physics unit 7-8

unit 7 - radioactivity

geiger-marsden experiment observation and hypothesis

1. a lot of alpha particles went straight through the gold paper

most of the atom is empty, and the nucleus must be held in a small region

2. some alpha particles are deflected by small angles

3. very small majority of the alpha particles repelled back more than 90°

the nucleus must be postively charged (because + repel from +)

things that affect the amount of deflection

• proximity to the nucleus of gold atoms from gold foil

• speed - the quicker the particle is, the less likely it will deflect

• charge - a larger charge on the nucleus creates a stronger electrostatic repulsive force

activity

the number of radioactive particles emitted per second (Bq)

radioactive decay

the spontaneous process in which unstable atomic nuclei lose excess energy by emitting radiation, which helps elements turn into a stable state

alpha particles

• 2 protons, 2 neutrons (same as helium nucleus)

• charge: 2+

• high ionising power - the ability to know electrons off atoms

• low penetration - the ability to pass through materials

• stopped by paper/skin

beta particles

• charge: 1-

• medium ionising power

• medium penetration

• stopped by 1-2 cm of aluminium

beta emission

• a form of radioactive decay

• a neutron will transform into a proton, with a high-speed electron emitted from the nucleus

• atomic number increase by 1 but mass stays the same

gamma rays

• has no charge or mass

• low ionising power

• high penetration

• stopped by several inches of lead

background radiation

the constant, low-level radiation that exists in the environment around us

testing for background radiation - geiger-muller tube

1. transmit electrical pulse to the machine

2. creates a “clicking” sound when radiation is detected

3. increase frequency of clicking when close to the radioactive region

testing for background radiation - photographic film

• gets dark when detected radiation (darker if more radiation)

• contains various materials that radiation must penetrate (aluminium, copper, lead, paper, plastic)

cosmic

• fusion process in stars

• all types of radiation

• average person’s annual radiation exposure: 10%-15%

man-made

• industrial, medical, nuclear weapon

• gamma rays

• average person’s annual radiation exposure: 13%

biological mass

• all living matter

• beta radiation

• average person’s annual radiation exposure: 10%-1115%

the ground

• radioactive materials in rocks

• all radiations

• average person’s annual radiation exposure: 15%

radon gas

• rocks

• alpha

• average person’s annual radiation exposure: 50%

half-life

the time it takes for the number of nuclei in an isotope to halve

use of radioactivity

• medical tracer

• non medical or industrial

• radiotherapy

• dating rocks

• dating archaeological speciments

medical tracer

• use a radioactive substance in a meal to track its progress in the body

• gamma

• high activity (detected by gamma cameras)

• short half-life

non medical / industrial

• measures the upstream and downstream of radioactivity in factories

• gamma

• low activity (measured by geiger-muller tubea)

• half-life of a few days

radiotherapy

• kill cancerous tumors that are difficult to remove in surgeries

• the beam of the gamma ray is focused on the tumor to prevent exposure to healthy cells

dating rocks

• measure how much of the isotopes is present in the formation of rocks

dating archaeological specimens

• measure the amount of carbon-14 to find its half-life

how does half-life work?

• carbon-14 contains a small fraction of CO2 that we breathed through/photosynthesised and ate

• we grow new cells, with some decaying and replaced by new ones through breathing

• by death, the body no longer take in carbon-14 and it keeps decaying

• eg after 5730 years, half of the carbon-14 is gone, after 11460 years…

things that can be can be dated

usually organic substances (those breathed, eaten, photosynthesised):
seeds, woods, rocks, bones, teeth, paper, textile, shells

radioisotopes

unstable isotopes

• the neutron transforms into a proton and decays, spitting out radiation to become stable

• this means that a beta particle is emitted from its nucleus

contamination

• A radioactive substance resides in the object

• make the object radioactive

irradation

• a process of exposing an object to radiation

• not radioactive itself

harmful effects of radiation

• cancer: increase low level of exposure

• radiation burn: high exposure, short time on small part

• radiation sickness: high exposure, long time to whole body

nuclear fission

• the spitting of large, unstable nucleus

• the process by which a large, unstable nucleus splits into two smaller “daughter” nuclei

• the process releases lots of energy

fission process

1. a neutron is fired at the unstable nuclei

2. the nuclei aborbs the the neutron, making it even less stable

3. this causes the nuclei to split: two daughter nuclei and 2-3 extra neutrons

nuclear fusion

• the process of combining small/light nuclei (eg. hydrogen) with large/heavy nuclei (eg. helium)

• this is to form a single larger nucleus

• usually takes place in the core of stars

fusion conditions

• high temperature

• high pressure

• high kinetic energy

→ to overcome the strong electrostatic repulsion between the positive nuclei so that they can fuse

unit 8 - astrophysics

universe

a collection of billions of galaxies

galaxies

a collection of billions of stars

gravitational field strength

the attractive force towards the centre of that planet

key facts

• gravity provides the force that allows objects to orbit around others

• objects are attracted to the centre of Earth due to gravitational potential energy

• the greater the mass of the planet, the grater gravitational field strength

orbital speed formula

orbital speed = 2 x pi x orbital radius / time

what orbits what

• moon orbits planets

• planets orbits the sun

• artificial satellite orbits the earth

planet sequence

My Very Easy Method Just Speeds Up Naming Planets

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto

stellar evolution (small stars - the sun)

nebula → Main Star Sequence → red giant → white dwarf

(clouds of gas and dust) → (most of star’s life) → (the star expands and cool) → (remaining hot core)

elaborated small stars evolution

1. nebula is a large cloud of gas and dust

2. a force of gravitational attraction pulls the gas and dust together, causing the cloud to collide inward

3. as the gas collapses, the temperature and pressure inside dramatically increases due to the collision of particles, forming a protostar

4. when the core becomes hot enough, nuclear fusion occurs, where hydrogen nuclei combines with helium nuclei

5. the energy released from the nuclear fusion creates an outward pressure that balances with the inward gravitational force, turning a star into a stable state, which is the main sequence star

stellar evolution (large stars - many times bigger than the sun)

nebula → Main Sequence Star → red supergiant → supernova → black hole or neutron star

(cloud of dust) → (most of star’s life) → (stars expand and cool) → (forms the collapse of the remaining core)

classification of stars

coolest → hottest

blue, blue white, white, yellow white, yellow, red orange, red

astronimical objects cool as they expands, heats up as they contract

comets

• comets have eliptical orbits, meaning that they travel really close to the sun

• this is because, when it is close to the sun, the GPE is transferred into KE

• this thus increases speed because KE is directly proportional to v²