Ch Energy. Energy = the ability to do work; energy is transferred by a force moving an object over a distance

Transcription

1 Ch Energy Energy = the ability to do work; energy is transferred by a force moving an object over a distance Work --> a transfer of energy (when work is done on an object, energy is transferred to the object) 1

2 2 Energy Types: 1) Kinetic energy --> energy of motion; depends upon mass and speed of the moving object. Kinetic Energy (KE) = ½ mv 2 units: kg. m 2 or Joules (N. m) s 2 Note that a change in speed will have a greater effect on the size of kinetic energy. 2

3 2) Potential Energy --> energy stored as a result of position or shape 2 types of potential energy: A) Gravitational potential energy = depends on an object s height PE = (mass)(gravity)(height) PE = mgh A diver is at the top of a 10.0 m. high diving platform and has a mass of 50.0 kg. What is her potential energy? Mass = 50.0 kg Gravity = 9.8 m/s 2 Height = 10 m. PE = mgh PE = (50.0 kg)(9.8 m/s 2 )(10.0 m) = 4900 J *Note that PE is usually given in reference to a level. 3

4 B) Elastic potential energy = Energy of an object that can be stretched or compressed Potential & kinetic energy are often changed from one to the other. Ex: A pendulum! 4

5 Forms of Energy Major forms of energy: 1. Mechanical energy = associated with the motion and position of everyday objects; usually of large scale objects Ex: moving cars, bouncing balls 5

6 2. Thermal energy = energy in moving atoms and molecules Ex: Heated gases have large amounts of thermal energy 3. Chemical energy = energy stored in chemical bonds; released when bonds are broken Ex: cars burning gasoline, wood burning 6

7 4. Electrical energy = energy from electric charges and moving electrons Ex: lightning, electricity to power things 5. Electromagnetic energy = form of energy that travels through space in the form of waves Ex: visible light, x-rays 7

8 6. Nuclear energy = energy stored in nuclei in the form of strong & weak nuclear forces Ex: nuclear fission produced energy by splitting the nucleus of an atom 8

9 Energy Conversion & Conservation Energy conversion --> the conversion from one energy form to another Lift your backpack! Lifting --> muscle energy (mechanical) --> food energy (chemical) --> plant energy (chemical) --> solar energy (electromagnetic) Light a match! chemical --> mechanical --> thermal energy --> chemical energy which produces more thermal energy and electromagnetic energy 9

10 Rube Goldberg devices are famous for the many energy transfers that take place!! 10

11 Law of Conservation of Energy --> energy cannot be created or destroyed; only converted from one form to another GPE to KE is one of the most common energy transformations Pendulum motion = GPE --> KE --> GPE --> KE Why does it eventually stop? Friction So the GPE is ultimately converted to thermal energy! 11

12 Pole vaulting = KE --> GPE (pole) --> GPE (height) --> KE --> sound/thermal/transferred to mat --> thermal Conservation of Mechanical Energy (assuming no loss due to friction) (KE + PE) beginning = (KE + PE) end 12

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14 Energy & Mass Einstein s Theory of Relativity: Energy and mass are equivalent and can be converted into each other. E = mc 2 E = energy m = mass c = speed of light Because the speed of light = 3.0 x 10 8 m/s, a very small amount of matter can produce a large amount of energy 14

15 Energy Resources Nonrenewable energy = exist in limited quantities; can t be replaced except over the course of millions of years. Ex: oil, natural gas, coal, uranium Fossil fuels = formed underground from the remains of once-living organisms; currently account for majority of world s energy use; aren t distributed evenly --> 60% of oil is located in Middle East, 2% in the US; 25% of world s coal supplies is in the U.S.; are relatively inexpensive but produce pollution when burned Renewable energy = can be replaced in a relatively short period of time; most originate from the sun; include hydroelectric, solar, geothermal, wind, biomass, and nuclear fusion (hopefully!) 15

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17 Types of Renewable Energy 1) Hydroelectric Energy --> energy obtained from flowing water; as water flows downhill its GPE is converted to KE which then turns turbines connected to generators; can occur naturally or by construction of a dam Advantages of dams - lack of pollution, creates recreational opportunities Disadvantages of dams - affect ecosystems, flooding above the dam, not a potential energy source everywhere 17

18 2) Solar energy --> sunlight converted into usable energy; passive systems = sunlight heats buildings naturally; active systems = solar panels that collect sunlight and convert it to electrical energy and flat collection plates that absorb sunlight that water then flows across to get heated. Advantages - nonpolluting Disadvantages - not practical in areas with many cloudy days; expensive to get started 18

19 3) Geothermal energy --> thermal energy beneath earth s surface; geothermal power plant pumps water into the ground, where it turns into steam which is then used to drive electric generators; Advantages - nonpolluting Disadvantages - not widely available 19

20 4) Biomass energy --> based on the chemical energy stored in living things; can be converted directly into thermal energy; includes burning of wood, peat & cornstalks Advantages - cheap, available Disadvantages - if not burned properly, can pollute 20

21 5) Hydrogen fuel cells --> reacts hydrogen with oxygen to produce electricity; can use electricity from solar cells to extract hydrogen from water Advantages - nonpolluting Disadvantages - costly, requires large solar cell along with fuel cell 21

22 6) Nuclear fusion - produced large amounts of energy by combining 2 atoms into 1 Advantages - nonpolluting Disadvantages - still trying to make it happen! 22

23 7) Wind energy - uses wind turbines to convert kinetic energy of wind to mechanical energy of moving wind turbines which then produce electricity Advantages - nonpolluting Disadvantages - expensive to start, bad when there is little wind 23

24 Conserving Energy Energy resources can be conserved by: 1. Reducing energy needs 2. Increasing the efficiency of energy use 24