An atom has a small positively charged nucleus surrounded by the same number of negatively charged electrons
In a neutral atom there are the same positive and negative charges
Electrostatic effects are due to the movement of electrons
When a polyethene rod is rubbed with a duster, electrons are transferred from the duster to the rod making it negatively charged
When an acetate rod is rubbed with a duster, electrons are transferred from the rod leaving it positively charged
In general an object has negative charge to due to excess electrons, positive charge due to lack of electrons
Atoms or molecules that become charged become ions
Electrostatic Shocks:
When inflammable gases or vapours are present, there is a high concentration of oxygen, where a spark from static electricity could ignite the gas or vapours cause an explosion
If someone touches something at high voltage, large amounts of electric charge flow through their body to the earth
Static electricity is annoying but not dangerous, such as: dust and dirt attracted to insulators (television screens), clothes made from synthetic materials cling to each other
Electric shocks can be avoided, if the object is earthed so all the charge flows down the earth wire, by wearing insulating shoes so the charge can't flow, fuel tankers to be connected to an aircraft by a conducting cable during refuelling
Anti-static sprays, liquid and clothes made from conducting materials carry away electrical charge
Dust Precipitators:
They remove harmful particles from chimneys of factories and power stations that pollute the atmosphere
Metal grid is placed in the chimney and given a large charge from a high-voltage supply
Inside the chimney the plates are earthed and gain opposite charges to the grid
Dust particles gain or lose electrons to become charged
Charge on the dust particles induce a charge on the earthed metal plate
Opposite charges attract so dust is attracted to the plate
Paint Spraying:
Static electricity is used in paint spraying
The spray gun becomes charged
All the paint particles become charged with the same charge
Like charges repel so the paint particles spread out giving a fine spray
The object to be painted is given the opposite charge to the paint
Opposite charges attract, so the paint is attracted to the object and sticks to it, therefore the object gets an even coat with limited paint wasted
If the object to be painted and not charged the object becomes charged from the paint gaining the same charge, and further paint droplets are repelled away from the object
Electrostatic is also used in crop spraying
Defibrillators:
Two paddles are charged from a high-voltage supply
They are then placed firmly on the patient's chest to ensure good electrical contact
Electric charge is induced through the patient to make their heart contract
Power = energy ÷ time
Resistance:
A variable resistor (rheostat) changes the resistance. Longer wires have more resistance, thinner wires have more resistance
Voltage is measured in volts (V) using a voltmeter connected in parallel - For a fixed resistor as the voltage across it increases, the current increases
For a fixed power supply, as the resistance increases, the current decreases, the opposite
Resistance = voltage ÷ current and measured in ohms
The live wire carries a high voltage around the house
The neutral wire completes the circuit, providing a return path for the current
The earth wire is connected to the case of an appliance to prevent it becoming live
The fuse contains wire which melts, breaking the circuit, if the current becomes too large
Earth wires and fuses stop a person receiving an electric shock if they touch a faulty appliance
A re-settable fuse (circuit breaker) doesn't need to be replaced to restore power, it can just be re-set
Power = voltage x current
The live wire is brown and carries the high voltage, the neutral wire (blue) completes the circuit and the earth wire is yellow/green and is a safety wire stopping the appliance becoming live
Appliances that are double insulated don't need to be earthed
Ultrasound:
Ultrasound is sound above 20000 Hz which is a higher frequency than humans can hear, it travels as a pressure wave it contains compressions and rarefactions
Compressions are regions of high pressure and rarefactions are regions of low pressure
Features of a longitudinal waves: - they can't travel through a vacuum (the denser the medium the faster the sound wave travels) - the higher the frequency the smaller the wavelength - the louder or more the powerful the ultrasound, the more energy carried by the wave and the larger the amplitude
In a longitudinal wave the vibrations of the particles are parallel to the direction of the wave
In a transverse wave the vibrations of the particles are at right angles to the direction of the wave
Uses of ultrasound:
To break down kidney stones, a high-powered ultrasound beam is directed at the kidney stones, they are then broken down into smaller pieces, so they can be excreted normally
Body scans, each boundary between tissue some ultrasound is reflected and is transmitted, the echoes that return are recorded and used to build up picture of the internal structure
To measure the speed of blood flow in the body
Ultrasound is preferred to x-rays as it can produce images of soft tissue and doesn't damage living cells
Radioactive decay:
Radioactive substances decay naturally, emitting alpha, beta and gamma radiation
Nuclear radiation causes ionisation, by removing electrons from atoms causing them to gain electrons
Decay is random and unpredictable when it will occur in the nucleus
The half-life of a radioisotope is the average time for the half nuclei to decay and the half-life cannot be changed
The nucleus:
Nucleon is the protons and neutrons found in the nucleus
Nucleus of an atom can be shown like this: AZX
A = atomic mass (nucleon number), Z = atomic number (proton number), X = chemical symbol of element
Alpha and Beta particle properties:
Alpha: - positively charged - large mass - helium nucleus - helium gas around it - 2 protons and 2 neutrons
Beta: - negatively charged - very small mass - travels very fast - it is an electron
Changes to the decaying atom:
Alpha: - mass number decreases by 4 - nucleus has two fewer neutrons - nucleus has two fewer protons - atomic number decreased by two
Beta: - mass number unchanged - nucleus has one less neutron - nucleus has one more proton - atomic number increases by one
Nuclear equations for decay:
Alpha: 23892U → 23490Th + 42He
Beta: 146C → 147N + 0-1e
Radiation:
Background radiation can occur because of radioactive substances which are present: rocks like granite, in the soil, cosmic rays, and manmade sources like hospitals
Some comes from nuclear power or nuclear weapon testing
Tracers:
A tracer can be used to locate a leak underground
A small amount of a gamma is emitted into the pipe (because gamma can penetrate through the surface), then a detector is passed along above the path of the pipe and an increase in activity is detected in the region of the leak
Smoke Detectors:
Smoke detectors contains the isotope Americium-241 which emits 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 present the alpha particles are partially blocked so there is less ionisation of the air, the change in current is detected and the alarm sounds
Dating rocks:
Some rocks such as granite contain radioactive substance like uranium, this goes through a series of decays to eventually form a stable isotope of lead
By comparing the amounts of uranium and lead present in rock, the approximate age can be found
Uranium-238 has a half-life of about 4500 million years
The proportion of lead increases as time increases, if there are equal quantities of 23892U and 20682Pb the rock is 4500 million years old
Carbon-14:
Is a radioactive isotope of carbon that is present in all living things, by measuring the amount of carbon-14 an approximate age can be found
Carbon dating can be used on objects that were once alive
When an object dies no more carbon 14 is produced, so as carbon 14 decays the activity of the sample decreases, so the ratio of current activity from living matter to the sample activity provides a reasonably accurate date
Treatment:
Radiation is emitted from the nucleus of an unstable atom which can be alpha, beta or gamma
Alpha is absorbed by the skin so it's not good for therapy, beta radiation passes through the skin but not the bone. It can be used to treat eyes, gamma is very penetrating and used in medicine. Cobalt - 60 is used to treat cancers
When nucleus radiation passes through a material it causes ionisation, this damages living cells causing cancer
Cancer cells can be destroyed with exposing them to radiation, this is known as radiotherapy
Materials can become radioactive when their nuclei absorbs extra neutrons in a nuclear reactor
X-Rays:
When a X-ray passes through the body some tissues actually absorb the ionising radiation, this amount depends on the thickness and density of the absorbing material
High-speed electrons are fired at metal targets, to produce and x-ray
Gamma rays and x-rays have similar wavelengths but are produced in different ways
X-ray machines allow the rate of production and energy of the x-rays to be controlled, but the gamma radiation emitted from a particular radioactive source cannot be changed
When a nucleus of an atom of a radioactive substance decays, it emits an alpha or a beta particle and loses any surplus energy by emitting gamma rays
Medical tracers:
Technetium-99m is commonly used, it only emits gamma radiation
Iodine-123 emits gamma radiation, it is used to investigate the thyroid gland
It is drunk/eaten/ingested/injected into the body, then allowed to spread through the body, and followed by a radiation detector on the outside (example: gamma camera)
Treating cancer:
Gamma rays focused on tumour with a wide beam is used, this is rotated round the patient with the tumour in the centre - this limits damage to non-cancerous tissue
Fission and fusion:
Fission is the splitting of nuclei
Fusion is the joining of nuclei.
Nuclear power station use uranium as a fuel
Uranium consists of two isotopes uranium-235 and uranium-238, uranium 235 is used as it is the enriched uranium, containing a greater proportion of uranium-235
Fission occurs when a large unstable nucleus is split up and energy is released as heat: - heat is used to boil water to produce steam - pressure of the steam acting upon the turbine blades makes it turn - the rotating turbine turns the generator producing electricity
When uranium fissions, a chain reaction starts releasing a lot of energy. An example of this would be a nuclear bomb
Nuclear fission produces radioactive waste
Controlling nuclear fission:
A nuclear reactor can be controlled by: - Graphite moderator between the fuel rods slows down the fast moving neutrons emitted during fission, the slow moving nuclei are likely to be captured by other uranium nuclei - Boron control rods can be raised or lowered, the boron absorbs neutrons so fewer neutrons are available to split more uranium nuclei. This controls the rate
Fusion:
Fusion requires extremely high temperatures which has proved difficult to do on Earth
Research is also very expensive so is under a international joint venture to share the costs, expertise and benefits
In stars fusion takes place, where hydrogen nuclei join together to form helium nuclei 11H + 21H → 32He
Fusion bombs are started with a fission reaction which creates exceptionally high temperatures
For power generation exceptionally high temperatures and/or pressures are required and this combination offers (to date) safety and practical challenges
Cold fusion isn't accepted by scientists yet as it is an unrealistic method at the moment