15.2 Energy Conversion and Conservation

1. 15.2 Energy Conversion and Conservation Page 453-459

2. Energy Conversion • Energy can be CONVERTED from one form to another • The process of changing energy from one form to another • Ex: When a spring unwinds goes from Elastic Potential Energy TO Kinetic Energy • Light bulb: Electric (the battery creates electricity) TO Thermal and Electromagnetic (heat of the bulb and the light it shines)

3. Energy conversion examples Chemical energy (in battery) Electrical energy (through the wires) Electromagnetic and thermal energy (from the light bulb) Electrical energy (from the wall socket to the electric stove) Thermal energy (the hot stove coil)

4. Law of Conservation of Energy • The law of conservation of energy states that Energy cannot be created or destroyed • In a closed system (where nothing can enter or leave) the amount of energy present at the beginning of a process is the same as the amount of energy at the end • Therefore, the pendulum in the picture would always reach the exact same height on either side and would never come to rest

5. Law of Conservation of Energy (cont.) • In the real world, energy can be lost to frictional forces (kinetic to thermal energy which is then lost) If you account for thermal energy lost to friction energy will be conserved overall • Friction can slow things • So the pendulum would eventually stop moving

6. Friction effects machinery • Friction within machinery reduces efficiency • Friction is a major cause of energy consumption in cars and factories because all the moving parts are subject to friction • You can reduce friction but you cant avoid it. Engine block over heating can be caused by excess friction. If a lot of energy is lost as heat more fuel is needed to do less work.

7. Energy Conversion • The GPE of an object is converted to KE of motion as it falls. • Ex: Clock Pendulum

8. Energy Conversion: gull vs oyster • Sea gulls use PE to obtain food • As the gull brings the oyster up into the air PE increases • At its highest point the gull releases the oyster at max PE • As it falls PE is converted to KE as the oyster increases is velocity toward the rocks • As it hits the rocks the oyster is at max KE and it shatters into the gulls lunch

9. Energy Conversion: pole vault • Pole-vaulter sprints = kinetic energy • Pole bends = elastic potential energy • Pole springs back into shape = kinetic energy • Pole-vaulter rises quickly into the air = kinetic to gravitational potential energy • Reaches highest point and begins to fall, max PE to KE http://www.popsci.com/how-it-works/article/2008-07/how-it-works-pole-vault

10. Energy Conversion Calculations • Mechanical energy = KE + PE • When friction is small enough to be ignored mechanical energy at the beginning equals the mechanical energy at the end which means mechanical energy remains constant • Conservation of Mechanical Energy • (KE + PE)begin = (KE + PE)end

11. Conservation of mechanical energy problem • At a construction site, a 1.5 kg brick is dropped from rest and hits the ground at a speed of 26 m/s. assuming air resistance can be ignored, calculate the GPE of the brick BEFORE it was dropped. • Given info: • Mass = 1.5 kg • Velocity = 26 m/s • But we know that • KEb + PEb = KEend + PEend • So if….. KEb= 0 and PEend = 0 • Then… 0 + Peb = KEend + 0 • And finally…. PEb = KEend • So we solve for KEend to find PEb • KEend = 1/2 mv2 • KEend = 1/2 (1.5 kg) (26m/s)2 • KEend = 1/2 (1.5 kg) 676 m2/s2 • KEend = 507 kgm/s2m • KEend = 507 N m • KEend = 507 J • So now we know KEend therefore we also know PEb • PEb = KEend = 507 J, so PEb = 507 J Cannot find PE with only this info Step 1 Step 2 Step 3 Final answer

12. Energy and Mass • Einstein’s equation, E = mc2, says that energy and mass are equivalent and can be converted into each other • Meaning energy is released as matter is destroyed, and matter can be created from energy • Therefore, the law of conservation of energy has been modified to say that mass and energy together are always conserved. • E = mc2 • m = mass and C = speed of light

13. The End