Online Teaching Evaluations

1
Online Teaching Evaluations

• Importance for evaluating the teaching
  effectiveness.
• The comments and suggestions are most welcome and
  valuable
• Please fill out the questionnaire!
• Your responses will be anonymous.
• Teaching evaluation summaries will not be
  released to instructors until after final grades
  have been submitted.
• Send e-mail to teacheval_at_science.ubc.ca if they
  have questions or encounter difficulties.

2

• A light bulb is connected to a real battery.
  Suppose a wire of low resistance R is connected
  across the bulb as shown. When the wire is
  connected, the brightness of the bulb
• increases.
• decreases to half of the initial
• brightness.
• stays the same.
• decreases depending on the resistance
• of the wire.
• increases or decreases depending on the
  resistance of the bulb.

3
Power stations

• Thermal
• Oil
• Coal
• Nuclear
• Hydroelectric
• Wind
• Solar
• Thermal
• Solar cells
• Piezoelectric (?)

4
Electrical energy consumption

• Yearly variation
• Daily variation
• Base, intermediate and peak consumption

5

• To convert thermal energy into any other kind of
  energy (for example electrical or mechanical) we
  need
• The temperature of the heat source to be higher
  than 100ºC
• A temperature difference
• Something burning
• All of the above

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Thermal Generating Power Plants

• Primary energy coming from combustion of coal,
  oil, or gas or from nuclear reactions is used to
  heat water, generate steam and drive a steam
  turbine.
• Efficiency is limited by the temperature
  difference
• (in K) between the pressurized steam and
  environment
• h (1 Tenvironment/Tsteam) lt 65,
• due to limitation of boiler materials to Tsteam
  ? 800 K. Including losses in the turbine h lt 45
  for generating electricity.
• Total efficiency can be increased when hot water
  coming out of power plants is used to heat homes.

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Oil Power Station
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Nuclear Power Station
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(No Transcript)
10
Converting Mechanical Energy into Electrical
Energy.

• Faradays Electromagnetic Lab
• http//phet.colorado.edu/new/simulations/sims.php?
  simFaradays_Electromagnetic_Lab

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Hydro Power Station
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Hydro Power

• Design of a tidal hydro power station in Severn
  Estuary on English Channel Coast
• 2 tides a day up to 14 m, average 8 m
• Area to be dammed 2 000 km2
• How much power can this power station provide?

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Q2. What is the potential energy of 1 cubic meter
of water 1 meter above water level as shown on
the picture

• E mgh g 9.8m/s2

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Q2. What is the potential energy of 1 cubic meter
of water 1 meter above water level as shown on
the picture

• E mgh g 9.8m/s2
• E ?wVgh
• ?w 1000 kg/m3 density of water
• V 1 m3 - volume of water
• h 0.5 m average height of water or height of
  center of mass of water
• E 1000 kg/m3 1 m3 0.5 m 9.8m/s2 4900J

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Hydro Power Station in Severn Estuary on English
Channel Coast Hydro Power Station in Estuary

• Potential energy of the water
• E mgh V?wgh
• Power PE/time
• 2 tides a day
• P E/12 hours
• E 2 000?106 m2?8 m?1000 kg/m3?10 m/s2?4 m
• 600 ?1012 J
• P 600 ?1012 J / 43 200 s 14 GW
• Planning 8.6 GW
• So the plan has to talk about peak power not the
  average power the station can not be 61
  efficient

16
Q3. What is the kinetic energy of 1 cubic meter
of air moving at the speed of 10 m/s?

• The density of air is 1.2 kg/m3
• E mv2/2
• 12 J
• 120 J
• 60 J
• 6 J

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Q3. What is the kinetic energy of 1 cubic meter
of air moving at the speed of 10 m/s?

• The density of air is ?a 1.2 kg/m3
• E mv2/2 V?av2/2
• 1m3 ( 1.2 kg/m3) (10m/s)2 60 J

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Wind Power
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Efficiency of the turbine at high speed

• Kinetic energy per second of a 51km/h wind
• 51 km/h 14 m/s
• Radius of the turbine 15 m
• Volume of air per second
• 14 mp152 m2 10 000 m3/s
• Mass of air per second
• 1.2 kg/m3 10 000 m3/s 12 000 kg/s
• Kinetic energy of this air is per second (power)
• K mv2/2 12 000 kg/s (14 m/s) 2/2 1.2 MW
• Efficiency 750 kW/ 1 200 kW 63

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Efficiency of the turbine at low speed

• Kinetic energy per second of a 15 km/h wind
• 15 km/h 4 m/s
• Radius of the turbine 15 m
• Volume of air per second
• 4 mp152 m2 3 000 m3/s
• Mass of air per second
• 1.2 kg/m3 3 000 m3/s 3 600 kg/s
• Kinetic energy of this air is per second (power)
• P mv2/2 3600 kg/s (4 m/s) 2/2 0.12 MW
• Efficiency 200 kW/120 kW 170!
• What is wrong? The published data!

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Example Vestas 3 MW Wind Turbine

• Maximum of P 3.0 MW at v 15 m/s.
• Wind Power P ½ r A v3 13.7 MW
• Overall efficiency 3.0/13.7 22

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Solar Power
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Solar Power

• Two methods Concentrating solar radiation to
  heat a fluid and produce steam or direct
  conversion into electricity using the photvoltaic
  effect in solar cells.
• In batteries, the work required to separate
  positive and negative charges is done by an
  electrochemical reaction.
• In solar cells, the work is done by the incoming
  solar radiation in quantum processes A visible
  photon from the sun has enough energy to separate
  an electron from an atom leaving behind a site
  that is positively charged (hole).

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http//inventors.about.com/library/inventors/blsol
ar3.htm
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Solar Cells

• When solar radiation hits the cell, electron/hole
  pair are created.
• Electrons are attracted to the positive side and
  holes move to the negative site, so we have a
  current that can do work on an external load.
• Remember You need current and voltage to have
  electric power.
• Most commercial cells have an efficiency of (5
  25)

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How efficient it is to use the solar energy to
produce electricity and than use electricity to
produce light

• 80
• 50
• 20
• 5
• 2

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Solar Power

• An off grid house on the golf islands
• Solar panel roof area 12 m2
• The energy of solar radiation reaching the roof
  surface 1000 W/m2 (why not 1400 W/m2 ?)
• On average 12 h of daylight
• 12h ? 1000 W/m2 12 kWh/m2/day
• For this area the expected output from the solar
  cells is only about 3.4 kWh/m2/day! (NASA data)
• Why the difference?

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Solar Power

• Solar cells efficiency 20
• Average angle of incidence ? 0