Energy, Power, and Climate Change - iknsphysicsib -...

Energy, Power, and Climate Change Topic 8

Energy conversions

!   Energy transferred from one form to another (conservation of energy)

!   Can an engine work endlessly?

Energy conversions

!   Energy transferred from one form to another (conservation of energy)

!   Can an engine work endlessly? – No

!   This is because some energy has to be trasnfered to the surrounding from the system as thermal energy

!   This energy can’t be harvested. It can’t do work.

!   Let’s look at this from a microscopic view. Open to page 170 in purple book.

Energy conversions

!   This unavailable energy is known as degraded energy.

!   Definition: thermal energy transferred to the surroundings that is no longer available to produce useful work.

Energy conversions

Energy conversions !   All energy eventually turns to heat

!   Eventually everything will be the same temperature… end of the universe???

Sankey diagrams

!   Used to represent energy conversions

!   Arrow represents conversions

!   Width power or energy consumed at the stage

!   Degraded energy is shown with an arrow up or down

Sankey diagrams

!   Examples on page 171 in purple book

Question

!   Draw a Sankey diagram for: !   A) an electric light bulb

!   B) a bicycle dynamo producing electricity to illuminate a light from mechanical energy

Question !   Draw a Sankey diagram for:

!   A) an electric light bulb

!   B) a bicycle dynamo producing electricity to illuminate a light from mechanical energy

Electrical generators

!   Converts mechanical energy to electrical

!   When charged particles move in an magnetic field, they produce a force

!   Explain what happens when the wire moves upward.

Electrical generators !   Electrons going up experience a force

!   Force will make them move to one side of the wire

!   Causing a potential difference and a current will flow.

!   In generators, coils are used instead of straight wires.

Generating electricity

!   Coal – fired power station !   Fuel is used to release thermal energy

!   This thermal energy is used to boil water to make steam

!   Steam used to turn turbines

!   Motion of turbine used to generate electricity

!   Transformers alter the potential difference.

!   Open to page 66 in study guide

Coal – fired power station

Generating electricity

!   Look at the Sankey diagrams of electricity generation on P.180 and 181 in purple book and 66 in study guide

Sources of energy

!   Renewable VS non-renewable !   Renewable: energy sources that can’t be used up

!   Non-renewable: energy sources that can be used up and eventually run out


!   Homework: read and summarize pages 173 – 177 in ur purple book.

Energy density !   Useful comparison between fuels

!   Definition: energy liberated per kg

!   Fuel choice is influenced by energy density.

!   A fuel with high energy density costs less to transport

!   The greater the mass of the fuel that needs to be transported, the greater the cost

!   Table of energy densities on page 67 in study guide.

Nuclear power

!   What happens in a fission reaction?

Nuclear fission !   What happens in a fission reaction?

!   Large nuclei split into smaller nuclei

!   173 MeV is released in this reaction. How did we calculate that?

!   How much Joules is that?

!   This amount of energy is just for 1 atom.. Do you usually only have 1 atom?

Nuclear chain reaction

!   To split nucleus, energy is required to overcome strong nuclear force

!   This energy can be supplied by adding a neutron to the uranium nucleus.

!   Binding energy is now increased and nucleus can’t get rid of energy, so it splits into two

!   New neutrons are now released and can be captured by other uranium nuclei.

!   This is what we call a chain reaction

Nuclear chain reaction

Factors affected chain reaction !   Speed of neutron

!   KE about 1eV !   If too fast it will pass by nuclei without binding !   It is slowed down by moderator nuclei (small nuclei)

!   Mass of radioactive nuclei !   If too small neutrons will have to be slowed down a lot..

And by then the neutron would have left the reacting uranium

!   The minimum amount of mass is called the critical mass

Nuclear fuel

!   99.3% of natural uranium is U – 238 .

!   It’s a weak alpha emitter

!   It absorbs the neutron but doesn’t split (prevents chain reaction).

!   U – 235 makes the other 0.7% and is much useful as it produces a chain reaction and energy can be harvested.

!   Therefore U – 235 needs to be increased by process called enrichment.

Nuclear fuel

!   3% of U – 235 used to fuel a nuclear reactor

!   Removed U – 238 (so that to increase the proportion of U – 235) is called depleted uranium.

!   This depleted uranium is very tough and heavy and used for making radiation shielding and weapons (armour piercing ammo).

Nuclear fuel

! Pu – 239 can be also used as a fuel for energy or in the manufacturing of weapons

Nuclear fuel !   fuel made from small cylinders stalked together to

make fuel rods

!   Fuel rods are bundled together

!   A reactor will contain many of these bundles

Nuclear reactor !   Three important components:

!   Moderator !   To slow down neutrons

!   Control rods !   Easily absorbed by neutron !   Can be added or removed to control chain reaction

!   Heat exchanger !   Reactor is in a sealed area !   The temperature increase because of reaction !   This thermal energy is transferred to water. !   The steam produced will turn the turbines and produce

electricity

Nuclear reactor !   Description

is on page 185 in purple book

Nuclear reactor

!   Look at Sankey diagram on page 69 in study guide

Controlling production

!   Controlling the number of neutrons

!   U -238 will absorb some of the neutrons therefore lower the reaction rate (not easily changed so hard to control)

!   Introduce rods (Boron) that are neutron absorbing.

!   Loss of control: mix of U – 235 with moderator (ATOMIC BOMB)

Advantages VS disadvantages

Advantages Disadvantages

Extremely high energy density

Process produces nuclear waste that is just stored

Reserves of uranium large compared to oil

Large risk if anything should go wrong

Nonrenewable (nonrenewable but lasts a long time)

Problems with nuclear energy

!   Extraction of uranium !   Radiation exposure to mine workers

!   Another method: leachingà solvent used to dissolve uranium and then pumped out à this though leads to groundwater contamination

!   Meltdown !   Overheat of fuel rods causing them to melt

!   Fuel then can’t be removed and pressure vessel will burst (Chernobyl)

!   Meltdown can be caused by malfunction in cooling system or leak in pressure vessel.

Waste

!   Low level waste !   Extraction, fuel enrichment, heat transfer from fuel rods

all leave some traces of radioactive material !   Material disposed should be in places away from human

contact for 100 – 500 years !   Old reactors – can’t be touched for many years then it is

demolished or kept in concrete.

!   High level waste !   Disposal of fuel rods (half life for thousands of years) !   Currently they are disposed of in:

!   water: to cool off then they are sealed in steel cylinders !   Reprocessed to remove uranium and plutonium.

homework

!   Solve questions 6 – 9 on page 187

!   Read about fusion on pages 187 – 189

!   Watch videos on website

Solar power !   Two ways to harvest radiated energy from the sun:

!   Photovoltaic cell

!   Active solar heater

Photovoltaic cell !   Solar energy à electrical energy

!   Uses semiconductors to do this

!   Produces small potential difference and not much current (used for low energy devices)

!   Use in series: more voltage / use in parallel: high current

Active solar heater

!   Also know as solar panel

!   Solar energy à thermal energy

!   hot water is typically produced and would cut down on use of electricity



Very ‘clean’ production (no harmful chemical by-products)

Can only be used during the day

Renewable source Unreliable (bad weather)

Free Low energy density

Solar constant !   Amount of energy that

falls per second on an area of 1 m2 above the Earth’s atmosphere that is at right angles to the sun’s rays.

!   About 1400 Wm-2

!   Look at diagram on page 71 in study guide

!   Different parts of the earth (different latitudes) will receive different solar radiation

Inverse square law of radiation

!   The longer the distance from radiation point source à the more area it will cover à less power is received.

!   What happened when you doubled the distance?

Inverse square law of radiation

!   Surface area of sphere is 4πr2

!   Intensity, power received per unit area, is:

!   I inversely proportional to r2

€

I =P4πr2

Wave power

!   Uses kinetic energy of the wave to generate electricity

!   Two main techniques: !   Oscillating water column (OWC)

! Pelamis

!   (let’s watch the videos)


Hydroelectric power

!   GPE water à KE water à KE turbines à electrical energy

!   Water can gain its PE from: !   Water cycle: rain stored in reservoirs as high up as possible

!   Tidal power: trap water at high tides and release at low tides

!   Water pumped up from low reservoir to high reservoir. They can get this energy from coal power station when coal power station is shut and excess heat can be used to pump water up.



Very ‘clean’ production (no harmful chemical by-products)

Only in certain areas

Renewable source Construction of dam will need land being buried under water

Free

Same as solar and wind (next)

Greenhouse effect

Greenhouse effect !   Short wavelength radiation causes earth surface temp

to go up

!   Some of radiation will be reflected (Infrared à longer wavelength)

!   Some of this radiation will be absorbed by gases in atmosphere and re-radiated in all directions

!   This is called greenhouse effect

– 13 – N11/4/PHYSI/SP2/ENG/TZ0/XX/M

B3. Part 1 Greenhouse effect

(a) effect caused by gas such as 2 3 4 2H O/NH /CH /CO /greenhouse gas in the atmosphere;

gas absorbs outgoing (long wave) radiation from Earth; gas re-radiates some of the energy back to Earth; [3]

(b) (i) 8

13

3.0 104.6(µm)

6.5 10

×=

×;

5(µm)≈ [1]

(ii) water vapour molecules have a natural frequency of oscillation;

if this frequency of oscillation is 136.5 10× / reference to frequency at X;

due to resonance this radiation is readily absorbed by the molecules / the radiation matches the natural frequency of oscillation; [3]

or X is a natural frequency (of oscillation) of water molecule; so resonance effects mean that molecules are excited at this frequency; and energy is removed/less energy transmitted from electromagnetic waves at

this (particular) frequency; (iii) energy gained by absorption needs to be re-emitted (as molecules de-excite); in other directions / some returns to Earth; [2] (iv) more greenhouse gases means that there is more absorption of outgoing

radiation; therefore more energy returns to Earth; leading to a further/greater increase in the temperature of the surface (of Earth); [3]

(c) change in volume 3 9 –4 82.3 10 10 0.5 2.1 10 2.4 10( )= × × × × × = × ;

change in water level3 9 –4

–3

5 6

2.3 10 10 0.5 2.1 10( 1 10 m)

2.4 10 10

× × × × ×= = ×

× ×;

clear statement that this value is similar to fall due to evaporation; [3]

Greenhouse effect !   Gases that absorb this radiation are called greenhouse

gases

!   Albedo: is the ratio of reflected over incident radiation

!   The net temp of earth surface goes up

!   Without this effect earth temp would be much cooler

!   Temp remains constant if !   rate earth radiates energy = rate earth absorbs energy

Equations—Core and AHL

10 © International Baccalaureate Organization 2007

Core AHL

Topic 8: Energy, power and climate change

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Greenhouse gases !   CH4: product from decay,

composition, fermentation, livestock and plants

!   Water: water vapour in upper atmosphere (clouds have condensed water vapour) – usually doesn't’t change level

!   CO2: from combustion of fuels – plants remove CO2 through photosynthesis reducing amount of CO2 – this is known as carbon fixation

!   N2O (nitrous oxide): from livestock and industries – remain in atmosphere for very long times

Greenhouse gases !   Other gases that contribute to

green house effect: !   Ozone O3: absorbs UV – it adds to

greenhouse effect

!   Chlorofluorocarbons (CFCs): used in refrigerators propellants, and cleaning solvents.. Depletes ozone layer

Greenhouse gases !   Gases absorb IR as a result of

resonance

!   Natural frequency of molecule are in IR region

!   Resonance occurs when !   driving frequency (radiation

coming from earth) = natural frequency of molecule

!   Amplitude of vibration increase therefore temp increases

!   Specific frequency for different molecular bonds

Greenhouse gases

Global Warming !   Temp increases over years.. Causes?

!   Changes in composition of greenhouse gases in atmosphere

!   Changes in intensity of radiation from sun – increased solar flare activity

!   Cyclical changes in Earth’s orbit and volcanic activity

!   Both natural and human activities contribute

!   Enhanced greenhouse effect: increase in the greenhouse effect caused by human activity.

Evidence of global warming

!   Ice core data

Evidence of global warming

!   Ice core data !   New snow fall adds layer to ice present over years

!   Isotopic analysis allows temp to be estimated and air bubbles trapped in ice to be measured

!   Temp and carbon dioxide changes are closely correlated.

Mechanisms that increase rate of global warming

!   1- Increase GW reduces ice/snow cover à reducing albedo à more heat being absorbed

!   2- Temp increases à reduces CO2 solubility in sea à increasing CO2 in atmosphere

!   3- Increased GW à increase evaporation of water – more water vapour in atmosphere

!   4- in Tundra (regions with frozen subsoil) has simple vegetation and with increased temp the CO2 trapped in subsoil may destroy vegetation

!   5- deforestation: release more CO2 in atmosphere and reduces carbon fixation

Mechanisms that increase rate of global warming

!   Mechanism 1 – 4: an increase in temp has gone to lead to a further increase in temp

!   This process is called positive feedback [not something positive for our Earth]

!   Can the problem be fixed by reducing the temp “negative feedback”??

Effects of GW !   Climate change and increase sea level

!   precise predictions are difficult to make due to factors such as: !   anomalous expansion of water

!   different effects of ice melting on sea water compared to ice melting on land.

Effects of GW !   Coefficient of volume expansion records fractional

change (volume per degree change in temp)

!   NOT IN DATA BOOKLET

!   ΔV : increase in volume (m3)

!   V0 : original volume (m3)

! Δθ: increase in temp (K or °C)

! γ: coeff. of volume expansion (K-1 or °C-1) €

γ =ΔVV0Δθ

Effects of GW !   Case 1: Temp changes from 0 to 4 degrees Celsius

! Coeff is negative

!   temp increased à volume decreased

!   When floating ice melts in seawater, the volume at first will decrease (eg. Arctic ice at N. Pole)

!   Case 2: ice that is on land !   Increase sea level (as it was not part of sea and not

displacing it)

!   E.g. glaciers, snow, Antarctic ice in S. Pole

€

γ =ΔVV0Δθ

Solutions to reduce enhanced greenhouse effect

!   Reduce consumption of fossil fuels

!   To do that: !   Advances in technology in order to have more efficient

power production, decarbonizing exhaust gases, fusion reactors made operational

!   Reduction in energy combustion by improving thermal insulation, transportation

!   Replace oil and coal use with renewable energy sources or natural gas for fewer greenhouse gas emissions

!   Planting new trees and maintain existing forests

Solutions to reduce enhanced greenhouse effect

!   Read last section on page 78 in study guide about protocols and international agreements

Black-body radiation

!   Radiation given out by hot objects

!   The ‘perfect’ emitter will be a ‘perfect’ black-body

!   Black body absorbs all light falling on it

!   In theory, radiation from a ‘perfect’ black emitter is known as black-body radiation

Black-body radiation !   Black-body doesn’t depend on nature of substance

!   Depends on its temperature

Black-body radiation

Black-body radiation !   At a given temp

!   Different wavelengths of radiation emitted

!   Some wavelength more intense than others

!   Area under graph between two wavelengths = intensity given by that range

!   Area of whole spectrum = total power radiated

!   The hotter the temp, the shorter the wavelength the most intense part of the spectrum will be

!   Stars and planets are not perfect emitters, but their radiation is similar to black-body radiation

Black-body radiation !   All object (higher than 0 K) will radiate EM waves

!   Most every objects radiate energy in IR range

!   At constant temp (not changing states) : rate at which an object radiates and absorbs is the same

!   Good radiator mean good absorber

!   Surfaces that are !   light in color/ smooth/ shiny à bad radiators/absorbers !   Dark and rough à good radiators/absorbers

!   Temp increases à frequency of radiation increases à energy radiated increases

!   E.g.: sun warmth on earth by radiation / white cloths in summer

Wien’s law

!   Wien’s displacement law relates the wavelength at which the intensity of the radiation is a maximum λmax to the temperature of the black-body T

!   Λmax T = constant

!   Constant found by experiment (2.9 x 10-3 mK)

!   T here has to be in KELVIN

Wien’s law

Wien’s law

!   From stars: !   We can calculate surface temperature

!   Surface temp less than core temp

!   Hot stars: give out all frequencies so we see as white

!   Cooler stars: lower frequencies (longer wavelength) so red

!   Planets: radiation emitted in IR region

Stefan – Boltzmann Law !   Links total power radiated by black-body (per unit area)

to the temperature of the black-body

!   Total power radiated directly prop. T4

!   A: surface area of emitter

!   T: temp of emitter (K)

!   For star: A = 4πr2



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© International Baccalaureate Organization 2007 1

Physics data booklet

Fundamental constants

Quantity Symbol Approximate value

Acceleration of free fall (Earth’s surface) g 29.81ms�

Gravitational constant G 11 2 26.67 10 N m kg� �u

Avogadro’s constant NA 23 16.02 10 mol�u

Gas constant R 1 18.31J K mol� �

Boltzmann’s constant k 23 11.38 10 J K� �u

Stefan–Boltzmann constant V 8 2 45.67 10 W m K� � �u

Coulomb constant k 9 2 28.99 10 N m C�u

Permittivity of free space 0H 12 2 1 28.85 10 C N m� � �u

Permeability of free space 0P 7 14ʌ 10 T m A� �u

Speed of light in vacuum c 8 13.00 10 ms�u

Planck’s constant h 346.63 10 J s�u

Elementary charge e 191.60 10 C�u

Electron rest mass me 31 29.110 10 kg 0.000549 u 0.511MeV c� �u

Proton rest mass mp 27 21.673 10 kg 1.007276 u 938MeV c� �u

Neutron rest mass mn 27 21.675 10 kg 1.008665u 940 MeV c� �u

Unified atomic mass unit u 27 21.661 10 kg 931.5MeV c� �u

Equilibrium

!   Thermal equilibrium: temp is constant if power absorbed = power radiated (e.g. by a planet)

!   Absorbs > radiates : temp goes up

!   If object not perfect black-body !   Emissivity: power radiated by unit area

power radiated per unit area by blackbody at same temp

Equilibrium

!   Surface heat capacity !   Energy required to raise the temp of unit area of planet’s

surface by one degree

!   Albedo is the ratio of power scattered over incident

!   So power absorbed = P(1 – albedo) πr2 (r is the radius of planet)

!   Total power radiated from planet surface = 4πr2



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Equilibrium

!   If radiation reflected and incident is not same, therefore change in temp

!   Temp change = (Incoming – outgoing radiation) x time

Cs

•  Simplifications of model:

Treats whole planet as single body

Ignores any processes that involve feedback



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