Ch' 5 Energy

1
Ch. 5 - Energy

• II. Thermal Energy
• (p.134-137, 141-144)
• Temperature
• Thermal Energy
• Heat Transfer

2
A. Temperature

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

3
B. 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

4
B. Thermal Energy

• Which beaker of water has more thermal energy?
• B - same temperature, more mass

5
C. 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

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C. 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.

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C. Heat Transfer

• Specific Heat (Cp)
• amount of energy required to raise the temp. of 1
  kg of material by 1 degree Kelvin
• units J/(kgK)or J/(kgC)

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C. Heat Transfer

• Which sample will take longer to heat to 100C?

• Al - It has a higher specific heat.
• Al will also take longer to cool down.

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C. Heat Transfer
Q m ? ?T ? Cp
Q heat (J) m mass (kg) ?T change in
temperature (K or C) Cp specific heat (J/kgK)
10
C. Heat Transfer

• Calorimeter
• device used to measure changes in thermal
  energy

• in an insulated system,

heat gained heat lost
11
C. Heat Transfer

• A 32-g silver spoon cools from 60C to 20C. How
  much heat is lost by the spoon?

GIVEN m 32 g Ti 60C Tf 20C Q ? Cp
235 J/kgK
WORK Q m?TCp m 32 g 0.032 kg ?T 20C -
60C 40C Q (0.032kg)(-40C)(235J/kgK) Q
301 J
12
C. Heat Transfer

• How much heat is required to warm 230 g of water
  from 12C to 90C?

GIVEN m 230 g Ti 12C Tf 90C Q ? Cp
4184 J/kgK
WORK Q m?TCp m 230 g 0.23 kg ?T 90C -
12C 78C Q (0.23kg)(78C)(4184 J/kgK) Q
75,061 J