nuclear/space/atp

Describe the structure of an atom

Positively charged nucleus; Negatively charged electrons orbit around nucleus

How are positive ions formed?

By an atom losing electrons

How are negative ions formed?

By an atom gaining electrons

What is an alpha (⍺) particle?

Helium nucleus (2 protons, 2 neutrons); Positively charged ion

Explain the results of firing alpha particles at a sheet of thin metal

Alpha particle is a helium nucleus (2 protons, 2 neutrons → overall positive charge); Most pass straight through metal → an atom is largely empty space; Some were deflected a little → nucleus is very small; Very few were deflected a lot → nucleus is positively charged (like charges repel) and contains most of atom’s mass

Where in an atom are protons and neutrons found?

Nucleus

Where are electrons found?

In shells around nucleus

Give the relative mass and charge of an electron, a proton and a neutron

Proton: relative mass 1, relative charge +1; Neutron: relative mass 1, relative charge 0; Electron: relative mass 1/1840, relative charge -1

Define atomic (proton) number (Z)

Number of protons in an atom; Also equal to the number of electrons; Proton number gives relative charge of a nucleus

Define mass (or nucleon) number (A)

Number of protons + number of neutrons in the nucleus of an atom; Nucleon number gives relative mass of a nucleus

How is the number of neutrons in an atom calculated?

Mass number minus atomic number (A - Z)

What is an isotope?

An atom of the same element with the same number of protons but different number of neutrons; Elements may have more than one isotope

What is nuclear fission?

Splitting of atomic nuclei

Where is nuclear fission carried out?

In a nuclear generator

How may a chain reaction be set up in a nuclear reactor?

Fast moving neutron collides with uranium nuclei → Nuclei split forming 2 radioactive daughter nuclei and 2-3 neutrons → Neutrons hit other uranium nuclei → Chain reaction set up

Describe what is meant by a chain reaction

Fission releases neutrons; Neutrons can be captured by other uranium nuclei; These nuclei then undergo fission

What are the products of each reaction during nuclear fission?

2 radioactive daughter nuclei; Small number of neutrons

State the type of energy released in a fission reaction

Kinetic energy

How does nuclear fission lead to the production of electricity?

Thermal energy released used to heat water to create steam → Steam turns a turbine → Turbine turns a generator

What is nuclear fusion?

Collision of atomic nuclei at high speed; Creates larger nuclei, resulting in the loss of mass from small nuclei (i.e. mass before > mass after); Releases energy

Why does nuclear fusion not occur at low temperatures?

Nuclear fusion involves 2 nuclei joining; Nuclei are positively charged → electrostatic repulsion between like charges; Needs extremely high temperature to overcome the repulsion; These temperatures are only currently found in stars

What is background radiation?

Radiation which is always present; Received by detectors all the time

What device is used to measure the amount of ionising radiation (radioactivity)?

Geiger-Muller detector connected to a counter

Give some sources of background radiation

Radon gas in air; Granite rocks in ground and buildings; Some food and drink (e.g. coffee, bananas); Cosmic rays from the Sun

Describe the procedure to measure background radiation in order to correct a count measurement

Remove the radioactive source → Measure background count → Repeat the measurement and calculate average → Scale background count to same time as count measurement → Subtract background count from the non-corrected measurement

What is the unit for measuring radioactivity?

Becquerel (Bq) (Note: Some questions may refer to counts per second instead)

Describe the emission of radiation from a nucleus

Spontaneous; Random in direction

What is a beta (β-) particle?

Fast moving electron

What is a gamma (γ) ray?

Electromagnetic wave

How are alpha, beta, and gamma radiation stopped?

Alpha: few centimetres of air, paper; Beta: 10cm air, aluminium foil; Gamma: several inches thick lead, metres of concrete

List alpha, beta and gamma radiation in order, starting with the most penetrating

Gamma > beta > alpha

Define ionising

Ability to remove an electron from an atom

List alpha, beta and gamma radiation in order, starting with the most ionising

Alpha > beta > gamma

Why is it relatively safe to be in the same room as an alpha-emitting substance?

Alpha radiation cannot penetrate skin

Explain why alpha has a stronger ionising effect than gamma

Alpha is charged, gamma is not (greater charge increases likelihood of ionisation); Alpha is large and has mass, gamma has no size or mass; Alpha has greater kinetic energy so makes more frequent, successful collisions; Gamma makes few collisions so not very ionising

Describe the deflection of alpha particles in electric and magnetic fields

Electric field: attracted to negatively charged plate because alpha particles are positively charged; Magnetic field: deflected in curved path as alpha particles are charged (direction worked out using Fleming’s left hand rule)

Describe the deflection of beta particles in electric and magnetic fields

Electric field: attracted to positively charged plate because beta particles are negatively charged; Magnetic field: deflected in curved path as beta particles are charged (direction worked out using Fleming’s left hand rule, deflected in opposite direction to alpha particles, deflected more than alpha particles due to very small mass)

Describe the deflection of gamma rays in electric and magnetic fields

Electric field: no deflection because gamma rays are not charged; Magnetic field: no deflection because gamma rays are not charged

Define radioactive decay

Change in unstable nucleus resulting in emission of ⍺-particles or β-particles and/or γ-radiation; Emission of ⍺-particles or β-particles changes nucleus to that of another element; Spontaneous and random process

What happens when a nucleus emits an alpha particle?

Alpha is a helium nucleus (2 protons and 2 neutrons are emitted); Mass number decreases by 4, atomic number (proton number) decreases by 2

What happens when a nucleus emits a beta particle?

A neutron turns into a proton and electron; Proton stays in the nucleus, fast moving electron (β- particle) emitted; Mass number unchanged, atomic number (proton number) increases by 1

What happens when a radioactive element emits a gamma ray?

High energy electromagnetic wave released from atom; No change to mass number or proton number

What causes isotopes of an element to be radioactive?

Excess of neutrons in the nucleus and/or the nucleus being too heavy

What happens to the stability of a nucleus after radioactive decay has occurred?

Stability increases because number of excess neutrons decreased

Define half life

Time taken for half the radioactive nuclei to decay

Describe the nature of radioactive decay

Random process; Different for different radioactive isotopes

Why is tossing a coin a good model for radioactive decay?

Random; Unable to predict whether you will land on a head or tail — with radioactive decay you don’t know when the nuclei will breakdown (decay)

What are the limitations of tossing a coin as a model for radioactive decay?

You can only toss the coin at most 1000 times really; In radioactive material there are millions of nuclei that have the potential to decay

Describe how alpha radiation is used in smoke alarms

Alpha radiation ionises the air creating small current picked up by a detector; In a fire, alpha particles are stopped by smoke; Current stops, alarm sounds

Explain why gamma radiation is used to irradiate food and sterilise medical equipment

Gamma is most penetrating type of radiation; Food can be irradiated to kill bacteria; Can be done once food is already in packaging; Can irradiate all sides of medical instruments without removing from packaging

How can radiation be used to measure and control thicknesses of materials?

Radiation emitter and detector placed either side of material; As material moves along, particles that penetrate it are detected; If material gets thicker → more particles absorbed → detection count drops; If material gets thinner → fewer particles absorbed → detection count increases; Machine can make adjustments to keep thickness of material constant

What type of radiation is used to measure and control thicknesses of materials?

Mostly commonly beta particles (more penetrating and less ionising than alpha particles); If material is very thin, alpha particles may be used (material may not be thick enough to absorb beta particles); If material is very thick, gamma radiation may be used (gamma rays are most penetrating)

Describe the properties of radioactive medical tracers

Short half-life (long enough to complete the procedure, but falls to safe level quickly); Usually emits gamma rays (gamma rays can penetrate out of the body and be detected)

Why can gamma rays be used to treat cancer?

High energy electromagnetic wave; Can penetrate body and kill living cells

What are the dangers of exposure to ionising nuclear radiation?

Cell death; Mutations; Cancer

Discuss precautions that must be taken when disposing of radioactive waste with a long half-life

Must use strong, lead-lined containers that can’t rust (could lead to contamination); Minimise amount of time spent handling sources; Radioactive waste can potentially be diluted in sea water, but must not leak into water table; Bury underground to minimise risk of contamination in a secure place not at risk from natural disasters

How can you reduce your risk of radiation?

Limit exposure time; Shielding e.g. lead, thick layer of concrete; Using tongs to handle radiation

How should radioactive sources be stored?

In shielded (e.g. lead-lined) containers; Lead absorbs most radiation

How should radioactive materials be moved?

Using tongs; Wearing protective gloves; Increases distance between living tissues and radioactive source

What is the Earth?

A planet

What is a day?

Time taken for Earth to rotate once on its axis; Takes approximately 24 hours for one rotation

Explain what causes day and night

Earth rotates on its tilted axis; Part of Earth facing the Sun → day; Part of Earth facing away from the Sun → night; As Earth rotates, Sun appears to rise and set (rises in East, sets in West, appears highest in sky at midday)

What is a year?

Time taken for the Earth to complete one orbit of the Sun

Explain the periodic nature of the seasons

Axis of Earth is tilted; As Earth orbits the Sun, either the northern or southern hemisphere will be tilted towards Sun; When northern hemisphere is tilted towards Sun → summer in northern hemisphere (warmer, with longer daylight hours); When northern hemisphere is tilted away from Sun → winter in northern hemisphere (colder, with shorter daylight hours)

Where is the asteroid belt located?

Between Mars and Jupiter

What is a satellite?

An object which orbits a planet; Can be either natural (moon) or artificial (man-made)

What is a comet?

Ball of ice and dust that orbits the sun

Compare the orbits of comets, planets and moons

Comets have very elliptical orbits, with a star at one focus; Planets have slightly elliptical orbits around a star (star not precise centre of orbit unless orbit is approximately circular); Moons have circular orbits around a planet

State the differences between an artificial satellite’s orbit and a planet’s orbit

Satellite orbits a planet, planet orbits a star; Orbital radius of planet is greater than satellite

Where is a comet’s kinetic energy greatest?

When it is nearest the star (gravitational force strongest)

Suggest why planets nearer to a star take less time to orbit the star

Smaller orbital path for close planets; Larger speed for close planets

State the similarities between an artificial satellite’s orbit and a moon’s orbit

Both orbit a planet; Both have same shape of orbit

Describe the structure of the four planets nearest the Sun

Rocky and small

Describe the structure of the four planets furthest from the Sun

Gaseous and large

What is a nebula?

Interstellar cloud of dust and gases (mostly hydrogen and helium, small percentage of heavier atoms e.g. carbon, magnesium, nitrogen)

Outline the accretion model for Solar System formation

Solar System formed 4.5 billion years ago; A nebula began to contract, collapsing in on itself under force of its own gravity; The atoms collided together, generating heat; Temperature became high enough for nuclear fusion to occur, forming the Sun; Material in nebula not absorbed into the Sun swirled around into flat disk of dust and gas (accretion disk); Accretion disk held in orbit by the Sun’s gravity

How does the accretion model explain the formation of the planets?

Each planet began as microscopic grains of dust in the accretion disk; Atoms and molecules began to accrete (stick together) into larger particles; As these objects grew, they were big enough to attract others by gravity; Eventually planets were formed

Why are the closest planets to the Sun smaller and rocky?

Stellar winds from the Sun blew off most of the gases from the closest planets; Left them smaller, with only rocks and metals intact

Why are the furthest planets from the Sun larger and gaseous?

Far from the Sun so stellar winds could not blow away ice and gases; Remained gaseous with only small rocky core; Primarily made of lighter elements (hydrogen and helium) to begin with; Heavier elements were closer to centre of original accretion disk due to Sun’s gravity

Why does the accretion model depend on gravity?

Caused nebula to collapse; Held accretion disc in orbit around Sun; Allowed planets to form; Means heavier elements found in planets closer to Sun

Describe the trend in surface temperature of the planets

The further from the Sun, the cooler the planet (further for infrared radiation to travel); Venus is the only exception (hotter than Mercury)

Describe the trend in orbital radius and orbital duration of the planets

The further from the Sun, the larger the orbital radius; Orbital duration increases with orbital radius

What does the strength of a gravitational field depend on?

Masses of objects (greater mass = stronger gravitational field); Distance between the objects (close object = stronger gravitational field)

Why do larger planets exert larger gravitational forces?

Larger masses (not larger size!)

Why does our weight vary on Earth and on the moon?

Gravitational field strength is weaker on the moon; Weight = mass x gravitational field strength; Mass is unchanged

What is the speed of light?

3.0 x 10^8 m/s

Where is most of the mass in the Solar System found?

The Sun

Why do the planets orbit the Sun?

The Sun has the most mass of all the objects in the Solar System; Strength of gravitational field increases with mass; The Sun has strongest gravitational field in the Solar System

Explain why gravitational force keeps objects in orbit

Gravitational force attracts objects together; Do not crash into each other as all objects are moving; Gravitational force keeps objects moving in curved paths called orbits (moons orbit planets, planets orbit the Sun, artificial satellites orbit the Earth, comets orbit the Sun)

Suggest why planets nearer to the Sun take less time to orbit the Sun

Smaller orbital path for close planets; Faster orbital speed for close planets; Due to stronger gravitational field strength being close to the Sun

Suggest why planets further away from the Sun take more time to orbit the Sun

Strength of Sun’s gravitational field decreases as distance increases; Orbital speed decreases

Why does an elliptical orbit travel faster when closer to the Sun?

Gravitational potential energy (Ep) decreases as planet gets closer to the Sun; As Ep decreases, kinetic energy (Ek) increases; According to the conservation of momentum / energy (Ek = ½mv²); So as Ek increases, so does the speed

Describe the structure of the Sun

Medium-sized star; Mostly hydrogen and helium

Which regions of the EM spectrum is most of the Sun’s energy radiated in?

Infrared, visible and ultraviolet regions

What is the source of energy for stars?

Nuclear fusion reactions; Hydrogen nuclei fuse to form helium in stable stars; Energy released

What is a galaxy?

Large collection of billions of stars

What is the name of our galaxy?

The Milky Way

What is the Sun?

A star in the Milky Way; Closest star to Earth; All other stars are much further away from Earth

What is a light-year?

Distance travelled in space by light in one year

How far is one light-year?

9.5 x 10^15 m