ENERGY

1. 1 ENERGY THE ABILITY TO CAUSE CHANGE

2. 2 Types of Energy Radiant energy mechanical energy

3. 3 types of energy • Thermal energy - from the motion of molecules • Chemical Energy – mostly among chemical bonds • Nuclear energy - from the nuclei of an atoms • Electrical and Magnetic energies – E&M interactions • Radiant energy -- carried by electromagnetic waves • Mechanical energy – usually by large bodies

4. 4 Energy CLIP • Energy is the capacity (ability) to do work.  • Kinetic Energy: Associated with an object in motion • Potential Energy: Is the interaction energy of an object wit another

5. Accounting for Energy - Part 1 Chapter 22

6. Objectives • Understand what energy is • Understand path energies • Work (specific classification; see next) • mechanical, shaft, hydraulic, electrical, photon… • Heat • conduction • blackbody radiation

7. What is energy? Let’s define in the most general way Work first Workis a change in the universe Energy – is the source or driving force able to perform such a work Thus, work done on x changes the xenergy

8. = Energy • Make an analogy for energy: money is a “unit of exchange”, • Example • 1 car = $20 k • 1 house = $100 k • 5 cars = 1 house

9. Energy Equivalents • 1 kg coal = 42,000,000 joules • 1 kg uranium = 82,000,000,000,000 joules (82x1012) • 1 kg uranium = 2,000,000 kg coal!!

10. Energy is a conserved quantity • Generally, energy is a conserved quantity, i.e., generation and consumption are zero. • In fusion and fission systems, energy can be generated (reactors, sun, stars, radioactive atoms, … • In particle accelerators, energy can be consumed, also everywhere in the universe cosmic rays became particles These are not common engineering systems • For most applications: Accumulation = Net Input

11. Types of Energy Accumulation = Net Input State Energies • Kinetic • Potential • Internal (Independent of Path) Path Energies • Work • Heat (Depend on Path)

12. Path Energies • Work is an energy flow across a boundary from a driving force • mechanical + shaft + hydraulic + electrical + chemical + laser • Heat is an energy flow resulting from a temperature driving force. • conduction + blackbody radiation

13. Mechanical Work • Results from a force applied over a linear distance

14. θ Shaft Work • Results from a twisting force (torque, T) applied over a circular distance

15. Pairs Exercise #1 • A lawn mower engine is started by pulling a cord wrapped around a sheave. The radius of the sheave is 8.0 cm and the cord is wrapped around the sheave twice. If a constant force of 90 N is applied to the cord, what work is done?

16. Shaft Power • Power = work/time • Units are usually horsepower (hp)

17. Pairs Exercise #2 • How much power (in hp) is produced if your automobile engine is producing 250 ft-lb of torque at 4000 rpm?

18. Hydraulic Work A (area of piston face) W = F Dx F = A(P2-P1) W = A(P2-P1)Dx Volume of fluid that flows: V=A Dx W = V (P2-P1) W = Qt (P2-P1) F P2 Dx P1 Q = V/t (flow rate)

19. Pairs Exercise #3 Atmospheric-pressure water is pumped at rate of 15 gal/min into a 60-psig storage tank. • How much work (in ft-lb) is done by the pump in one day assuming it pumps constantly for the 24-h period? • What “size” pump (i.e., power requirements in hp) is required for this job?

20. Electrical Work • In electricity, the driving force is a voltage potential difference. • Electrons move from lower to higher potential; i.e., they flow uphill, just opposite to positive charges • The force required to move a charge, q, a distance Dx from voltage V1 to V2 is F = q(V2-V1)/Dx

21. Electrical Work • Current [Amperes] is charge/time, thus i = q/t, or q = it • Therefore electrical work is:

22. Pairs Exercise #4 • You buy a light bulb that uses 0.545 A (current) for your porch light (110 V). You leave your light on all weekend (48 hours). The electric company charges you 5.75 cents per kW-h (kilowatt-hour). • How much does it cost to leave your light on all weekend?

23. Lasers and Photons LASER: Light Amplification by Stimulated Emission of Radiation The energy (E) of a photon (quantum of light) with frequency n is E = hnJoules n = c/l E = hc/lJoules h = Planck’s constant = 6.62 x 10-34 J s c = speed of light = 3.0 x 108 m/s

24. Heat • Heat is energy flow driven by temperature differences • Two major types of heat flow: • Conduction • Blackbody radiation

25. k A Q/t x T1 T2 Conduction • Conduction equation k = thermal conductivity Rate of heat conduction

26. Pairs Exercise #5 • A 4-cm-thick insulator (k=2x10-4 cal cm/(s cm2oC) has an area of 1000 cm2. If the temperature on one side is 170 oC and the temperature on the other side is 50 oC, what is the heat transfer by conduction?

27. Black Body and Blackbody Radiation • A black body adsorbs all incident electro-magnetic radiation, regardless of frequency or angle of incidence. • A black body in thermal equilibrium (that is, at a constant temperature) emits electromagnetic radiation called black-body radiationwith a spectrum that is determined by the temperaturealone, not by the body's shape or composition. • Heat transfer between blackbodies: