What is Energy? Energy- is the ability to do work. Energy is the ability to cause a change. Energy can change an object s:

Transcription

1 Energy & Work

2 What is Energy? Energy- is the ability to do work. Energy is the ability to cause a change. Energy can change an object s: motion temperature shape color

3 Energy THERMAL internal motion of particles NUCLEAR changes in the nucleus ENERGY The ability to cause change. MECHANICAL motion of objects CHEMICAL bonding of atoms joules (J) ELECTRICAL motion of electric charges

4 Energy Kinetic Energy (KE) energy in the form of motion depends on mass and velocity KE = 1/2mv 2 Which has the most KE? 80 km/h truck Which has the least KE? 50 km/h motorcycle 50 km/h 80 km/h

5 Energy Potential Energy (PE) stored energy depends on position or configuration of an object PE = mgh Which boulder has greater gravitational PE?

6 Types of Potential Energy Gravitational Due to position above Earth s surface Elastic- Stored in something stretchy Chemical Result of a chemical reaction

7 Bell Ringer What is Energy? What is Potential Energy? What is Kinetic Energy?

8 Work W Work F d transfer of energy through motion force exerted through a distance W = Fd W: work (J) F: force (N) d: distance (m) 1 J = 1 N m J = Joules Distance must be in direction of force!

9 Work Brett s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W =? WORK: W = F d W = (30 N)(1.5 m) W = 45 J W F d

10 Work A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W =? W F d WORK: W = F d F = m a F =(40kg)(9.8m/s 2 )=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. d is not in the direction of the force.

11 Conservation of Energy Law of Conservation of Energy- Energy may change forms, but it cannot be created or destroyed under ordinary conditions. EX. s of Changing Forms: PE KE mechanical thermal chemical thermal

12 Mechanical Energy The motion of some objects contain a combination of only kinetic and potential energies. ME is the sum of the KE and forms PE associated with a system Therefore, ME = KE + PE

13 Conservation of Energy TOTAL ENERGY OF THE SYSTEM REMAINS THE SAME

14 Conservation of Energy TOTAL ENERGY OF THE SYSTEM REMAINS THE SAME

15 Bell Ringer Solve the following: W = F d & F = m a 1. A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina 2. Jake s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back?

16 Bell Ringer The diagram below shows a toy popper that has been stretched and placed on the floor. It is ready to launch. Once it launches, position C is the highest point the popper reaches before it begins descending back to the floor.

17 (a) What type of energy does the popper have at its starting position? (b) What type of energy does the popper have at the halfway up point? (c) At what position(s) does the popper have 0 KE? (d) At what position(s) does the popper hav 0 GPE? (e) At what position(s) does the popper hav the greatest GPE?

18 Power W Power How fast can work be done The faster work is done, more powerful horsepower = 746 watts P t P: Power (w) W: Work (J) t: time (s) P = W/t w = watts = 1 J/s

19 Energy Thermal Energy

20 Temperature Temperature measure of the average KE of the particles in a sample of matter

21 Thermal Energy Thermal Energy the total energy of the particles in a material KE - movement of particles PE - forces within or between particles due to position depends on temperature, mass, and type of substance

22 Thermal Energy Which beaker of water has more thermal energy? B - same temperature, more mass 80ºC 80ºC A B 400 ml 200 ml

23 Heat Transfer Heat thermal energy that flows from a warmer material to a cooler material Like work, heat is... measured in joules (J) a transfer of energy

24 Heat Transfer Why does A feel hot and B feel cold? Heat flows from A to your hand = hot. Heat flows from your hand to B = cold. 80ºC 10ºC A B

25 Heat Transfer Specific Heat (C p ) amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin units: J/(kg K) or J/(kg C) Specific Heat Values (J/(kg K)) Water Alcohol Aluminum Carbon (graphite) Sand Iron Copper Silver

26 Heat Transfer Which sample will take longer to heat to 100 C? 50 g Al 50 g Cu Specific Heat Values (J/(kg K)) Water Alcohol Aluminum Carbon (graphite) Sand Iron Copper Silver Al - It has a higher specific heat. Al will also take longer to cool down.

27 Heat Transfer Q = m T C p Q: heat (J) m: mass (kg) T: change in temperature (K or C) C p : specific heat (J/kg K) T = T f - T i Q = heat loss + Q = heat gain

28 Heat Transfer Calorimeter device used to measure changes in thermal energy in an insulated system, Coffee cup Calorimeter heat gained = heat lost

29 Heat Transfer A 32-g silver spoon cools from 60 C to 20 C. How much heat is lost by the spoon? GIVEN: m = 32 g T i = 60 C T f = 20 C Q =? C p = 235 J/kg K WORK: Q = m T C p m = 32 g = kg T = 20 C - 60 C = 40 C Q = (0.032kg)(-40 C)(235J/kg K) Q = 301 J

30 Heat Transfer How much heat is required to warm 230 g of water from 12 C to 90 C? GIVEN: m = 230 g T i = 12 C T f = 90 C Q =? C p = 4184 J/kg K WORK: Q = m T C p m = 230 g = 0.23 kg T = 90 C - 12 C = 78 C Q = (0.23kg)(78 C)(4184 J/kg K) Q = 75,061 J