Types of Energy

1
Types of Energy

• Heat Chemical
• Light Gravitational
• Sound Elastic/strain
• Kinetic Nuclear
• Electric
• Stored/potential

2
The Law of Conservation of Energy

• Energy can be changed (transformed) from one
  type to another, but it can never be made or
  destroyed.

3
This means that the total amount of energy in the
Universe stays the same!
4
Energy Flow diagrams

• We can write energy flow diagrams to show the
  energy changes that occur in a given situation.
• For example, when a car brakes, its kinetic
  energy is transformed into heat energy in the
  brakes.
• Kinetic heat

sound
5
Other examples

• When a rocket launches.
• Chemical kinetic gravitational

heat
heat
sound
6
Energy degradation!

• In any process that involves energy
  transformations, the energy that is transferred
  to the surroundings (thermal energy) is no longer
  available to perform useful work.

7
Energy transfer (change)

• A lamp turns electrical energy into heat and
  light energy

8
Sankey Diagram

• A Sankey diagram helps to show how much light
  and heat energy is produced

9
Sankey Diagram

• The thickness of each arrow is drawn to scale to
  show the amount of energy

10
Sankey Diagram

• Notice that the total amount of energy before is
  equal to the total amount of energy after
  (conservation of energy)

11
Efficiency

• Although the total energy out is the same, not
  all of it is useful.

12
Efficiency

• Efficiency is defined as
• Efficiency useful energy output
• total energy input

13
Example

• Efficiency 75 0.15
• 500

14
Energy efficient light bulb

• Efficiency 75 0.75
• 100

Thats much better!
15
Energy Density

• The energy that can be obtained from a unit mass
  of the fuel
• J.kg-1
• If the fuel is burnt the energy density is simply
  the heat of combustion

16
Energy density

• Coal - 30 MJ.kg-1
• Wood - 16 MJ.kg-1
• Gasoline 47 MJ.kg-1
• Uranium 7 x 104 GJ.kg-1 (70000000 MJ.kg-1)

17
Hydroelectric energy density?

• Imagine 1 kg falling 100m.
• Energy loss mgh 1x10x100 103 J
• If all of this is turned into electrical energy
  it gives an energy density of the fuel of 103
  J.kg-1

18
Electromagnetic induction

• If a magnet is moved inside a coil an electric
  current is induced (produced)

19
Electromagnetic induction

• A electric current is induced because the
  magnetic field around the coil is changing.

20
Generator/dynamo

• A generator works in this way by rotating a coil
  in a magnetic field (or rotating a magnet in a
  coil)

21
Non-renewable

• Finite (being depleted will run out)
• In general from a form of potential energy
  released by human action

22
Fossil fuels Coal, oil, gas
23
Nuclear fuels
24
Renewable

• Mostly directly or indirectly linked with the sun
• The exception is tidal energy

25
Photovoltaic cells (photoelectric effect)
26
Active solar devices
27
Wind
28
Wave
29
Tidal
30
Biomass
31
World energy production
Fuel total energy production CO2 emission g.MJ-1
Oil 40 70
Natural gas 23 50
Coal 23 90
Nuclear 7 -
Hydroelectric 7 -
Others lt 1 -
32
Electricity production

• Generally (except for solar cells) a turbine is
  turned, which turns a generator, which makes
  electricity.

33
Fossil fuels

• In electricity production they are burned, the
  heat is used to heat water to make steam, the
  moving steam turns a turbine etc.

34
Fossil fuels - Advantages

• Relatively cheap
• High energy density
• Variety of engines and devices use them directly
  and easily
• Extensive distribution network in place
• Gas power stations are about 50 efficient

35
Fossil fuels - Disadvantages

• Will run out (finite)
• Burning coal can cause acid rain
• Oil spillages etc.
• Contribute to the greenhouse effect by releasing
  carbon dioxide

36
A coal powered power plant has a power output of
400 MW and operates with an overall efficiency of
35

• Calculate the rate at which thermal energy is
  provided by the coal
• Efficiency useful power output/power input
• Power input output/efficiency
• Power input 400/0.35 1.1 x 103 MW

37
A coal powered power plant has a power output of
400 MW and operates with an overall efficiency of
35

• Calculate the rate at which coal is burned (Coal
  energy density 30 MJ.kg-1)
• 1 kg of coal burned per second would produce 30
  MJ. The power station needs 1.1 x 103 MJ per
  second. So
• Mass burned per second 1.1 x 103/30 37
  kg.s-1
• Mass per year 37x60x60x24x365 1.2 x 109
  kg.yr-1

38
A coal powered power plant has a power output of
400 MW and operates with an overall efficiency of
35

• The thermal energy produced by the power plant is
  removed by water. The temperature of the water
  must not increase by moe than 5 C. Calculate the
  rate of flow of water.
• Rate of heat loss 1.1 x 103 0.400 x 103
  740 MW
• In one second, Q mc?T
• 740 x 106 m x 4200 x 5
• m 35 x 103 kg
• So flow needs to be 35 x 103 kg.s-1

39
Nuclear Fission

40
Uranium

• Uranium 235 has a large unstable nucleus.

41
Capture

• A lone neutron hitting the nucleus can be
  captured by the nucleus, forming Uranium 236.

42
Capture

• A lone neutron hitting the nucleus can be
  captured by the nucleus, forming Uranium 236.

43
Fission

• The Uranium 236 is very unstable and splits into
  two smaller nuclei (this is called nuclear
  fission)

44
Free neutrons

• As well as the two smaller nuclei (called
  daughter nuclei), two neutrons are released (with
  lots of kinetic energy)

45
Fission

• These free neutrons can strike more uranium
  nuclei, causing them to split.

46
Chain Reaction

• If there is enough uranium (critical mass) a
  chain reaction occurs. Huge amounts of energy are
  released very quickly.

47
Bang!

• This can result in a nuclear explosion!YouTube -
  nuclear bomb 4

48
Controlled fission

• The chain reaction can be controlled using
  control rods and a moderator. The energy can then
  be used (normally to generate electricity).

49
Fuel rods

• In a Uranium reactor these contain Enriched
  Uranium (the percentage of U-235 has been
  increased usually by centrifuging)

50
Moderator

• This slows the free neutrons down, making them
  easier to absorb by the uranium 235 nuclei.
  Graphite or water is normally used.

1 eV neutrons are ideal)
51
Control rods

• These absorb excess neutrons,making sure that
  the reaction does not get out of control. Boron
  is normally used.

52
Heat

• The moderator gets hot from the energy it
  absorbs from the neutrons.

53
Heat

• This heat is used to heat water (via a heat
  exchanger), to make steam, which turns a turbine,
  which turns a generator, which makes electricity.

54
Useful by-products

• Uranium 238 in the fuel rods can also absorb
  neutrons to produce plutonium 239 which is itself
  is highly useful as a nuclear fuel (hence breeder
  reactors)

It makes more fuel!!!
55
Nuclear power - Advantages

• High power output
• Large reserves of nuclear fuels
• No greenhouse gases

56
Nuclear power - disadvantages

• Waste products dangerous and difficult to dispose
  of
• Major health hazard if there is an accident
• Problems associated with uranium mining
• Nuclear weapons
• Expensive to build and maintain

57
Solar power
58
Photovoltaic cells (photoelectric effect)
59
Active solar devices
60
The solar constant
61
The solar constant

• The suns total power output is 3.9 x 1026 W!

62
The solar constant

• The suns total power output is 3.9 x 1026 W!
• Only a fraction of this power actually reaches
  the earth, given by the formula
• I (Power per unit area) P/4pr2
• For the earth this is 1400 W.m-2 and is called
  the solar constant

63
The solar constant

• For the earth this is 1400 W.m-2 and is called
  the solar constant
• This varies according to the power output of the
  sun ( 1.5), distance from sun ( 4), and angle
  of earths surface (tilt)

64
The solar constant

• This 1400 W.m-2 can only shine on the cross
  sectional area of the earth as seen from the sun.
  Area pre2

65
The solar constant

• This 1400 W.m-2 can only shine on the cross
  sectional area of the earth as seen from the sun.
  Area pre2

However, as the earth turns this is spread over
the TOTAL surface area of the earth 4pre2
66
The solar constant

• Therefore the average intensity of the sun
  falling on the earth (pre2/4pre2) 1400 W.m-2
  350 W.m-2

67
Solar power - advantages

• Free once built
• Renewable
• Clean

68
Solar power - disadvantages

• Only works during the day
• Affected by cloudy weather
• Low power output
• Requires large areas
• Initial costs are high

69
Hydroelectric power
70
Water storage in lakes

• High water has GPE. AS it falls this urns to
  KE, turns a turbine etc.

71
Pumped storage

• Excess electricity can be used to pump water up
  into a reservoir. It acts like a giant battery.

72
Tidal water storage

• Tide trapped behind a tidal barrage. Water turns
  turbine etc.
• YouTube - TheUniversityofMaine's Channel

73
Hydroelectric - Advantages

• Free once built
• Renewable
• Clean

74
Hydroelectric - disadvantages

• Very dependent on location
• Drastic changes to environment (flooding)
• Initial costs very high

75
Wind power

• Calculating power

76
Wind moving at speed v, cross sectional area of
turbines A
V
A
77
Wind moving at speed v, cross sectional area of
turbines A
V
Volume of air going through per second Av Mass
of air per second Density x volume Mass of air
per second ?Av
A
78
Wind moving at speed v, cross sectional area of
turbines A
V
Mass of air per second ?Av If all kinetic
energy of air is transformed by the turbine, the
amount of energy produced per second ½mv2
½?Av3
A
79
Wind power - advantages

• Free once built
• Renewable
• Clean
• Ideal for remote locations

80
Wind power - disadvantages

• Works only if there is wind!
• Low power output
• Unsightly (?) and noisy
• Best located far from cities
• High maintainance costs

81
Wave power
82
OWC

• Oscillating
• water column

83
Modeling waves

• We can simplfy the mathematics by modeling square
  waves.

2A
84
Modeling waves

• If the shaded part is moved down, the sea becomes
  flat.

2A
85
Modeling waves

• The mass of water in the shaded part Volume x
  density Ax(?/2)xLx? A?L?/2

2A
86
Modeling waves

• Loss of Ep of this water mgh (A?L?)/2 x g x
  A A2gL?(?/2)

2A
87
Modeling waves

• Loss of Ep of this water mgh A2gL?(?/2)
• of waves passing per unit time f v/?

2A
88
Modeling waves

• Loss of Ep per unit time A2gL?(?/2) x v/?
• (1/2)A2L?gv

2A
89
Modeling waves

• The maximum power then available per unit length
  is then equal to (1/2)A2?gv

2A
90
Power per unit length

• A water wave of amplitude A carries an amount of
  power per unit length of its wavefront equal to
• P/L (?gA2v)/2
• where ? is the density of water and v stands for
  the speed of energy transfer of the wave

91
Wave power - Advantages

• Free once built
• Reasonable energy density
• Renewable
• Clean

92
Wave power - disadvantages

• Only in areas with large waves
• Waves are irregular
• Low frequency waves with high frequency turbine
  motion
• Maintainance and installation costs high
• Transporting power
• Must withstand storms/hurricanes

93
Radiation from the Sun
http//www.youtube.com/watch?NR1v1pfqIcSydgE
94
Black-body radiation

• Black Body - any object that is a perfect emitter
  and a perfect absorber of radiation
• object does not have to appear "black"
• sun and earth's surface behave approximately as
  black bodies

95
Black-body radiation
Need to learn this!

• http//phet.colorado.edu/sims/blackbody-spectrum/b
  lackbody-spectrum_en.html

96
Wiens law

• ?maxT constant (2.9 x 10-3 mK)

97
Example

• The sun has an approximate black-body spectrum
  and most of its energy is radiated at a
  wavelength of 5.0 x 10-7 m. Find the surface
  temperature of the sun.
• From Wiens law
• 5.0 x 10-7 x T 2.9 x 10-3
• T 5800 K

98
Stefan-Boltzmann law

• The amount of energy per second (power) radiated
  from a body depends on its surface area and
  absolute temperature according to
• P esAT4
• where s is the Stefan-Boltzmann constant (5.67 x
  10-8 W.m-2.K-4) and e is the emissivity of the
  surface ( e 1 for a black object)

99
Example

• By what factor does the power emitted by a body
  increase when its temperature is increased from
  100ºC to 200ºC?
• Emitted power is proportional to the fourth power
  of the Kelvin temperature, so will increase by a
  factor of 4734/3734 2.59

100
The Sun

• The sun emits electromagnetic waves (gamma
  X-rays, ultra-violet, visible light, infra-red,
  microwaves and radio waves) in all directions.

101
The earth

• Some of these waves will reach the earth

102
Reflected

• Around 30 will be reflected by the earth and
  the atmosphere. This is called the earths albedo
  (0.30). (The moons albedo is 0.12) Albedo is the
  ratio of reflected light to incident light.

103
Albedo

• The Albedo of a body is defined as the ratio of
  the power of radiation reflected or scattered
  from the body to the total power incident on the
  body.

104
Albedo

• The albedo depends on the ground covering (ice
  high, ocean low), cloud cover etc.

105
Absorbed by the earth

• Around 70 reaches the ground and is absorbed by
  the earths surface.

70
106
Absorbed by the earth

• This absorbed solar energy is re-radiated at
  longer wavelengths (in the infrared region of the
  spectrum)

Infrared
107
Temperature of the earth with no atmosphere?

• Remember the solar constant is around 1360 W.m-2.
  This can only shine on one side of the Earth at a
  time, and since the silhouette of the earth is a
  circle, the power incident 1360 x pr2
• 1360 x p x (6.4 x 106)2 1.75 x 1017 W

108
Temperature of the earth with no atmosphere?

• Power incident on earth 1.75 x 1017 W
• Since the albedo is 30, 70 of the incident
  power will be absorbed by the Earth
• 70 of 1.75 x 1017 W 1.23 x 1017 W

109
Temperature of the earth with no atmosphere?

• Power absorbed by Earth 1.23 x 1017 W
• At equilibrium,
• the Power absorbed Power emitted
• Using the Stefan Boltzmann law
• 1.23 x 1017 esAT4

110
Temperature of the earth with no atmosphere?

• Using the Stefan Boltzmann law
• 1.23 x 1017 esAT4
• 1.23 x 1017 1 x 5.67 x 10-8 x 4pr2 x T4
• This gives T 255 K (-18C)

111
Temperature of the earth with no atmosphere?

• T 255 K (-18C)
• This is obviously much colder than the earth
  actual temperature. WHY?

112
Absorbed by the earth

• This absorbed solar energy is re-radiated at
  longer wavelengths (in the infrared region of the
  spectrum) http//phet.colorado.edu/en/simulation/g
  reenhouse

Infrared
113
Absorbed

• Various gases in the atmosphere can absorb
  radiation at this longer wavelength (resonance)

H
O
They vibrate more (become hotter)
O
C
H
H
C
H
H
H
O
114
Greenhouse gases

• These gases are known as Greenhouse gases. They
  include carbon dioxide, methane, water and N2O.

O
H
O
H
H
C
H
H
C
H
O
115
Re-radiated

• These gases in the atmosphere absorb the
  infra-red radiation and re-emit it, half goes
  into space but half returns to the earth.

116
Its complex!!!
117
Balance

• There exists a balance between the energy
  absorbed by the earth (and its atmosphere) and
  the energy emitted.

Energy in
Energy out
118
Balance

• This means that normally the earth has a fairly
  constant average temperature (although there have
  been big changes over thousands of years)

Energy in
Energy out
119
Balance

• Without this normal greenhouse effect the
  earth would be too cold to live on.

Energy in
Energy out
120
Greenhouse gases

• Most scientists believe that we are producing
  more of the gases that absorb the infra-red
  radiation, thus upsetting the balance and
  producing a higher equilibrium earth temperature.
  This is called the enhanced greenhouse effect.

121
What might happen?

• Polar ice caps melt

122
What might happen?

• Higher sea levels and flooding of low lying areas
  as a result of non-sea ice melting and expansion
  of water

123
Coefficient of volume expansion

• Coefficient of volume expansion is defined as the
  fractional change in volume per unit temperature
  change

124
Coefficient of volume expansion

• Given a volume V0 at temperature ?0, the volume
  after temperature increase of ?? will increase by
  ?V given by
• ?V ?V0??

125
Definition

• Coefficient of volume expansion is the
  fractional change in volume per unit temperature
  change.
• ?V ?V0??

126
Example

• The area of the earths oceans is about 3.6 x
  108 km2 and the average depth is 3.7 km. Using ?
  2 x 10-4 K-1, estimate the rise in sea level
  for a temperature increase of 2K. Comment on your
  answer.

127
Example

• The area of the earths oceans is about 3.6 x
  108 km2 and the average depth is 3.7 km. Using ?
  2 x 10-4 K-1, estimate the rise in sea level
  for a temperature increase of 2K. Comment on your
  answer.
• Volume of water approx depth x area
• 3.6 x 108 x 3.7
• 1.33 x 109 km3 1.33 x 1018 m3
• ?V ?V0??
• ?V 2 x 10-4 x 1.33 x 1018 x 2 5.3 x 1014 m3
• ?h ?V/A 5.3 x 1014/3.6 x 1014 1.5 m
• Evaporation? Greater area cos of flooding?
  Uniform expansion?

128
What else might happen?

• More extreme weather (heatwaves, droughts,
  hurricanes, torrential rain)

129
What might happen?

• Long term climate change

130
What might happen?

• Associated social problems (??)

131
Evidence?

• Ice core research
• Weather records
• Remote sensing by satellites
• Measurement!
• How do ice cores allow researchers to see climate
  change? GrrlScientist Science
  guardian.co.uk

132
Other possible causes of global warming?

• Increase in solar activity
• Volcanic activity increasing CO2 concentrations
• Earth orbitting closer to sun?!

133
Surface heat capacitance Cs

• Surface heat capacitance is defined as the
  energy required to increase the temperature of 1
  m2 of a surface by 1 K. Cs is measured in
  J.m-2.K-1.
• Q ACs?T

134
Example

• Radiation of intensity 340 W.m-2 is incident on
  the surface of a lake of surface heat capacitance
  Cs 4.2 x 108 J.m-2.K-1. Calculate the time to
  increase the temperature by 2 K. Comment on your
  answer.
• Each 1m2 of lake receives 340 J.s-1
• Energy needed to raise 1m2 by 2 K Q ACs?T 1
  x 4.2 x 108 x 2 8.4 x 108 J
• Time Energy/power 8.4 x 108/340 2500000
  seconds 29 days
• Sun only shines approx 12 hours a day so would
  take at least twice as long