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Thermal Energy

Thermal Energy. Thermal energy. Thermal energy is the same as heat or heat or heat energy Temperature is not the same thing as heat We use different units of measurement for these two things: Temperature is measured in _______ Heat is measured in ________. Temperature and Heat.

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Thermal Energy

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  1. ThermalEnergy

  2. Thermalenergy • Thermal energy is the same as heat or heat or heat energy • Temperature is not the same thing as heat • We use different units of measurement for these two things: • Temperature is measured in _______ • Heat is measured in ________

  3. Temperature and Heat • Temperature is a measure of the AVERAGE amount of energy in a substance,and is measured in °C ⁰F. and K • Conversions: ° C = K – 273 : K = C + 273 • If the temperature increases by 10°C, how much does it increase in K? • Heat is related to temperature but the two are not the same. • Which has a higher temperature? A space heater or a candle? • Which one can make a room warmer?

  4. Differencebetweenheat and temperature You add the same amount of heat energy (for example 50,000J) to both you would find that the temperature of the cup of water increases by much more than the temperature of the bucket of water.

  5. Heat transfer • Heat can be transferred in three ways. • Conduction - by a substance which does not move (solids). • Convection - by  substance which moves (liquids and gasses). • Radiation - infra-red radiation is exchanged between all substances. Heat is transferred naturally from a substance with a higher temperature to a substance with a lower temperature. If two objects have the same temperature, they will not transfer energy and are said to be tin thermodynamic equilibrium

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  7. Thermalenergy • The energy required to raise a kg of an object by 1 degree is different for different substances. • For example: • To heat 1kg of copper from 20°C to 21°C requires 390J, • To heat 1kg of aluminium from 20 to 21°C takes 910J. • This is known as the SPECIFIC HEAT CAPACITY of a substance. • Specific heat units:

  8. Specificheatcapacity • The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of the substance unit mass by a unit change in temperature. • The unit mass is usually 1kg, while the unit change in temperature is 1K or 1°C. The key formula is: • Energy = specific heat capacity × mass × change in temperature

  9. Specificheatcapacity • Calculatethefollowing: • How much energy is needed to increase the temperature of 500 g of lead from 20 ºC to 45 ºC? The specific heat capacity of lead is 128 J/kg/ºC.

  10. Heat Transfer Complete the following definition using the words below: • ___________ energy or heat is always transferred from a _________ place to a ___________ place. The amount of energy _____________ depends on how big the _______________ difference is • cooler thermal transferred • hotter temperature

  11. Specific Heat capacity • Specific Heat : C • Change in Temperature: ∆T , • Mass : m • Energy, Heat: Q • Rearrange this equation to solve for m, C, Tf, Ti

  12. Review • Use the table on page 318 of the textbook to assist you in answering this question: • How much heat is required to raise the temperature of a 2.3kg copper pipe from 20⁰C to 80⁰C? Q = 53,130J

  13. Review • Use the table on page 318 of the textbook to assist you in answering this question: • 50,000J of heat are added to a 10kg bucket of water. How much would you expect the temperature to increase? 50,000J = 10kg = 1.2K

  14. Review • Use the table on page 318 of the textbook to assist you in answering this question: • The temperature of a block of Aluminum increases from 250K to 300K as it absorbs 4000J of heat. What is the mass of the block? m = Q/(C∆T) = 4000J/(897J/kgK*50K) = .089kg = 89g

  15. Conservation of Thermal Energy • As with any isolated system, the total amount of energy in a closed system is constant. • If a system involved contact between two different objects, the total energy of the two blocks would remain constant: • For this to be true when energy is exchanged, • Total energy in the system does not change

  16. When heat is the only form of enerygy exchanged • What must be the relationship between the temperature changes of A and B? • Objects will exchange energy until they reach the same final temperature • =

  17. Calculations of Equilibrium Temperature • A 200g sample of water at 80C is mixed with 300g of water at 10C. What is the final temperature of the mixture? = = =311K

  18. A .5kg lead weight at a temperature of 100⁰C is dropped into 1kg of water at 35⁰C. What is the final temperature of the water and weights? = Tf = 36⁰C

  19. Phase Change • What is the temperature of ice water ? • What is the temperature of Boiling water? • How does the device in the picture work? • Why do we sweat?

  20. States of Matter • Solid (fixed volume, shape) • Liquid(fixed volume, variable shape) • Gas (variable volume, variable shape) • At various temperatures, and pressures, substances can change between these three states of matter.

  21. States of matter • Melting point- Substance changes from solid to liquid • Boiling point – Substance changes from liquid to gas • Sublimination- Substance changes directly from solid to gas.

  22. Energy • The more energy a molecule has, the more kinetic energy it has. • The more kinetic energy, the more a molecule is free to move. Resulting in changes in its state. • When heat is added to a system, some of the energy is converted into kinetic energy of the molecules instead of thermal energy.

  23. Boiling A kg of Water

  24. Energy of Phase Change • During a phase change Temperature does not increase even though thermal energy exchanged • Melting- 273K ice becomes 273K water • Heat is Absorbed • Freezing – Heat is Released • Boiling - Heat is absorbed • Condensing – Heat is released

  25. Heat of Vaporization • The energy required to convert Ikg of a substance between liquid and gaseous states • Liquid to Gas - • Gas to liquid - is negative

  26. Implications of Phase Change • When a liquid evaporates, it absorbs heat from its environment. ( Cooling ) • When liquid condenses, it releases heat to the environment (Warming)

  27. Heat of Fusion • The energy required to change a1kg of a substance between solid and liquid states • Solid to Liquid – Positive • Liquid to solid - Negative

  28. Practice • How much heat is absorbed by a 50 g ice cube as it melts to become liquid water at 273K? Q = .05kg*3.34* Q = 16,700J

  29. Practice • Do you expect a soda can to get warmer or colder as water condenses on the outside? • Warmer

  30. Kinetic Energy and Heat • A .005kg bullet is travelling at 200m/s. What is its kinetic energy? • If the bullet is stopped by a block of ice, how much water is formed? Q = m* 1,000J = m*334,000J/kg m = .003kg = 3g of water

  31. Multiple Steps • How much energy is required to completely boil 2kg of water initially at a temperature of 50⁰C? • Step 1: Heat water to 100⁰C : Q = mC∆T • Step 2 Evaporate water to steam at • 100 ⁰C : Q = Q = 2kg*2180J/kgK*50K + 2kg*2.26*

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