Types of Energy Heat Chemical Light Gravitational Sound Elastic/strain Kinetic Nuclear Electric Stored/potential
Dec 25, 2015
Types of Energy
Heat Chemical
Light Gravitational
Sound Elastic/strain
Kinetic Nuclear
Electric
Stored/potential
The Law of Conservation of Energy
Energy can be changed (transformed) from one type to another, but it can never be made or destroyed.
This means that the total amount of energy in the Universe stays the same!
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
Other examples
When a rocket launches.
Chemical kinetic gravitational
sound
heat heat
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.
Energy transfer (change)
A lamp turns electrical energy into heat and light energy
Sankey Diagram
A Sankey diagram helps to show how much light and heat energy is produced
Sankey Diagram
The thickness of each arrow is drawn to scale to show the amount of energy
Sankey Diagram
Notice that the total amount of energy before is equal to the total amount of energy after (conservation of energy)
Efficiency
Although the total energy out is the same, not all of it is useful.
Efficiency
Efficiency is defined as
Efficiency = useful energy output
total energy input
Example
Efficiency = 75 = 0.15
500
Energy efficient light bulb
Efficiency = 75 = 0.75
100 That’s much better!
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
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)
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
Electromagnetic induction
If a magnet is moved inside a coil an electric current is induced (produced)
Electromagnetic induction
A electric current is induced because the magnetic field around the coil is changing.
Generator/dynamo
A generator works in this way by rotating a coil in a magnetic field (or rotating a magnet in a coil)
Non-renewable
• Finite (being depleted – will run out)
• In general from a form of potential energy released by human action
Fossil fuels – Coal, oil, gas
Nuclear fuels
Renewable
• Mostly directly or indirectly linked with the sun
• The exception is tidal energy
Photovoltaic cells (photoelectric effect)
Active solar devices
Wind
Wave
Tidal
Biomass
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 < 1 -
Electricity production
Generally (except for solar cells) a turbine is turned, which turns a generator, which makes electricity.
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.
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
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
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
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
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 MWIn one second, Q = mcΔT
740 x 106 = m x 4200 x 5m = 35 x 103 kg
So flow needs to be 35 x 103 kg.s-1
Nuclear Fission
Uranium
Uranium 235 has a large unstable nucleus.
Capture
A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236.
Capture
A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236.
FissionThe Uranium 236 is very unstable and splits into two smaller nuclei (this is called nuclear fission)
Free neutrons
As well as the two smaller nuclei (called daughter nuclei), two neutrons are released (with lots of kinetic energy)
Fission
These free neutrons can strike more uranium nuclei, causing them to split.
Chain Reaction
If there is enough uranium (critical mass) a chain reaction occurs. Huge amounts of energy are released very quickly.
Bang!
This can result in a nuclear explosion!YouTube -
nuclear bomb 4
Controlled fission
The chain reaction can be controlled using control rods and a moderator. The energy can then be used (normally to generate electricity).
Fuel rods
• In a Uranium reactor these contain Enriched Uranium (the percentage of U-235 has been increased – usually by centrifuging)
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)
Control rods
These absorb excess neutrons,making sure that the reaction does not get out of control. Boron is normally used.
Heat
The moderator gets hot from the energy it absorbs from the neutrons.
HeatThis heat is used to heat water (via a heat exchanger), to make steam, which turns a turbine, which turns a generator, which makes electricity.
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!!!
Nuclear power - Advantages
• High power output
• Large reserves of nuclear fuels
• No greenhouse gases
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
Solar power
Photovoltaic cells (photoelectric effect)
Active solar devices
The solar constant
The solar constant
The sun’s total power output is 3.9 x 1026 W!
The solar constant
The sun’s 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/4πr2
For the earth this is 1400 W.m-2 and is called the solar constant
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 earth’s surface (tilt)
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 = πre2
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 = πre2
However, as the earth turns this is spread over the TOTAL surface area
of the earth = 4πre2
The solar constant
Therefore the average intensity of the sun falling on the earth = (πre
2/4πre2) 1400 W.m-2
= 350 W.m-2
Solar power - advantages
• “Free” once built
• Renewable
• Clean
Solar power - disadvantages
• Only works during the day
• Affected by cloudy weather
• Low power output
• Requires large areas
• Initial costs are high
Hydroelectric power
Water storage in lakes
“High” water has GPE. AS it falls this urns to KE, turns a turbine etc.
Pumped storage
• Excess electricity can be used to pump water up into a reservoir. It acts like a giant battery.
Tidal water storage
• Tide trapped behind a tidal barrage. Water turns turbine etc.
• YouTube - TheUniversityofMaine's Channel
Hydroelectric - Advantages
• “Free” once built
• Renewable
• Clean
Hydroelectric - disadvantages
• Very dependent on location
• Drastic changes to environment (flooding)
• Initial costs very high
Wind power
Calculating power
Wind moving at speed v, cross sectional area of turbines = A
V
A
Wind moving at speed v, cross sectional area of turbines = A
V
AVolume of air going through per second = Av
Mass of air per second = Density x volume
Mass of air per second = ρAv
Wind moving at speed v, cross sectional area of turbines = A
V
AMass of air per second = ρAv
If all kinetic energy of air is transformed by the turbine, the amount of energy produced per second = ½mv2 = ½ρAv3
Wind power - advantages
• “Free” once built
• Renewable
• Clean
• Ideal for remote locations
Wind power - disadvantages
• Works only if there is wind!
• Low power output
• Unsightly (?) and noisy
• Best located far from cities
• High maintainance costs
Wave power
OWC
Oscillating
water column
Modeling waves
• We can simplfy the mathematics by modeling square waves.
λ
L
2A
Modeling waves
• If the shaded part is moved down, the sea becomes flat.
λ
L
2A
Modeling waves
• The mass of water in the shaded part = Volume x density = Ax(λ/2)xLxρ = AλLρ/2
λ
L
2A
Modeling waves
• Loss of Ep of this water = mgh = = (AλLρ)/2 x g x A = A2gLρ(λ/2)
λ
L
2A
Modeling waves
• Loss of Ep of this water = mgh= A2gLρ(λ/2)
• # of waves passing per unit time = f = v/λ
λ
L
2A
Modeling waves
• Loss of Ep per unit time = A2gLρ(λ/2) x v/λ
• = (1/2)A2Lρgv
λ
L
2A
Modeling waves
• The maximum power then available per unit length is then equal to = (1/2)A2ρgv
λ
L
2A
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 = (ρgA2v)/2
where ρ is the density of water and v stands for the speed of energy transfer of the wave
Wave power - Advantages
• “Free” once built
• Reasonable energy density
• Renewable
• Clean
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
Radiation from the
Sun
http://www.youtube.com/watch?NR=1&v=1pfqIcSydgE
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
Black-body radiation
• http://phet.colorado.edu/sims/blackbody-spectrum/blackbody-spectrum_en.html
Need to “learn” this!
Wien’s law
• λmaxT = constant (2.9 x 10-3 mK)
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 Wien’s law
5.0 x 10-7 x T = 2.9 x 10-3
T = 5800 K
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 = eσAT4
where σ 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)
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
The Sun
The sun emits electromagnetic waves (gamma X-rays, ultra-violet, visible light, infra-red, microwaves and radio waves) in all directions.
The earth
Some of these waves will reach the earth
Reflected
Around 30% will be reflected by the earth and the atmosphere. This is called the earth’s albedo (0.30). (The moon’s albedo is 0.12) Albedo is the ratio of reflected light to incident light.
30%
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.
Albedo
The albedo depends on the ground covering (ice = high, ocean = low), cloud cover etc.
Absorbed by the earth
Around 70% reaches the ground and is absorbed by the earth’s surface.
70%
Absorbed by the earth
Infrared
This absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum)
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 πr2
= 1360 x π x (6.4 x 106)2 = 1.75 x 1017 W
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
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 = eσAT4
Temperature of the earth with no atmosphere?
Using the Stefan Boltzmann law;
1.23 x 1017 = eσAT4
1.23 x 1017 = 1 x 5.67 x 10-8 x 4πr2 x T4
This gives T = 255 K (-18°C)
Temperature of the earth with no atmosphere?
T = 255 K (-18°C)
This is obviously much colder than the earth actual temperature. WHY?
Absorbed by the earth
Infrared
This absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum) http://phet.colorado.edu/en/simulation/greenhouse
Absorbed
• Various gases in the atmosphere can absorb radiation at this longer wavelength (resonance)
C
O
O
C
H
H
H
HThey vibrate more (become hotter)
HH
O
Greenhouse gases
• These gases are known as “Greenhouse” gases. They include carbon dioxide, methane, water and N2O.
C
O
O
C
H
H
H
HHH
O
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.
It’s complex!!!
Balance
There exists a balance between the energy absorbed by the earth (and its atmosphere) and the energy emitted.
Energy in Energy out
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
Balance
Without this normal “greenhouse effect” the earth would be too cold to live on.
Energy in Energy out
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.
What might happen?
• Polar ice caps melt
What might happen?
• Higher sea levels and flooding of low lying areas as a result of non-sea ice melting and expansion of water
Coefficient of volume expansion
• Coefficient of volume expansion is defined as the fractional change in volume per unit temperature change
Coefficient of volume expansion
Given a volume V0 at temperature θ0, the volume after temperature increase of Δθ will increase by ΔV given by
ΔV = γV0Δθ
Definition
Coefficient of volume expansion is the fractional change in volume per unit temperature change.
ΔV = γV0Δθ
Example
The area of the earth’s oceans is about 3.6 x 108 km2 and the average depth is 3.7 km. Using γ = 2 x 10-4 K-1, estimate the rise in sea level for a temperature increase of 2K. Comment on your answer.
Example
The area of the earth’s oceans is about 3.6 x 108 km2 and the average depth is 3.7 km. Using γ = 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
ΔV = γV0Δθ
ΔV = 2 x 10-4 x 1.33 x 1018 x 2 = 5.3 x 1014 m3
Δh = ΔV/A = 5.3 x 1014/3.6 x 1014 = 1.5 m
Evaporation? Greater area cos of flooding? Uniform expansion?
What else might happen?
• More extreme weather (heatwaves, droughts, hurricanes, torrential rain)
What might happen?
• Long term climate change
What might happen?
• Associated social problems (??)
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
Other possible causes of global warming?
• Increase in solar activity
• Volcanic activity increasing CO2 concentrations
• Earth orbitting closer to sun?!
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 = ACsΔT
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 = ACsΔT = 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