Heat Energy

1
Chapter 4

• Heat Energy
• ( energy transferred from one object to
  another)
• A. Temperature Scales
• (Temperature measures kinetic energy of
  particles)
• Fahrenheit BP of water212oF/FP of water 32oF
• Celsius BP of water100oC/FP of water 0oC
• F (9/5 C)32 C 5/9(F-32)
• Kelvin absolute zero no kinetic energy 0 K
• K C 273 C K - 273

2

• Examples
• Normal body temperature is 98.6oF.
• What is this temperature in Celsius Kelvin?
• 2. Room temperature is about 23oC.
• What is this temperature in Fahrenheit?

3

• B. Methods of Heat Transfer
• 1. Conduction by direct contact,
• touches
  directly
• 2. Convection by currents or
• movement of fluids (liquids gases)
• (heat rises/cold sinks)
• 3. Radiation by electromagnetic waves,
• not through materials
• (examples x-rays, microwaves,
• ultraviolet waves, infrared waves)

4

• C. Calculating Heat Energy
• a. Heat to change Temperature m x Cp x ?T
• m mass of substance in kg
• Cp Heat Capacity
• ?T change in temperature (T2-T1)
• Usually measured by transferring heat
• to/from water, whose Cp 4.2 kJ/kgoC

5

• Examples
• 1. A 5 g piece of brass at a temperature of
  100oCis placed into a cup containing 11.2 g of
  water at a temperature of 22oC. The final
  temperature of both the brass and the water is
  25oC. What is the heat capacity of the brass?

6

• 2. A 10 kg stone is dropped into a 1000 g puddle
  of water from a height of 100 m. If all of the
  energy changes to heat, by how much will the
  temperature of the water in the puddle raise?

7

• b. Heat to Change State
• Solids particles packed in rigid structures,
• attracted by opposite charges
• Liquids particles move around, but still remain
  close
• attracted by opposite charges
• Gases particles far apart, little or no
  attraction
• All energy used to pull molecules apart when
  changing state,
• No temperature change until done

8

• Heat of Fusion (Hf) energy gained/lost when
  changing between solid liquid
• Hf for water 335 kJ/kg
• Heat of Vaporization (Hv) energy gained/lost
  when changing between gas liquid
• Hv for water 2260 kJ/kg
• Break down problems into sections
• Heat to Melt Heat to change Temp Heat to
  vaporize
• Heat to condense Heat to change Temp Heat to
  Freeze

9

• Example
• How much heat energy is needed to change
• 25 g of ice at 0oC to steam at 100oC?

10

• Physics Test Next Week
• You will be given the sheet of Physics Formulas
  Tables
• Study all material on Homework Assignments from
  Weeks 1-4
• Test is mostly problems!