**Energy**

**ENERGY** • Energy- the ability to do work • SI unit for energy is Joules (J) • 2 basic forms • Potential energy- energy of position • Kinetic energy- energy of motion • Energy is classified by its source • Mechanical potential energy • Common example is gravitational potential energy

**Energy** • Gravitational potential energy • GPE= mgh • EX. How high has a 7.26 kg shot put been thrown if it has a potential energy of 240.2 J? • h= GPE/(mg) • GPE=240.2 J • m= 7.26 kg • g= 9.81 m/s2 • h=? • h= 240.2 J/ [(7.26 kg)(9.81 m/s2)] • h= 3.37 m

**Kinetic Energy** • Kinetic energy- energy of motion • KE= ½ mv2 • What is the kinetic energy of a 14.5 g rifle bullet moving at 625 m/s? • KE=? • m= 14.5 g (convert to kg)= 0.0145 kg • v= 625 m/s • KE= ½(0.0145 kg)(625 m/s)2 • KE= ½(0.0145 kg)(390,625 m2/s2) • KE= 2832 J or 2830 J

**Chapter 6 Energy** Thermal Energy- the sum of all of the kinetic energies of all the particles in an object Theoretically, At 0 Kelvin all particle movement stops, but this is not achievable on earth Acoustic Energy- involves random vibrations and motion of particles Produces sound must travel through matter-cannot exist in a vacuum Moves in periodic motion- back and forth vibrations

**Energy** • Electrical Energy- forces acting on other electrical charges • Opposite attract, like repel • Natural sources- lightning, electric rays and eels • Man made sources- batteries, generators, etc • Magnetic Energy-ability to do work through the influence of a magnetic field • Opposite attract, like repel • Radiant Energy- also called electromagneticenergy- does not require matter through which to travel, can travel in a vacuum • Most common form is visible light

**Energy** • Chemical Energy- potential energy stored in chemical bonds • During chemical reactions atoms rearrange their bonds and energy is released or absorbed • Energy can be released as thermal, radiant, or acoustic energy • Note: we will go over this in more detail in Chapters 18 and 19

**Energy** • Nuclear Energy- this is the energy associated with the nucleus of an atom • Can be released in 2 ways • Fusion- is the combining of 2 or more nuclei to form a larger nuclei • stars 2 H atoms combine to form He • Fission- is the splitting apart of a nucleus into 2 more smaller nuclei • Man made nuclear power plants to generate power Nuclear energy always releases radiation-DANGER REMEMBER- energy cannot be created nor destroyed only transferred- all energies in the end must equal that with which you started!!

**Energy** • Mass Energy- this is the largest source of potential energy in the universe • It is the energy equivalent to all matter itself • This is where Einstein’s theory is applied • E= mc2 • Energy is equal to the mass times the speed of light squared

**E=mc2** • What is the energy equivalent of the mass of a 0.01 g plant mite, a tiny spider-like animal? (remember that c= 3.0 x 108 m/s) • E=mc2 • E=? • m= 0.01g (you have to convert to kg, divide by 1000) • m= 0.00001 kg or 1.0 x 10-5 kg • c= 3.0 x 108 m/s • E= 1.0 x 10-5 kg (3.0 x 108 m/s)2 • E= 1.0 x 10-5 kg (9.0 x 1016 m2/s2) • E= 9.0 x 1011 kg m2/s2 • E= 9.0 x 1011 J

**Example 6-3** • How much mass energy, in joules, could be obtained from the complete conversion of a compact 138 g media player? • m= 138 g = 0.138 kg • c= 3.00 x 108 m/s2 • E= ? • E= mc2 E= 0.138 kg (3.00 x 108 m/s)2 E= 1.242 x 1016 kg*m2/s2 E= 1.24 x 1016 J

**Energy Conservation** • 1st Law of Thermodynamics- conservation of energy • energy cannot be created nor destroyed only transferred, therefore the energy before the transformation must equal the energy after • Sometimes when energy is transferred some energy is ‘lost’ to the surrounding, • Because of this we measure the efficiency • This a comparison of the measurement of the amount of usable energy produced with the amount available before the transformation

**Energy Conservation** • Energy can be transferred to one or more forms of energy • Chemical energy from food becomes thermal energy in our cells and mechanical energy in our muscles • The 1st Law of thermodynamics is easily demonstrated by a pendulum • Consists of a heavy weight at the end of an arm that swings back and forth on a pivot point at its upper end

**Collisions and Energy** • As we have discussed before momentum and kineticenergy are properties of motion • Momentum is a vector defining a system’s quantity of motion • Kinetic energy is a scalar quantity describing the mechanical energy of a moving system • Collisions……………………….

**Collisons** • There are 3 types of collisions • 1. Elastic collisions- occurs when 2 objects collide and bounce (rebound) off from one another • The sum of their momentums and the sum of their kinetic energies are the same before and after the collision • 2. Partially elastic collisions – occurs when one or both objects in a collision are deformed before rebounding • 3. Inelastic collision- occurs when two objects collide and stick together REMEMBER ENERGY IS NOT LOST BUT TRANSFERRED