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Warm-up 2/3/14

How much energy is needed to run a washing machine which uses 55 kg of water and heats the water from 15  C to 53  C, if the specific heat of water is 4,184 J/ kg o C ? How much will it cost if the power company charges $0.13 per kW-hr? (1 kW-hr = 3.6 x 10 6 J ). Warm-up 2/3/14 .

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Warm-up 2/3/14

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  1. How much energy is needed to run a washing machine which uses 55 kg of water and heats the water from 15C to 53C, if the specific heat of water is 4,184 J/kgoC? How much will it cost if the power company charges $0.13 per kW-hr? (1 kW-hr = 3.6 x 106 J) Warm-up 2/3/14 Q = m cT m = 55 kg Q = (55 kg)(4184 J/kgC)(38 C) c = 4,184 J/kgC T = 53C - 15 C = 38 C Q = 8,740,000 J or 8.74 MJ Q = ?

  2. HEAT TRANSFER Conduction – heat is transferred through a material or between 2 materials. Metals are the best conductors. This is due to loosely bound electrons that can transfer energy easily. We insulate to keep the heat in not the cold out. Heat conduction occurs by electron and atomic collisions. Example: pan on a stove

  3. Conduction

  4. HEAT TRANSFER Convection – heating occurs because of currents in a fluid. Convection occurs in liquids and gases. Air is heated, expands, becomes less dense causing it to rise up. Convection occurs in winds, rising smoke, and above candles. Example: weather Example: furnace in room

  5. Convection What happens with the food coloring when put into the convection tube on the right?

  6. HEAT TRANSFER Radiation– the sun’s energy gets to us by radiation. Radiation is electromagnetic waves like light, radio, and infrared waves. Next time your eyes get hot next to a fire put on some glasses it will block a lot of infrared waves. Example: sun’s energy Only radiation is possible in empty space.

  7. Radiation What happens with the radiometer when put in the sun? Which way does it spin?

  8. A convection D radiation B conduction C convection current

  9. Thermal Expansion Most substances expand when they are heated and contract when they are cooled (except water to ice). The lines in concrete, expansion joints in bridges, and sagging electric lines all are due to expansion. Different materials expand at different rates (useful for removing lids)

  10. It bends as one metal expands more than the other. Question: What happens when a bi-metallic strip is heated? Some thermostats contain bimetallic strips how does this make them work? The metallic strip will bend to make contact and complete the circuit when it gets hot or cold.

  11. Water expands when it freezes. Water is the most dense at 4ºC (ice is less dense due to the crystal structure ice forms) Why is this good for fish?

  12. Thermal expansion ( and ) • = L / (LiT) and = V / ViT. •  = coefficient of linear expansion. Found on a table.  = coefficient of volume expansion found on table. • L = change in length. Lf – Li or V = change in volume. Units cancel so just have to be in same units. • Li is initial length and Vi = initial volume. • T = Change in Temperature. In Celsius or Kelvin.

  13. QuestionHow much longer will a 300.0 m steel bridge be on a 30.0 C day in August versus a -10C night in January?  = ΔL/(LiΔT) ΔL =  (LiΔT) ΔL = 13 x 10-6 /C (300 m x 40 C ) ΔL = 0.156 m or 15.6 cm Li = 300 m  = 13 x 10-6 /C T = 30- -10C = 40 C L = ?

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