Physics

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SPARKS

Atom: small positively charged nucleus surrounded by an equal number of negatively charged electrons

  • all electrostatic effects are due to the movement of electrons
  • polythene rubbed with duster - electrons transfer from duster to polythene - so polythene = negative 
  • acetate rubbed with duster - electrons transfer from acetate to duster - so acetate = positive

Ions: atoms or molecules that have become charged (positive/negative)

Static electricity is a nuisance:

  • dust and dirt are attracted to insulators such as television screens
  • clothes made from synthetic material often sling to each other and the body

How to avoid electric shocks:

  • earth wires to connect things that may become charged to earth
  • operator stands on insulating rubber mat when operating machinery that may become charged
  • shoes with insulating soles are are worn by workers so charge can't flow through them to earth
  • anti-static sprays, liquids and cloths carry away electric charge to prevent build-up
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USES OF ELECTROSTATIC

Dust precipitators:

  • removes harmful particles from chimneys in factories and power stations
  • metal grid placed in chimney and given large charge from high voltage supply
  • plates inside chimney are earthed and gain opposite charge to grid
  • as dust particles pass the grid they become the same charge as the grid so repel
  • at intervals the plates are vibrated and the dust falls down to a collecter
  • the charge on the dust particles induce a charge on the metal plate

Paint spraying:

  • spray gun is charged and paint particles become charged with the same charge
  • like charged repel so paint particles spread out giving a fine spray
  • the object is given opposite charge to paint (opposites attract)
  • if object is not oppositely charged, the paint particles will give the object their charge and other particles will be repelled from that charge
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USES OF ELECTROSTATIC 2

Defibrillators:

  • procedure to restore regular heart rhythm by delivering electric shock through chest to heart
  • two paddles are charged from a high voltage supply, they are then placed on patient to ensure good electrical contact
  • the operator doesn't receive shoch due to plastic handles being insulators
  • power = energy / time
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SAFE ELECTRICALS

  • variable resistor or rheostat changes resistance, longer wire and thinner wire = more resistance
  • fixed resistor - as voltage increases so does current
  • fixed power supply - as resistance increases, current decreases
  • resistance = voltage / current
  • resistance measured in ohms

wires:

  • live wire - carries high voltage around house
  • neutral wire - completes curcuit providing return path for current
  • earth wire - is connected to the case of an appliance to prevent it from becoming live
  • fuse - contains wire which melts if current becomes too large, breaking the current, so it can't overheat and cause damage
  • re-settable fuse doesn't need to be replaced to restore power - it can be reset

power = voltage x current

power can be used to decide which fuse to have in an electrical device

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ULTRASOUND

  • we can't hear ultrasound because the frequency is too high!
  • it travels as a pressure wave containing compressions and rarefactions

Longitudinal waves:

  • can't travel though a vacuum, the denser the medium, the faster the sound wave travels
  • higher the frequency = smaller the wavelength
  • louder the sound or more powerful the ultrasound = more energy carried by wave and larger amplitude
  • vibrations of particles are parallel to direction of wave (transverse wave - vibrations are at right angles to wave)

Uses of ultrasound:

  • break down kidney stones - high powered beam aimed at stone so energy breaks down stone into small pieces and then we excrete it from the body in normal ways
  • body scans - pulse of ultrasound sent into body, at each boundary between different tissues, some is reflected and some is transmitted, the returning echoes are recorded and used to build up an image of internal structure
  • ultrasound is preferred to x-rays - able to produce images of soft tissue and doesn't damage cells
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WHAT IS RADIOACTIVITY?

  • radioactive substances decay naturally giving out alpha, beta and gamma radiation
  • nuclear radiation causes ionisation by removing electrons from atoms or causing them to gain electrons
  • there are many atoms in a radioisotope that the average count will always be about the same, they have an unstable nuclei because their particles aren't held together strongly enough 
  • half-life of radioisotope = average time for half the nuclei to decay (can't be changed)

Nucleon: particles found in the nucleus, eg: protons and neutrons

  • number of neutrons = atomic mass - atomic number

Alpha particles are good ionisers, they are the largest particles emitted in radioactive decay, they are more likely to strike atoms of the material they are passing through and ionising them

Alpha = positively charged, large mass, helium nucleus, has helium gas around it, consists of 2 protons and 2 electrons

Beta = negatively charged, small mass, travels very fast, is an electron

Alpha in decay: mass no. decreases by 4, nucleus has 2 fewer neutrons + protons, atomic number decreases by 2

Beta in decay: mass no. is unchanged, nucleus has 1 less neutron, BUT 1 more proton, atomic number increases by 1

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USES OF RADIOISOTOPES

Background radiation:

  • due to radioactive substances present in rocks (especially granite) and soil, cosmic rays from space, man made sources (radioactive waste from industry and hospitals)
  • most from natural sources but some from human activity eg nuclear weapon testing and power

Tracers:

  • locating leaks in underground pipes: detector passed along ground above pipe, an increase in activity is detected at the region of the leak
  • gamma sources used as tracers because radiation can penetrate the surface

Smoke detectors (contains isotope which emitts alpha particles):

  • without smoke the alpha particles ionise the air, which creates a tiny current that can be detected by the circuit in the smoke alarm
  • with smoke the alpha particles are partially blocked so less ionisation of the air, the change in current is detected and alarm sounds
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USES OF RADIOISOTOPES 2

Dating rocks:

  • some rocks such as granite contain traces of uranium (radioactive material)
  • the uranium isotopes go through a series of decays before eventually forming a stable isotope of lead
  • by comparing the amounts of uranium and lead present in rock sample, approx age can be found
  • uranium-238 decays with half life of 4500 million years
  • proportion of lead increases as time increases, if there are equal quantities, the rock is 4500 million years old (1 half life)

Radiocarbon dating:

  • carbon-14 is a radio-active isotope of carbon that is present in all living things
  • by measuring the amount of carbon-14 present in an archaeological find, it's approx age can be found
  • when an object dies, no more c-14 is produced
  • as it decays, the activity of sample decreases
  • ratio of current activity from living matter to the sample activity provides a reasonably accurate date
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TREATMENT

  • alpha radiation absorbed by skin so no use for diagnosis or therapy
  • beta passes through skin but not bone - medical treatment limited but can treat eyes
  • gamma is penetrating and is used in medicine - cobalt-60 is a gamma emitting radioactive material that is used widely to treat cancer
  • ionising radiation increases risk of cancer
  • cancer cells can be destroyed by exposing to large amounts of radiation - radiotherapy consists of 3 sources of radiation aimed at tumour, healthy tissue receives only 1/3 of radiation so limits damage

X-rays vs. gamma rays:

  • when x-rays pass through the body, tissues absorb some ionising radiation
  • gamma rays and x-rays have similar wavelengths but are produced in different ways
  • x-rays made by firing high speed electrons at metal targets
  • x-ray machine allows rate of production and energy to be controlled but you can't change gamma radiation
  • when nucleus of an atom of a radioactive substance decays, it emitts an alpha or beta particle and loses extra energy by emitting gama rays

Tracers: eg technetium-99m and iodine-123 (used to investigate thyroid gland)

  • emitts gamma radiation, mixed with food or drink or injected into body
  • it's progress is monitored using a gamma camera conncted to a computer
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FISSION AND FUSION

Nuclear power stations:

  • uranium used as fuel in power station contains a greater proportion of uranium-235 compared to uranium-238 than occurs naturally
  • fission: occurs when large unstable nucleus splits up and energy is released as heat, heat used to boil water for steam, the pressure of the steam makes turbine blades turn, rotating turbine turns generator so produces electricity
  • nuclear bomb is an example of a chain reaction that is not controlled, this produces lots of energy and extra neutrons

Nuclear fission:

  • output of nuclear reactor controlled by: graphite moderator between fuel rods slows down fast moving neutrons emitted during fission (slow moving neutrons are more likely to be captured by other uranium nuclei)
  • boron control rods - raised or lowered, boron absorbs neutrons so fewer can split uranium nuclei, this controls rate of fission
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FISSION AND FUSION 2

Fusion:

  • happens when 2 light nuclei join together and release large amounts of heat energy (requires extremely high temp that are difficult to achieve)
  • in stars, fussion takes place due to high temp and pressure
  • fussion bombs are started with a fission reaction which creates exceptionally high temperatures
  •  so far attempts to replicate these conditions safely have been unsuccessful
  • cold fussion is still not accepted as realistic since any results are impossible to verify so far
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