 13 1 eV = 1.602·10-19 J (electron in previous example has ~5 eV kinetic energy) ! The energy you eat is measured in food calories 1 Cal = 4186 J (1 Cal > 40 times the kinetic energy of the baseball in previous example) ! For very large energy measures one need a large unit. The energy released by one million tons of TNT is large 1 Mt = 4.0·1015 J September 30, 2006 Physics for Scientists&Engineers 1 14 E x a m p le : F a llin g V a s e Question: ! A crystal vase (mass 2.40 kg) is dropped from a height of 1.30 m and falls to the floor. What is its kinetic energy just before impact? Answer: ! Once we know the velocity of the vase just before impact, we can put it into our equation for the kinetic energy. To obtain this velocity, we remind ourselves of the kinematics of free-falling objects. ! Use: (since v0 = 0) 2 v 2 = of kinetic ! y0 ) 2 g obtain ) ! Now use definition v0 ! 2 g( y energy=and ( y0 ! yin this case: E x a m p le : F a llin g V a s e - O b s e r v a t io n s ! In the case of the falling vase, the kinetic energy is a function of the height (linearly dependent on height) from which the vase was released, K = mg( y0 ! y) ! Numbers: K = 1 mv 2 = 1 m ( 2 g( y0 ! y)) = mg( y0 ! y) 2 2 K = (2.40 kg) ! (9.81 m/s 2 ) ! (1.30 m " 0 ) = 30.6 J ! Gravitational force, Fg = gm, accelerated vase during free fall, and thus increased its kinetic energy ! Kinetic energy is proportional to the magnitude of the gravitational force, too. ! Here the kinetic energy gained by the vase is simply the product of the magnitude of the gravitational force and the displacement. September 30, 2006 Physics for Scientists&Engineers 1 15 September 30, 2006 Physics for Scientists&Engineers 1 16 ...
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