The Story of Energy Forms and Functions
What are 5 things E helps us do?
Batteries store energy! This car uses a lot of energy Even this sleeping puppy is using stored energy. We get our energy from FOOD!
What are 5 transfers or transformations of energy happening right now in the classroom? examples
Energy O is the ability to cause CHANGE or MOTION
All Energy potential kinetic with an example of each
Potential Energy Stored-up energy, or energy held in readiness potential" simply means the energy has the ability to do something useful later on
For example: When a roller coaster slows to a stop at the top of a hill, it has potential energy because of where it is (position in space). It has the potential to move because it is above the ground and has somewhere to go.
OSubstances like wood, coal, oil, and gasoline have stored energy because of their chemistry they can burn OStored energy is potential energy
Potential forms of energy Chemical: E stored in bonds of molecules Nuclear: E stored in nucleus of atoms Mechanical: E stored in objects by applying force Gravitational: E of place or position
Chemical PE examples: You can connect a battery to a circuit and a reaction between chemicals takes place inside the battery and it produces electricity. You cannot see the energy in the battery when the battery is just sitting around; it is when the electricity is produced that the energy is seen Chemical energy is stored in the chemical bonds of atoms and molecules. It can only be seen when it is released in a chemical reaction. When chemical energy is released, the substance from which the energy came is often changed into an entirely different substance.
Nuclear PE examples:
Nuclear Fusion ONuclear fusion is the process by which multiple nuclei join together to form a heavier nucleus. OIt is accompanied by the release or absorption of energy depending on the masses of the nuclei involved.
Fusion Changes Mass to Energy E=mc 2.993 kg Helium 1kg Hydrogen
Example: Mechanical PE Elastic potential energy E stored in objects by applying force o Potential energy due compression or expansion of an elastic object.
other examples? gravitational potential energy Potential energy as a function of position.
Gravitational Potential Energy o If an object is elevated and has the force of gravity acting on it, the object has PE. o P.E. = mass x height x accel. due to gravity
Potential Energy (joules) PE = mgh Mass (kg) Height (m) Acceleration of gravity (9.8 m/sec 2 ) U g = mgh
There are a lot of advantages of knowing some physics but this one is definitely not one of them.
try a calculation What is the potential energy of a 50 kg gorilla on top of a skyscraper if he is 480 m above the street below? 50 kg PE = mgh 480 m PE = (50 kg)(9.8 m/s 2 )(480 m) PE = 235 kj
U g = (102 kg)(9.8 m/s 2 )(4 m) = 3,998 J O A cart with a mass of 102 kg is pushed up a ramp. O The top of the ramp is 4 meters high. 3,998 50 = 80 seconds to push the cart up the ramp. O How much potential energy is gained by the cart? O If an average student can do 50 J of work per second, how much time does it take to get up the ramp?
Kinetic Energy The energy of motion O If an object is moving, it has KE.
Examples of Kinetic Energy:
Kinetic forms of energy Radiant: E traveling in waves Thermal: or heat, the internal energy in substances Motion: movement of a substance from one place to another Sound: movement of E through substances in longitudinal waves Electrical: movement of electrons
O The kinetic energy of a moving object depends on two things: mass and speed. Kinetic Energy (joules) KE = 1 mv 2 2 Mass (kg) Speed (m/sec) Energy is measured in the same units as work because energy is transferred during the action of work.
Try a couple: What is the kinetic energy of a 5-g bullet traveling at 200 m/s? 5 g 200 m/s K mv 1 2 1 2 2 2 K = 100 J (0.005 kg)(200 m/s) What is the kinetic energy of a 1000-kg car traveling at 14.1 m/s? K 1 2 1 2 2 mv (1000 kg)(14.1 m/s) 2 K = 99.4 J
O Kinetic energy becomes important in calculating things like braking distance
Law of Conservation of Energy
O Potential Energy can be changed into Kinetic Energy O Also Kinetic Energy can be changed into Potential Energy
Energy in a system is Conserved O Don t we create energy at a power plant? O Oh that this were true no, we simply transform energy at our power plants O Doesn t the sun create energy? O Nope it exchanges mass for energy The Law of Conservation of Energy means energy is neither created nor destroyed.
Law of Conservation of Energy O As energy takes different forms and changes things by doing work, nature keeps perfect track of the total. O No new energy is created and no existing energy is destroyed. energy can neither be created nor destroyed, but can be change from one form to another.
Energy Exchange O Though the total energy of a system is constant, the form of the energy can change O A simple example is that of a simple pendulum, in which a continual exchange goes on between kinetic and potential energy pivot KE = 0; PE = mgh h PE = 0; KE = mgh KE = 0; PE = mgh height reference
PE = mass x g x height The total amount of mechanical energy in a system is the sum of both potential and kinetic energy, (measured in Joules (J)). E total = KE + PE KE = ½ mass x velocity 2
Energy Conversion/Conservation Example 10 m 8 m 6 m 4 m P.E. = 98 J K.E. = 0 J P.E. = 73.5 J K.E. = 24.5 J P.E. = 49 J K.E. = 49 J O Drop 1 kg ball from 10 m O starts out with PE = (1 kg)(9.8 m/s 2 )(10 m) = 98 J of gravitational potential energy O halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy 2 m 0 m P.E. = 24.5 J K.E. = 73.5 J P.E. = 0 J K.E. = 98 J O at floor (0 m), all potential energy is given up to kinetic energy
Energy Conversion/Conservation Example 10 m 8 m 6 m 4 m P.E. = 98 J K.E. = 0 J P.E. = 73.5 J K.E. = 24.5 J P.E. = 49 J K.E. = 49 J O Drop 1 kg ball from 10 m O starts out with PE = 98 J O at floor (0 m), all potential energy is given up to kinetic energy. KE = 98 J O What is its speed (velocity) when it hits the floor? 2 m 0 m P.E. = 24.5 J K.E. = 73.5 J P.E. = 0 J K.E. = 98 J KE = ½mv 2 = 98 J Rearrange the formula for v 2
Energy Conversion/Conservation Example 10 m 8 m 6 m 4 m 2 m 0 m P.E. = 98 J K.E. = 0 J P.E. = 73.5 J K.E. = 24.5 J P.E. = 49 J K.E. = 49 J P.E. = 24.5 J K.E. = 73.5 J P.E. = 0 J K.E. = 98 J O Drop 1 kg ball from 10 m O What is its speed (velocity) when it hits the floor? KE = ½mv 2 = 98 J Rearrange the formula for v 2 v 2 = 2 (98) m v 2 = 196 m 2 /s 2 Take the square root of both sides v = 14 m/s v = 14 m/s
Example 1 An 80 kg diver drops from a board 10.0 m above the water surface. Find his speed 5.00 m above the water surface. (Neglect air resistance.) Step 1 diagram!
Example An 80 kg diver drops from a board 10.0 m above the water surface. Find his speed 5.00 m above the water surface. (Neglect air resistance.) Step 2 decide the formula and rearrange if needed Step 3 solve PE = mgh = (80kg)(9.8m/s 2 )(5m) = 3920 J
Example An 80 kg diver drops from a board 10.0 m above the water surface. Find his speed 5.00 m above the water surface. (Neglect air resistance.) Step 3 solve PE = mgh = (80kg)(9.8m/s 2 )(5m) = 3920 J KE = PE 3920 J = (1/2)(80 kg)(v 2 ) (3920)(2) = v 2 80 98 = v = 9.9 m/s
Example 2 An 180 g softball is thrown at a speed of 25 m/s. Find its kinetic energy. (Neglect air resistance.) Step 1 diagram! 180 g = 0.18 kg 25 m/s
Example 2 An 180 g softball is thrown at a speed of 25 m/s. Find its kinetic energy. (Neglect air resistance.) Step 2 choose the formula and rearrange if needed KE = 1/2mv 2 Step 3 solve 25 m/s 180 g = 0.18 kg KE = (1/2)(0.18 kg)(25 m/s) 2 = (.09)(625) = 56.25 J
Conservation of Energy PE f KE f PE i KE i Conservative forces: Gravity, electrical, KE PE Non-conservative forces: friction, air resistance Non-conservative forces still conserve energy! Energy just transfers to thermal energy