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Energy-further topics

Energy-further topics. How will we measure energy?. Kinetic Theory of Molecular Motion. Absolute zero- The absence of motion, and therefore the absence of kinetic energy. If the temperature of any substance is above absolute zero, then there must be motion (and heat).

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Energy-further topics

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  1. Energy-further topics How will we measure energy?

  2. Kinetic Theory of Molecular Motion • Absolute zero- The absence of motion, and therefore the absence of kinetic energy. • If the temperature of any substance is above absolute zero, then there must be motion (and heat). • All substances are in motion, even solids. • Kinetic energy of a system is the result of collisions between molecules. • Temperature is the measure of the average kinetic energy of a system (a body, a block of wood, the atmosphere)

  3. The measurement of energy • You have observed how different materials react to energy differently • Metals transfer heat rapidly • Water reacts to heat more slowly • Glass tends to retain heat

  4. Specific Heat Capacity ( c ) • The way a substance responds to energy is specific and unique to that substance: i.e., a characteristic property. • Definition: The amount of heat necessary to raise one gram of the substance 1°C • Reported in J/g-°C or cal/g-°C

  5. The measurement of energy II • You may have noticed that a large pot of water tends to take longer to boil than a small pot • You may have also noticed that a large block of ice takes much longer to melt than a small ice cube • You may have noticed that a pot of hot water takes much less time to boil than a pot of cold water • What does this mean?

  6. The measurement of energy III • The way a substance responds to energy is also dependent on the amount of substance present; i.e., its mass • We can calculate the amount of energy transferred by measuring the change in temperature.

  7. Putting it all together • The heat contained in a substance can be calculated by applying the following equation: • Heat content (q) = • mass • x specific heat capacity • x change in temperature • Or q = m x c x Dt (D means “change in”) • Simpler form q = mcDt

  8. Calorimetry • If q = mcDt, then we can manipulate this relationship to determine all other quantities in the relationship. • m = q/cDt • c = q/mDt • Dt = q/mc • If we know 3, the fourth can be calculated.

  9. Sample Problem • How much heat was transferred to 200.0g of water if its temperature increased by 15.0°C? • The specific heat capacity of water is 4.184J/g-°C • Q = mcDt so: • 200g x 4.184J/g-°C x 15°C = • 12552J or 12600J

  10. Assignment • Heat problem set due Thursday 9/16 • Heat transfer lab Friday 9/17

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