Energy Production

Question 1

Law of conservation of energy

Answer 1

energy can not be created nor destroyed, only changed from one form to another

Question 2

Sankey diagrams

Answer 2

they show the flow of energy through a system and identify where energy is lost to the surroundings

Question 3

Specific energy

Answer 3

the energy liberated per unit mass of fuel consumed
units = Jkg-1
useful when considering which fuel to use as greater the mass needed the greater the transport costs
uranium has a very very high specific energy value in comparison to other fuels

Question 4

Energy density

Answer 4

the energy liberated per unit volume of fuel consumed
units = Jm-3

Question 5

Primary sources of energy

Answer 5

is one that has not been transformed or converted before use by a consumer
e.g. burning coal to release heat or using Ek of wind to turn a windmill to grind corn

Question 6

Secondary sources of energy

Answer 6

one that results from the transformation of a primary source
e.g. electrical energy generated by coal-fired power stations

Question 7

Non-renewable energy sources

Answer 7

those that can be used up

Question 8

Renewable energy sources

Answer 8

those that cannot be used up or are used at a rate that effectively means the source will not run out (e.g. biofuels)

Question 9

Which is the most globally used fuel and fuel used most to produce energy?

Answer 9

most used fuel = oil (38%)
most used for electrical energy = coal (40%)

Question 10

How can electrical energy be generated?

Answer 10

if a conductor is moved relative to a magnetic field or within a charging magnetic field a fore is exerted on the electrons in the conductor and a voltage different will be induced across the conductor
if the conductor is part of a circuit a current will flow
this happens inside a generator
electricity companies use turbines to convert flow of fluids into mechanical motion within a generator

Question 11

kilowatt-hour

Answer 11

the amount of energy used by a 1kW device in one hour

Question 12

Pros and cons of fossil fuels

Answer 12

pros:
- very high energy density
- easy to transport
- cheap
- can be used directly for heat
- power stations can be built anywhere

cons:
- combustion products produce pollution & greenhouse gases
- fuel extraction can damage the environment
- non-renewable
- power stations need large amounts of fuel

Question 13

Efficiency of different fossil fuels

Answer 13

coal - 35-40%
oil - 38-45%
natural gas - 45-50%

Question 14

Basic process of nuclear power

Answer 14

use the energy released from nuclear fission to turn water into steam which then turns a turbine connected to generators to produce electrical energy
most common fuel source is U-235
fission is induced by collision with a neutron
the nucleus splits into Ba-141, K-92 + 3n
these 3n can then collide with more U nuclei
a neutron source is e.g. Americium-241 or Be-9 is used to initiate the fission process
if the mass of U is large enough then enough neutrons by fission will collide with further U nuclei to sustain fission

Question 15

Components in a nuclear power station

Answer 15

Moderator - makes fission happen by slowing down the high energy neutrons so that collisions can occur

Control rods - control rate of fission by absorbing neutrons

Heat exchanger - transfers the thermal energy to cold water to convert to steam

Question 16

Energy flow diagram for nuclear power

Answer 16

Ek of fission prods
thermal energy of water
Ek of turbines
Ek of generator
electrical energy

30% effective

Question 17

Pros and cons of nuclear power

Answer 17

pros:
- U has very high specific energy
- there are relatively large reserves of U
- no greenhouse gases produced

cons:
- radioactive waste is produced
- high risk if things go wrong
- non-renewable

Question 18

How does wind power work?

Answer 18

use the Ek of wind to turn large rotors which are connected to generators which produce electrical energy

Question 19

Energy flow diagram for wind power

Answer 19

Ek of wind
Ek of blades
Ek of generator
electrical energy

30% effective

Question 20

Pros and cons of wind power

Answer 20

pros:
- clean
- renewable
- free source of energy

cons:
- visually undesirable
- often need to be built far from population centres
- unreliable
- low energy density so wind turbines need to be large and numerous

Question 21

How does hydroelectric power work?

Answer 21

uses gravitational potential energy of water
the water can be stored in one of 3 ways:
- in a lake using a dam
- capturing the water at high tide and releasing it at low tide
- by pumping water from a low to high reservoir when energy demands are low
when the water is released it ganes Ek which is used to turn turbines which spin generators

Question 22

Energy flow diagram for hydroelectric power

Answer 22

Ep of water
Ek of water
Ek of turbines
Ek of generator
electrical energy

Question 23

More detail on pumped storage hydroelectric systems

Answer 23

involves use of 2 reservoirs
- when demand for electricity is high, water is allowed to run from upper to lower reservoirs and electrical energy is generated
- when demand for electricity is low, the turbines operate in reverse and electrical energy is used ti pump water back up to the higher reservoir
these are good as they can meet a rapid increase in demand for electrical energy by releasing a large volume of water through the turbines

Question 24

Pros and cons of hydroelectric energy

Answer 24

pros:
- clean
- renewable
- free source of energy

cons:
- only possible in certain areas
- building damns involves land being covered by water

Question 25

Photovoltaic cells

Answer 25

converts solar energy to electrical energy using the photoelectric effect
only produce small voltages so only used for electrical devices that do not need much energy
for higher voltages connect in series
for higher currents connect in parallel

Question 26

Solar heating panels

Answer 26

converts solar energy to thermal energy
water in black coper pipes is heated by the sun
used to provide hot water for domestic use

Question 27

Pros and cons of solar power

Answer 27

pros:
- clean
- renewable
- free source of energy

cons:
- only available during the day
- unreliable
- low energy density
- seasonal variations in light

Question 28

Conduction

Answer 28

most effective in solids
where thermal energy is transferred without any overall movement of the molecules/atoms
Ek is passed from molecule to molecule
two mechanisms:
atomic vibrations - at higher temps the speed and amplitude of vibrations increases, the particles then collide with neighbouring atoms transferrin internal energy
free electrons - metals, when heated the e in metals gain energy then transfer it to the atomic lattice

Question 29

Convection

Answer 29

thermal energy is transferred in bulk movements of the molecules/atoms
in liquids or gases
the heated region of fluid will expand as molecules move further apart as they gain internal energy
causes lower density so they rise and a colder, denser region of fluid fills the space left behind
process repeats
convection currents are formed

Question 30

Thermal radiation def.

Answer 30

the emission of electromagnetic radiation by an object
no medium required
the amount and tyoe pf EM rad. emitted will depend on the temp of object + surface area + nature of surface

Question 31

Black body radiation

Answer 31

a black body is one that is a perfect emitter and perfect absorber hence amount of radiation does not depend on the nature of its surface but it does temp
as temperature of black body increases:
- peak wavelength of radiation decreases (peak freq. increases)
- overall intensity of each wavelength increases
- total intensity of radiation increases

Question 32

Stefan-Boltzmann Law

Answer 32

shows that power emitted is proportional to T^4 (where T in K)
power = eσAT^4

Question 33

Wien’s Displacement Law

Answer 33

relates the wavelength at which the intensity of the radiation is a maximum to the temperature of the black-body
wavelength max = 2.90 X 10^-3/T

Question 34

Emissivity (e)

Answer 34

def. = ratio of power radiated by an object per unit area to the power radiated per unit area by a black body at the same temperature

it is a measure of how close to a blackbody an object is in terms of power emitted or absorbed

max value = 1.0 (blackbody)

Question 35

Albedo (a)

Answer 35

is the proportion of incident radiation reflected

if:
a = 1 all incident radiation is reflected
a = 0 all incident radiation if asorbed

Question 36

Solar constant

Answer 36

the amount of solar energy that falls per second on an area of 1 m^2 above the earths atmosphere

average value is 1360 Wm^-2

amount of energy that arrives on earths surface is much lower as:
- radiation is absorbed by the atmosphere
- radiation is reflected by clouds

Question 37

Why is average intensity over the entire upper atmosphere only 340 not 1360 Wm-2

Answer 37

viewed from the sun, the earth is a flat disc and so total energy received by the earth is 1360 X piR^2
but the earth is actually a rotating sphere so energy is spread out over the entire surface
therefore:
average intensity of the surface of the entire upper atmosphere = total energy incident/total surface area
= (1360 X piR^2)/(4piR^2)
= 1/4 X 1360
= 340 Wm^-2

Question 38

Functioning of green house gases

Answer 38

they absorb the infra-red radiation emitted by the earths surface and re-emit it in all directions
some of the re-emitted radiation will be absorbed byt the earths surface causing further heating

they absorb infra-red radiation because the frequency of this radiation matches the natural frequency at which g.g. molecules vibrate (resonant frequency)