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Heating of the house problem

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... example, a spoon in a cup of hot soup becomes warmer because the heat from the ... rod is at a higher temperature, then energy will be transferred down the rod ... – PowerPoint PPT presentation

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Title: Heating of the house problem


1
Heating of the house problem
  • System as seen by a physicist

House Energy converted into thermal energy inside
Energy flow in
Energy flow Out
2
Reminders
  • Units of energy or work Joules J
  • Other energy units
  • Calorie cal 4.184 J
  • Calorie kcal 4.184 kJ
  • kilo-watt-hour kWh 1000W3600s 36105 J

3
Reminders Energy flow
  • Power energy flow per unit time or work per unit
    time
  • Unit W (Watt) J/s
  • Also J/d (Joule per day) 1/86400 W
  • MJ/d (Megajoule per day) 12 W
  • GJ/d (Gigajoule per day) 12 kW (kilowatt)
  • Horsepower hp 745.7 watts

4
Power Rate of Energy Transfer
Computer 200 W TV on standby a few W
Refrigerator 1.2 kWh/day Electrical stove 3
5 kW Person on standby lt 100W
5
Sources of Energy Flowing In
  • Electrical Energy
  • External radiation coming in
  • Chemical energy
  • Natural gas supply
  • Heating oil supply
  • Fireplace
  • Food

6
Electrical Energy
  • Production (details much later)
  • How is it delivered?
  • Electrical Potential (voltage) is always there
  • Current is zero if we are not using the energy
  • Accounting for electrical energy use price per J
    is 1.7 10-8 /J
  • Unit used by utilities companies kWh
  • 0.0615 per kWh
  • kWh 36105 J

7
Heat and Chemical Energy
Non-mechanical energy transfer
8
Chemical Energy
  • Propane 50 MJ/kg 0.5 kg/m3
  • Natural gas 47 MJ/kg 0.8 kg/m3
  • Heating oil 43 MJ/kg
  • Coal 29 MJ/kg
  • Wood 16 MJ/kg
  • Food (eaten not burned!) 16MJ/kg
  • Why food?

9
Heat flow of thermal energy
  • How it flows?
  • Conduction
  • Convection
  • Radiation

10
Water heated in microwave oven
11
Temperature
  • Measure of thermal energy energy of vibrating
    molecules, atoms
  • Units ºC, K, ºF

12
Convection
  • Convection is the transfer of heat by the actual
    movement of the warmed matter. Heat leaves the
    coffee cup as the currents of steam and air rise.
    Convection is the transfer of heat energy in a
    gas or liquid by movement of currents.
  • The heat moves with the fluid. Consider this
    convection is responsible for making macaroni
    rise and fall in a pot of heated water. The
    warmer portions of the water are less dense and
    therefore, they rise. Meanwhile, the cooler
    portions of the water fall because they are
    denser.
  • From http//www.mansfieldct.org/schools/mms/staff
    /hand/convcondrad.htm

13
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14
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15
  • Why some area of the ground could be warmer?
  • Why there is strong wind on the ground in the
    area of the forest fire?

16
Conduction
  • Conduction is the transfer of energy through
    matter from particle to particle. It is the
    transfer and distribution of heat energy from
    atom to atom within a substance. For example, a
    spoon in a cup of hot soup becomes warmer because
    the heat from the soup is conducted along the
    spoon.
  • Fun fact Have you ever noticed that metals tend
    to feel cold? Believe it or not, they are not
    colder! They only feel colder because they
    conduct heat away from your hand. You perceive
    the heat that is leaving your hand as cold.
  • From http//www.mansfieldct.org/schools/mms/staff
    /hand/convcondrad.htm

17
Conduction
  • Heat Conduction
  • Conduction is heat transfer by means of molecular
    agitation within a material without any motion of
    the material as a whole. If one end of a metal
    rod is at a higher temperature, then energy will
    be transferred down the rod toward the colder end
    because the higher speed particles will collide
    with the slower ones with a net transfer of
    energy to the slower ones.

18
Conduction
  • Heat Conduction
  • For heat transfer between two plane surfaces,
    such as heat loss through the wall of a house,
    the rate of conduction heat transfer is
  •           H kA (T2 - T1)/L                  
  • H heat transferred in time in J/s
  • k thermal conductivity of the barrier
  • A area
  • T2 - T1 temperature difference
  • L thickness of barrier

19
Example
  • A little brick house with the walls 15 cm thick
    has ten 100 W lights on. The temperature outside
    is 18 ºC. The house is 10 m long and 10 m wide
    and 4 m high. Both floor and roof are very well
    insulated. What is the temperature inside?
  • How will it change if two people go in?
  • For brick, k 0.84 J/(s m 0C) or 0.84 W/(m K)

20
  • What assumptions do we have to make about the
    system if we want to use the formula
  • The heat loss per second through the walls must
    be equal to the power delivered by the light
    bulbs
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