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Energy – you can’t have change without it!

Energy – you can’t have change without it!. Unit 2. What is energy – no, it is not coffee!. Energy is the ability to do work, it comes in many different forms: Chemical – the potential energy that is stored in chemical substances Heat – measure of energy of molecules & atoms

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Energy – you can’t have change without it!

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  1. Energy – you can’t have change without it! Unit 2

  2. What is energy – no, it is not coffee! • Energy is the ability to do work, it comes in many different forms: • Chemical – the potential energy that is stored in chemical substances • Heat – measure of energy of molecules & atoms • Potential – energy of position (think top of the mountain) • Kinetic – energy of motion • Light • Electrical – the flow of electrons • Nuclear – the energy released in fusion or fission

  3. The big rule! • Energy can not be created or destroyed, it just changes forms… • Example: • Mechanical  Heat: what happens when you rub a block of wood with sand paper • Heat  Mechanical: When steam is used to turn turbines in a steam locomotive • Chemical  Heat: what happens when you burn oil to heat your home

  4. Heat vs. Temperature • Heat is the amount of kinetic energy that molecules have, the more motion the more heat, heat is measured in Joules • Temperature is just a relative scale of comparing how hot or cold something is to something else measured in Fahrenheit, Celsius, Kelvin • Heat and Temperature are NOT synonymous!

  5. Temperature Scales • Fahrenheit – First Temperature Scale, based on freezing and boiling points of water (32º - 212º) • Celsius – Also based on freezing and boiling points of water, on a 100 point scale (0º - 100º) used to be called centigrade • Kelvin – not based on a physical object, but on the complete lack of molecular motion (absolute zero)

  6. Conversions: Celsius to Fahrenheit • F = 9/5 (temp in C) + 32 • Example • What is 20ºC in Fahrenheit? F = 9/5(20) + 32 = 68º Now you! What is –15ºC in Fahrenheit?

  7. Fahrenheit to Celsius • Answer to prior slide = 5ºF • C = 5/9 (temp in F – 32) • Example • What is 75ºF in Celsius C = 5/9(75 – 32) = 24ºC Now you! What is 20ºF in Celsius?

  8. Celsius Kelvin • Answer to prior slide: -7ºC • To change Celsius to Kelvin: Celsius + 273 = Kelvin Example: 31ºC in Kelvin is… 31+ 273 = 304 K, Note! There is NO degree symbol for Kelvin Now you… -23ºC to Kelvin

  9. Kelvin  Celsius • Answer to prior slide: 250K • To change Kelvin to Celsius : Kelvin - 273 = Celsius Example: 403K in Celsius 403K- 273 = , 130 ºC Now you… 159K is what in Celsius?

  10. Heat - it is needed for change • Answer to prior slide: -114ºC • Heat needs to be added or removed to change temperature or change the state of a substance. Heat = Mass º Specific Heat º Change in temperature Q = m Cp ΔT or Q = m Cp (Tf – Ti) Specific Heat (Cp)is a constant for the substance with the unit of J/kgºC

  11. Example # 1 • What is the heat needed to raise the temperature of a bottle of water (mass of 0.750kg) from 45.0 to 90.0 degrees Celsius if the specific heat of water is 4.19 J/kgºC? Q = m Cp (Tf – Ti) Therefore… = 0.75 kg (4.19 J/kgºC)(90.0 – 45.0) = 141J

  12. Example # 2 • What is the heat released when the temperature of a block of steel (mass of 2.50kg) is cooled from 145.0 to 30.0 degrees Celsius if the specific heat of steel is 450J/kgºC? Q = m Cp (Tf – Ti) Therefore… = 2.50 kg (450 J/kgºC)(30.0 -145.0 ) -1.3x105J (the negative sign means released)

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