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Heat Light Sound Kinetic Electric Types of Energy Chemical Gravitational Elastic/strain Nuclear 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 heat kinetic sound heat gravitational

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 = 75 500 = 0.15

Energy efficient light bulb Efficiency = 75 100 = 0.75

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 10 4 GJ.kg

-1 ( 70000000 MJ.kg

-1 )

Hydroelectric energy density?

• Imagine 1 kg falling 100m.

• Energy loss = mgh = 1x10x100 = 10 3 J • If all of this is turned into electrical energy it gives an “energy density” of the “fuel” of 10 3 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 Oil Natural gas Coal Nuclear Hydroelectric Others % total energy production 40 23 23 7 7 < 1 CO 2 emission g.MJ

-1 70 50 90 -

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 10 3 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 10 3 MJ per second. So Mass burned per second = 1.1 x 10 3 /30 = 37 kg.s

-1 Mass per year = 37x60x60x24x365 = 1.2 x 10 9 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 10 3 – 0.400 x 10 3 In one second, Q = mcΔT = 740 MW 740 x 10 6 = m x 4200 x 5 m = 35 x 10 3 kg So flow needs to be 35 x 10 3 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.

Fission The 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.

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.

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 10 26 W!

Only a fraction of this power actually reaches the earth, given by the formula I (Power per unit area) = P/4πr 2 For the earth this is 1400 W.m

-2 the solar constant and is called

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 = πr e 2

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 = πr e 2

However, as the earth turns this is spread over the TOTAL surface area of the earth =

4πr e 2

The solar constant Therefore the average intensity of the sun falling on the earth = (

πr e 2 /4πr e 2 )

= 350 W.m

-2 1400 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 A

Volume 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 A

Mass of air per second = ρAv If all kinetic energy of air is transformed by the turbine, the amount of energy produced per second = ½mv 2 = ½ρAv 3

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

2A Modeling waves • We can simplfy the mathematics by modeling square waves.

L λ

2A Modeling waves • If the shaded part is moved down, the sea becomes flat.

L λ

2A Modeling waves • The mass of water in the shaded part = Volume x density = Ax(λ/2)xLxρ = AλLρ/2 L λ

2A Modeling waves • Loss of E p of this water = mgh = = (AλLρ)/2 x g x A = A 2 gLρ(λ/2) L λ

2A Modeling waves • Loss of E p of this water = mgh= A 2 gLρ(λ/2) • # of waves passing per unit time = f = v/λ L λ

2A Modeling waves • Loss of E p per unit time = A • = (1/2)A 2 Lρgv 2 gLρ(λ/2) x v/λ L λ

2A Modeling waves • The maximum power then available per unit length is then equal to = (1/2)A 2 ρgv L λ

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 = (ρgA 2 v)/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 Need to “learn” this!

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

Wien’s law • λ max T = 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 = eσAT 4

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 473 4 /373 4 = 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 This absorbed solar energy is re-radiated at longer wavelengths (in the infrared region of the spectrum) Infrared