IB SL Physics Topic 8

Explain why some of the radiation by the ocean is returned to the oceans from the atmosphere.

Explain why some of the radiation by the ocean is returned to the oceans from the atmosphere.

• most of the radiation emitted by the oceans is infrared;

• radiation is absorbed by greenhouse gases in the atmosphere;

• gases re-radiate/re-emit;

• partly back towards oceans/in all directions/awareness that radiation in other directions is also present;

Discuss how the frequency of the radiation emitted by a black body can be used to estimate the temperature of the body.

• the (peak) wavelength of emitted em waves depends on temperature of emitter/reference to Wein’s Law; so frequency/color depends on temperature;

Explain two sources of energy loss in a power station. (2)

• friction/resistive losses in pipe/fluid resistance/turbulence/turbine or generator «bearings»

• sound energy losses from turbine/water in pipes;

• thermal energy/heat losses in wires/components;

• water requires kinetic energy to leave the system so not all can be transferred;

Outline why greenhouse gases result in an increase in the temperature of the Earth’s surface.

• radiation is emitted by Earth in the infrared region;

• frequency corresponds to the resonance frequency of greenhouse gases;

• atmosphere absorbs more (IR) energy and heats up;

• radiation absorbed by the atmosphere is re-radiated in all directions;

• Earth’s surface absorbs (IR) radiation hence temperature increases;

Outline, with reference to the vibrational behaviour of molecules of carbon dioxide, what is meant by a greenhouse gas. (3)

• frequency of vibration is close to that of the frequency of infrared radiation;

• (atmospheric) carbon dioxide absorbs the infrared radiated by the surface of Earth;

• the part of the radiation that is re-radiated back to Earth will cause the temperature of the surface to rise / re-radiated at a different frequency

Outline the difference between renewable and non-renewable energy sources. {2014}

Alt 1

• renewable sources: rate of use/depletion of energy source; is less than rate of production/regeneration of source;

{Accept equivalent statement for non-renewable sources.}

Alt 2

• mention of rate of production / usage; comparison of sources in terms of being used up/depleted/lasting a long time etc;

{Award [1] if answer makes clear the difference but does not address the rate of production.}

Some waste products in nuclear reactors are good absorbers of neutrons. Suggest why the formation of such waste products requires the removal of the uranium fuel rods well before the uranium is completely used up.

• proportion of waste builds up in fuel rod as uranium is consumed;

• increasing numbers of neutrons will be absorbed;

• this reduces the number available to sustain the chain reaction;

• build up of waste deforms fuel rod (which can then be difficult to remove);

Outline why uranium ore needs to be enriched before it can be used successfully in a nuclear reactor.

• U-238 is much more common than U-235 in ore;

• U-235 is more likely to undergo fission / critical amount of U-235 required to ensure fission;

• U-238 absorbs neutrons;

• U-238 reduces reaction rate in reactor;

State two reasons why it is impossible to convert all the available wind energy to electrical energy.

• the speed of the air/wind cannot drop to zero;

• wind turbulence / frictional losses in turbine/any moving part / resistive heating in wires;

State an appropriate domestic use for a solar heating panel and a photovoltaic cell.

• solar heating panel: water heater / any specific use such as swimming pool/bath;

• photovoltaic cell: powering TV/radio/lighting/any domestic electrical appliance;

Outline the role of the moderator. {2010}

• reduces neutron speed to (thermal) lower speeds;

• so that chance of initiating fission is higher;

Describe how and why some reactors are used to produce plutonium-239

• U-238 is present;

• U-238 captures neutron (to produce plutonium);

• plutonium is also fissionable/can be used as fuel for (fast breeder) reactor;

State three possible causes of global warming. {2009}

• enhanced greenhouse effect

• increase in solar (flare) activity;

• Earth orbits closer to Sun;

• volcanic activity increases global warming (through CO2 emission)/decreases global warming (particles emitted reflect sunlight);

State two examples of fossil fuels. (2){2009}

• peat;

• coal;

• oil;

• gas;

{Do not accept derived fuels, e.g. diesel.}

Explain why fossil fuels are said to be non-renewable. (2) {2009}

• rate of production of fuel much smaller than rate of usage;

• so fuel will be exhausted/run out;

Explain how, in a reactor, the production of energy in a chain reaction is controlled {2008}

• use of control rods to absorb excess neutrons;

• some further detail e.g. lowering or raising control rods to control rate of absorption;

In a nuclear reactor: {2008}

(a) Explain why it is necessary to slow down the neutrons.

(b) Energy is liberated in the fission of uranium.

(i) State the form of the energy produced in the fission reaction.

(ii) Identify the energy transfers by which the energy in (b)(i) passes to the coolant.

(a) • when neutrons collide with nuclei of U-235 they can undergo fission;

• low energy/slow neutrons are much more likely to produce fission;

• so maintaining the nuclear fission;

(b) (i) • kinetic (energy);

(ii) • thermal (energy) in pile/moderator;

• thermal energy in coolant;

Distinguish specific energy and energy density. (1)

• specific energy: the energy that can be liberated / stored per kg / per unit mass;

• energy density: the energy that can be liberated / stored per m³ / per unit volume;

State two reasons advantages and disadvantages of using fossil fuels.

advantages

• wide availability / a great deal available (in short term);

• ease of transportation;

• power stations can be built close to the source;

• high energy density;

• cheaper production of electrical energy;

disadvantages

• non-renewable fuel source;

• greenhouse gas emissions;

State two advantages and disadvantages of using nuclear power.

advantages

• no greenhouse gases are emitted / does not add to the greenhouse effect/global warming;

• high energy density / small mass is required;

• fuel source will last longer

disadvantages

• waste products are radioactive / no safe method of disposal / long half-life;

• power plant is more expensive;

• exposure to radiation for miners and workers;

• nuclear weapon production;

• radiation leaks / meltdown risks;

• radioactive waste processing/handling/storage;

{do not accept “lasts a long time” or “explosion”}

State two advantages and disadvantages of the use of wind power. {2006}

advantages

• renewable energy source;

• no carbon dioxide emissions;

{Do not allow “pollution free” / cost. Award [1] each for any two sensible suggestions}

disadvantages

• a large number of turbines required covering a large area of land;

• output dependent on wind speed hence unreliable;

• change in the local climate as a result of turbulence;

{Award [1] each for any two sensible suggestions and [1] for each explanation.}

State two advantages and disadvantages of using solar panels. {2002}

advantages

• renewable energy source;

• no carbon dioxide emissions;

disadvantages

• not much use if the Sun is not shining;

• large area needed for reasonable power output;

A nuclear power station uses uranium-235 (U-235) as fuel. Outline the:

(i) processes and energy changes that occur through which thermal energy is produced. (4)

(ii) role of the heat exchanger of the reactor and the turbine in the generation of electrical energy. (3)

(i) • U-235 nuclei undergo fission;

• products have a high energy/are fast-moving

• nuclei transfer kinetic energy to (reactor) core/neutrons transfer kinetic energy to moderator;

• core/moderator energy transferred to coolant;

(ii) • heat exchanger allows the transfer of thermal energy between reactor and coolant;

• coolant transfers (thermal) energy to steam/other named fluid;

• steam/fluid allows the turbine to drive a generator to create electrical energy;

Outline how electrical energy is obtained from the energy of wind.

• kinetic energy of wind transferred to (rotational) kinetic energy of turbine/blades;

• kinetic energy changed to electrical energy in generator;

Outline the operation of a pumped storage hydroelectric system.

• gravitational potential energy of water is converted to kinetic energy of moving water/turbine to electrical energy in generator;

• 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);

Describe the difference between photovoltaic cells and solar heating panels in terms of energy transfer and operation.

• solar heating panels convert solar/radiation/photon/light energy into thermal energy and photovoltaic cells convert solar/radiation/photon/light energy into electrical energy;

• solar heating cells store/circulate hot liquid while photovoltaic cells generate emf/potential difference

Outline a mechanism by which part of the radiation radiated by the Earth’s surface is absorbed by greenhouse gases in the atmosphere. (3)

Alt 1:

• the Earth radiates radiation in the infrared region of the spectrum;

• the greenhouse gases have energy level differences (in their molecular energy levels) corresponding to infrared energies;

• and so the infrared photons are absorbed;

Alt 2:

• the Earth radiates photons of infrared frequency;

• the greenhouse gas molecules oscillate/vibrate with frequencies in the infraredregion;

• and so because of resonance the photons are absorbed;

Suggest why the incoming solar radiation is not absorbed by greenhouse gases. (2)

• most incoming radiation consists of photons in the visible/ ultraviolet region / photons of much shorter wavelength than those radiated by the Earth / photons of different wavelength of that radiated by Earth;

• and so these cannot be absorbed;