IB Physics Topic 8

Albedo

(of a body) is the ratio between the power of radiation reflected (or scattered) from the body to the total power incident on it

Chain reaction (nuclear)

* The splitting was triggered by a single neutron that had just the right energy to excite the nucleus. * During the split, two or more neutrons were released. * If each of these neutrons splits subsequent nuclei, we have what is called a *chain reaction*.

Emissivity of black-body

*A black-body is a perfect emitter / absorber of radiation and has an emissivity of e = 1*

The rate of global warming might be reduced by A. Replacing the use of coal and oil with natural gas. B. A reduction in the Earth's albedo. C. A reduction in carbon fixation. D. An increase in deforestation.

*A. Replacing the use of coal and oil with natural gas.*

Which of the following is likely to increase greenhouse gas concentrations in the atmosphere? A. Using natural gas instead of coal to generate electrical energy. B. Incineration of waste to generate electrical energy. C. Increased use of wind turbines to generate electrical energy. D. Carbon dioxide capture and storage at the power station.

*B. Incineration of waste to generate electrical energy.*

Carbon dioxide is a greenhouse gas because A. It absorbs infrared light radiated by the Sun. B. It absorbs ultraviolet light radiated by the Earth. C. Its natural frequency molecular oscillation lies in the infrared region. D. Its natural frequency molecular oscillation lies in the ultraviolet region.

*C. Its natural frequency molecular oscillation lies in the infrared region.*

Sankey diagram of power plant

*Primary energy:* Chemical or nuclear energy *Useful/ un-useful energy:* 30% useful electricity while 70% is lost energy *Energy lost:* 1. Exhaust gas (15%) 2. Waste energy (50%) 3. Friction (5%)

Carbon dioxide is a greenhouse gas. State *one* source and *one* sink of this gas

*Source:* Emissions from volcanoes/ burning of fossil fuels in power plants/cars/breathing. *Sink:* Oceans/ rivers/lakes/seas/ trees

Convection

*The mode of transfer f heat energy by the actual movement of the particles.* Requires a medium of heat transfer. - As it rises it cools, and so becomes denser and sinks. (hence obtains a cycle called a convection current.)

Thermal radiation

*The mode of transfer of heat energy by electromagnetic waves (do not require a medium in between)* Only thermal radiation transfers heat without a physical medium such as solid, liquid or gas.

Conduction + equation to memorize

*The mode of transfer of heat energy without the actual movement of particles* When two solids of different temperatures touch, thermal energy are transferred from the hotter to the cooler object. (Vibrations of atoms from one to another.) *∆Q/ ∆t = kA∆t/d*

Benefits of hydroelectric power

1. A clean fuel source 2. Won't pollute 3. Renewable 4. Cost-competitive 5. Reliable 6. Base-load power 7. Flood control 8. Water supply

Which of the following has the lowest albedo and which has the highest? 1. A polar ice cap 2. Desert 3. Ocean 4. White cliffs

1. A polar ice cap has the highest 3. Ocean lowest

Describing fossil fuel power stations (Coal-burning power plant)

1. Chemical energy in coal is released by burning. 2. heat boils water and makes heat energy. 3. Steam produces kinetic energy and turns the turbine. 4. The turbine turns a coil of wire in a magnetic field. 5. Electric power is produced.

A nuclear reactor requires both control rods and a moderator to operate. Outline, with reference to neutrons, one similarity and two differences in the function of each of these components.

1. Control rods absorb neutrons. 2. Moderates slow down neutrons. 3. Both affect the rate of reaction. 4. Both rely on the neutrons colliding with their nuclei.

State one reason why the power of solar radiation in any particular region does not have a constant value?

1. Could cover variation 2. Season

Concerns with hydroelectric power

1. Dam failures can be catastrophic 2. Daming a river can change wildlife habitats. 3. Not completely free of greenhouse gas emissions - released during construction.

Explain why less energy can actually be generated by the wind generators than the value deduced...

1. Energy is los to thermal energy due to friction in the turbine. 2. Turbine will suffer downtime when no wind is present.

Explain why energy is wasted in the power station

1. Energy wasted due to inefficient burning of oil. 2. Heat energy lost to surroundings/ environment. 3. Energy is lost due to frictional losses in the turbine/generator. 4. Electrical energy is used to run the power station's systems.

Describing wind generators

1. Heated land air becomes less dense, and rises. 2. Cooler air then fills the low pressure left behind. 3. A convection current forms. 4. Wind turbines can use the wind to make electricity.

Explain how the energy is lost in the transmission (power station)

1. Heating of wires by electric current. 2. Inefficient transformers.

Distinguish, with reference to molecular motion and energy, between solid ice and liquid water.

1. In water, vibration and translation of molecules possible, in ice only vibration. 2. In liquid there is sufficient energy to break and re-form inter-molecular bonds.

The details of nuclear fission in nuclear power stations

1. Nuclear fission is the splitting of a large nucleus into two smaller (daughter) nuclei. 2. U-235 is hit by a neutron, and capturing it becomes excited and unstable. 3. It quickly splits into two smaller daughter nuclei, and two neutrons, each of which can split another nucleus of U-235.

Steps of a nuclear power plant

1. Nuclear reactor creates heat that is used to make steam. 2. The steam turns a turbine connected to an electromagnet, called a generator. 3. The generator produces electricity.

Describe briefly how the energy produced by the nuclear reaction is extracted from the reactor pile and then transferred to electrical energy

1. Reactor coolant from the pile is circulated through a heat exchanger. 2. The heat exchanger heats up water to boiling to run a steam turbine. 3. The steam turbine turns a generator to produce electricity.

Describing origin of fossil fuels

1. Sun makes biomass through photosynthesis 2. Biomass gathers and grows over time 3. Biomass burried under great pressure 4. Biomass becomes oil, coal, and natural gas over time. 5. Coal, oil, and natural gas are extractred. 6. Coal, oil. and natural gas are used as fuels.

Sun-driven hydroelectric systems

1. Sun-driven evaporation and rainfall place water at high potential energy. 2. During times of less energy demand, excess power plant electricity can be used to pump water back up into the reservoir for later use. 3. This is called the *pumped storage scheme*

Absorption of infrared radiation by greenhouse gases

1. The exciting gases will eventually de-excite and release photons. 2. The absorption and release of photons by the gases is called *scattering* 3. Scattering does NOT produce a new increase in heat energy in the atmosphere. However, some of the scattered photons may be in the infrared region - able to be absorbed as internal energy and kinetic energy.

What two trends are observed when a black-body is heated to incandescence?

1. The higher the temperature the greater the intensity. 2. The higher the temperature the smaller the wavelength of the maximum intensity.

Forms of tidal power

1. Tidal barrages (DAM - most efficient) 2. Tidal fences 3. Tidal turbines

greenhouse effect

1. When solar radiation strikes a planet that has a gaseous atmosphere, the gases comprising the atmosphere can absorb infrared radiation (heat). 2. The remainder of the incoming radiation then reaches the ground to wither be scattered back into the atmosphere, or absored. 3. Solids can absorb all frequencies of radiation, and conver them ti infrared wavelengths. 4. The heated ground can them emit infrared radiation back into the atmosphere, which then intercepts more of the energy on the way out. 5. The result is that the atmosphere traps heat and causes the temperature of the plane to rise.

Describing solar energy - heating panels

1. the heating panels converts sunlight directly into heat. 2. The slower the water is circulated, the hotter it can get.

Sankey diagram

A diagram that represents energy transfers by arrows. The width of each arrow depends on the amount of energy it represents.

The greenhouse effect can be explained by the fact that infrared radiation emitted by the surface of the Earth: A. Is absorbed by the atmosphere and then re-radiated in all directions. B. Raises the temperature of the upper atmosphere. C. Is trapped by the upper atmosphere. D. Is absorbed by the atmosphere and then all of it is re-radiated back to the surface of Earth.

A. Is absorbed by the atmosphere and then re-radiated in all directions.

Explain the function of control rods

Absorb neutrons. The control rods absorb neutrons produced during fission. This allows a reactor to just maintain its reaction rate at the self-sustaining level, rather than becoming a dangerous chain reaction.

A black-body object

Absorbs all wavelengths. As it heats up it emits all wavelengths, called black body radiation.

State two advantages and two disadvantages of using wind turbines to replace fossil-fuel generating stations.

Adv. 1. Wind power does not produce greenhouse gases. 2. Wind power is a renewable resource Dis. 1. Wind depends on the weather 2. A lot of wind turbines need to be used to replace a fossil-fuel generating station.

Solar power advantages and disadvantages

Adv. 1. Would be possible to power the entire world with these. 2. Betting better, and cheaper 3. Renewable 4. No greenhouse emissions. 5. Low maintenance costs Dis. 1. Unevenly distributed across the planet 2. Inconsistent (cloudy days) 3. Efficiency of the cell itself (reflections)

Nuclear advantages and disadvantages

Adv. 1. no air pollution 2. cheap to operate 3. little depletion of resources Dis. 1. nuclear waste 2. risk of accident 3. expensive to build

Fossil Fuels advantages and disadvantages

Adv. 1. no nuclear waste 2. less risk (than nuclear) 3. cheaper to build (than nuclear) Dis. 1. air pollution 2. expensive to build 3. depletes resources

Which of the following describes the role of the atmosphere in the greenhouse effect? A. The atmosphere is transparent to all solar radiation. B. The atmosphere absorbs infrared radiation from the ground. C. The atmosphere scatters red light more than blue light. D. Clouds in the atmosphere prevent absorption of infrared radiation.

B. The atmosphere absorbs infrared radiation from the ground.

An active solar heater (solar panel) is installed on the roof of a building in the southern hemisphere. Suggest why the panel should be facing northwards at noon.

Because of the tilt of Earth's axis southern exposures get more sun in the northern hemisphere, and northern exposures get more sun in the southern hemisphere.

Tital power benefits and concerns

Benefits: 1. Easy to install 2. Renewable 3. No direct greenhouse gas emissions 4. Low environmental impact 5. Very predictable energy source Concerns: 1. Can only be installed along coastlines. 2. Very specific site requirements 3. Tide cycles do not always match the consumption patterns

List 5 types of renewable energy sources

Biomass, Solar, Hydropower, Geothermal and wind

The greenhouse gases

CH4, H2O, CO2, N2O

Explain why countries are being asked to decrease their dependence on fossil fuels

CO2 is linked to greenhouse gas (greenhouse effect) which enhances global warming. (Climate change) *The release of CO2 into the atmosphere contributes and enhances effects of global warming*

What is the correct order of energy transformations in a coal power station?

Chemical - thermal - kinetic - electrical

Basic power plant

Chemical energy (nuclear) - Heat energy (boiler) - Steam w/ kinetic - Turbine (kinetic) - ELECTRICITY

Role of control rods in a nuclear reactor

Control rods are made of cadmium or boron steel. Retracting the control rods will increase the reaction rate. Inserting the control rods will decrease the reaction rate. Maintain a constant rate of fission.

Greenhouse gases: A. Reflect infrared radiation but absorb ultraviolet radiation. B. Reflect ultraviolet radiation but absorb infrared radiation. C. Transmit infrared radiation but absorb ultraviolet radiation. D. Transmit ultraviolet radiation but absorb ultraviolet infrared radiation.

D. Transmit ultraviolet radiation but absorb ultraviolet infrared radiation.

Describe the SANKEY diagram for a photovoltaic cell

Energy in incident sunlight = useable electricity + wasted heat

Describe the SANKEY diagram for sun- or tide-driven power

Energy in sunlight /tide = potential energy in reservoir + evaporation from reservoir = Kinetic energy of turbine + friction = electricity + friction

Describe the SANKEY diagram for nuclear power stations

Energy in uranium ore = energy used in processing fuel + energy in enriched uranium = electrical + energy remaining in spent fuel + heat loss.

Fuels

Energy sources that use chemical or nuclear reactions to provide thermal energy or mechanical power.

State the name given to this type of nuclear reaction. U-235 + neutron - Ba-144 + Kr-90 + 2 neutrons

Fission reaction

Energy is liberated in a fission reaction. In what form does this energy appear?

Heat and some kinetic energy of the product neutrons.

Define Intensity

Intensity is the rate at which energy is being gained per unit area.

Primary energy sources

Is a source which occurs naturally and has not been converted in any way. Energy contained in natural resources (solar radiation, coal, oil, sunlight, wind, uranium and crude oil)

Renewable energy source

Is one which is being continuously replaced (over a relatively short timescale) by natural processes.

Describe in terms of energy transfer, how thermal energy of the burning gas becomes electrical energy.

KE of the steam turns into KE of turbine which transfers to KE of generators to produce electrical energy.

Describe how the neutrons in the fission reaction may initiate a chain reaction.

Looking at the fission reaction we see that one neutron initiates one fission... but each reaction produces two neutrons. This the fission process can produce a self-sustaining chain reaction.

Describe how the neutrons produced in this reaction may initiate a chain reaction

Looking at the reaction we see that one neutron initiates one fission... but each reaction produces two neutrons. This the fission process can produce a self-sustaining chain reaction.

Suggest why the incoming solar radiation is not affected by the mechanism by which part of the radiation radiated by the Earth's surface is absorbed by greenhouse gases in the atmosphere.

Most incoming radiation consists of photons in the visible region/ultraviolet region and so have much shorter wavelengths than those radiated by the Earth and so these cannot be absorbed.

Suggest the conditions that would make use of wind generators in combination with either oil or nuclear fuel suitable for the islanders.

Needs to be windy and have space for wind turbines. It need to be able to import oil or nuclear fuel, and potentially dispose of nuclear waste.

What does it mean that all bodies radiate?

No body is at absolute zero (K = 0)

What is degraded energy?

No longer available for the performance of useful work

List 4 types of non-renewable energy sources

Nuclear, Oil, Coal and gas

Why do ocean current generate more energy than air currents?

Ocean water is 832 times more dense than air and therefore applies greater force on the turbines

Outline, with reference to energy changes, the operation of a pumped storage hydroelectric system.

PE of water is converted to KE of moving water/turbine to electrical energy «in generator/turbine/dynamo» idea of pumped storage, ie: pump water back during night/when energy cheap to buy/when energy not in demand/when there is a surplus of energy

Explain why it is necessary to slow down the neutrons

Product neutrons are too fast to cause fissioning. The moderator slows them down to the right speed.

Outline the mechanism by which geenhouse gases contribute to global warming.

Radiation emitted by the Earth in infrared region. Frequency corresponds to the resonant frequency of greenhouse gases. Radiation absorbed by greenhouse gases is re-radiated back to Earth.

Describe briefly how the energy produced by the nuclear reactions is extracted from the reactor pile and then transferred to electrical energy.

Reactor coolant from the pile is circulated though a heat exchanger. The heat exchanger heats up water to boiling to run a stream turbine. The stream turbines turns a generator to produce electricity.

What is the phenomenon that best explains why greenhouse gases absorb infrared radiation?

Resonance

Role of moderators (nuclear)

Slows down neutrons so that they are not absorbed by u-238. Moderators such as graphite, light water and heavy water slow down these fast neutrons to about 0.02 eV so that they can contribute to the fission process.

Wein's Displacement Law

Tells uses the wavelength of the maximum intensity (λmax) for black bodies at temperature T in Kelvin.

Outline a mechanism by which part of the radiation radiated by the Earth's surface is absorbed by greenhouse gases in the atmosphere.

The Earth radiates radiation in the infrared region of the spectrum. The greenhouse gases have energy level differences corresponding to infrared energies and so the infrared photons are absorbed. OR The earth radiates photons of infrared frequency. The greenhouse gas molecules vibrate with frequencies on the region and so because of resonance the photons are absorbed.

Stefan-Boltzmann Law

The Stefan Boltzmann law shows the relationship between the temperature of a black-body and the power emitted by the black body's surface area. *A black-body emits as much power as it absorbs and therefore the law works for both emission and absorption problems.*

Describe heat capacity of land

The land has a low heat capacity and therefore heats and cools quickly.

Outline why fuel enrichment is needed.

The percentage of U-235 in naturally occurring ores is too low to support fission or naturally, occurring U-238 does not undergo fission. Percentage of U-235 is increased.

Describe photovoltaic cell

The photovoltaic cell converts sunlight directly into electricity. The cell is made of crystalline silicon (a semiconductor) doped with phosphorus and boron impurities. Silicon is very stable (4 electrons in outer shell)

Nuclear energy

The potential energy stored in the nucleus of an atom

Explain why the solar intensity is different for different parts of the earth. State another reason for variation?

The same intensity is spread out over more areas the higher the latitude. Intensity also varies with the seasons, due to the tilt of the Earth

Suggest why the average power received per unit area of the earth is 350 Wm-2 if the intensity of the Sun's radiation at the position of the Earth is approximately 1400 Wm-2?

The solar radiation is captured by a disc of area πr^2 where R is the radius of the earth. but is distributed over the entire Earth's surface which has an area four times as large.

Electricity

The usefulness and versatility of electricity cannot be underestimated. Not only can electricity be converted to or from most other forms of energy relatively easily, it can be transferred around the country by cables with minimal energy dissipation. A form of energy caused by the movement of electrons. Electrons - magnetic field - electromotive force (emf) - electrical current

What is true about non-renewable fuels?

Their rate of consumption is much greater than the rate at which they are being produced.

Thermal energy

Thermal energy can be transferred from a warmer mass to a cooler mass by three means; conduction, convection, and radiation

Describing hydroelectric systems

Two groups: sun-derived (sun-driven potential energy) & moon-derived (tidal-driven potential energy)

Which fuel has the highest energy density?

Uranium (not coal, gas or oil)

State the three ways CO2 can store kinetic energy

Via vibration, translation and rotation. *Vibration:* refers to the molecules oscillating in conjunction with the springs. The *Natural frequency of greenhouse gases is in the infrared region of the spectrum and thus greenhouse gases are prone to absorb such frequencies.* *Translation:* refers to the molecule moving as a unit in a straight line. *Rotation:* refers to the molecule spinning about its center of mass.

Describe heat capacity of water

Water has a high heat capacity and therefore heats and cools slowly.

Critical mass (nuclear fission)

We call the minimum mass of a fissionable material which will sustain the fission process by itself *critical mass.* * U-238 is not even on the list. * We must enrich naturally-occurring uranium for reactor usage.

Energy density

a measure of the energy a food provides relative to the weight of the food. *Energy/Volume* = Joules/area = J/m^3

Emissivity

e of a body is a number between 0 and 1 that quantifies the emission and absorption properties of that body as compared to a blackbody of equal size. *it is the ratio of the power emitted by the body, to the power emitted by a black-body of the same size.*

Specific energy

energy released from fuel/ mass of fuel consumed. *Energy/Mass* = Joules/Kilogram = J/Kg^1

Not all the stored energy can be retrieved because of energy losses in the system. Explain one such loss.

friction/resistive losses in walls of pipe/air resistance/turbulence/turbine and generator bearings thermal energy losses, in electrical resistance of components water requires kinetic energy to leave system so not all can be transferred

Secondary energy sources

is one that has been converted from a primary source. Is an energy source, such as electricity or hydrogen, which has been transformed from a primary energy source before use by the consumer.

Describe the difference between photovoltaic cells and solar heating panels.

solar heating panel converts solar/radiation/photon/light energy into thermal energy AND photovoltaic cell converts solar/radiation/photon/light energy into electrical energy

The average surface temperature of the Earth is actually 288 K. Suggest how the greenhouse effect helps explain the difference between the temperature estimated in (c) and the actual temperature of the Earth.

the emitted radiation is in the infrared/IR/long wavelength/low frequency region «greenhouse» gases in the atmosphere absorb «infrared» radiation radiated in all directions «including back down to Earth» warming the Earth

After the upper lake is emptied it must be refilled with water from the lower lake and this requires energy. Suggest how the operators of this storage system can still make a profit.

water is pumped back up at times when the demand for/price of electricity is low

Explain two reasons why the number of turbines required is likely to be greater than your answer to (c)(i).

«efficiency is less than 100% as» not all KE of air can be converted to KE of blades OR air needs to retain KE to escape thermal energy is lost due to friction in turbine/dynamo/generator Allow velocity of air after turbine is not zero.