Static is Caused by Friction
- Electrons have a negative charge
- When two insultating materials rub together, electrons will move from one to another
- This leaves a postive charge on one of the materials due to the lack of electrons
- It also leaves a negative charge on the other due to the excess of electrons
- It depends on the two materials involved to know how the electrons are being transferred
- If an object is electrically charged it will attract small neutral objects near them
- An example of this is polythene and acetate rods being rubbed with a cloth duster.
Only Electrons Move
- Both postive and negative electrostatic charges are only produced by the movement of electrons
- The postive charges do not move. A postive static charge is caysed by electrons moving elsewhere
- If there is enough static charge built up, it can suddenly move causing a spark or a shock
- A charged conductor can be discharged safely by connecting it to earth with a metal strap. This is earthing
- The electrons flow down the strap to the ground if the charge is negative and flow up the strap fromthe ground if the charge is postive.
Like Charges Repel, Oppostie Charges Attract
- Two things with oppostie electric charges are attracted to each other
- Two things with the same electic charge will repel each other
- These forces get weaker the further apart the two things are
- Atoms or molecules that become charged are known as ions
Objects Can be Earthed or Insulated to Prevent Spa
- Sparks can be prvented by connecting a charged object to the ground using a conductor - this is earthing and it provides an easy route for the static charges to travel to the ground without it creating a shock or a spark.
- Static charges are a problem in places where sparks could ignite inflammable gases, or where there is a high concerntration of oxygen.
- Fuel tankers must be earthed to prevent any sparks that could cause the fuel to explode.
- Anti-static sprays and liquids work by making the surface of a charged object conductive, providing an easy path to the ground.
- Anti-static cloths are conductive, so they can carry charge from objects they're used to wipe.
- Insultating mats and shoes with insulating soles prevent static electricity from moving through the, so the stop you from getting as a shock.
Uses of Static Electricity - Paint Sprayers
Uses of Static Electricity - Dust Precipitators
Factories and power stations produce loads of smoke, which is made up of tiny particles. Fortunately, the smoke can be removed with a precipitator;
- As smoke particles reach the bottomof the chimney, they meet a wire fgrid or rods with a high voltage and negative charge.
- The dust particles gain electrons and become negativley charged.
- The dust particles then induce a charge on the earthed metal plates. (The engatively charged dust particles repel electrons on the plates, sp that the plates become positively charged.)
- The dust particles are attracted to the metal plates, where they stick together to form layer particles.
- When heavy enough, the particles fall off the plates or are knocked off by a hammer.
- The dust falls to the bottom of the chimney and can be removed.
- So the gases coming out of the chimney have very few smoke particles in them.
Uses of Static Electricity - Defibrillators
- The beating of your heart is controlled by tiny little electrical puses inside your body. So an electric shock to a stopped heart can make it start beating again.
- Hospitals and ambulances have machines called defibrillators which can be used to shcok a stopped heart back into operation.
- The defibrillator constits of two paddles connected to a power supply.
- The paddles of the defibrillator are placed firmly on the patient's chest to hget a good electrical contact and then the defibillator is charged up.
- Everyone moves away from the patient except for the defibillator operator who holds insulated handles - so only the patient gets a shock.
- The charge passes through the paddles to the paitent to make the heart contract.
Charge Flows Aroud a Circuit
- Is the flow of elctrical charge around a circuit - basically the flow of electrons. It's measured in amps, A. More charge passses around a circuit when a higher current flows. Current will only flow through a component if there is a voltage across that component (unless the component is a superconductor).
- Is the driving force that pushes the current round - electrical pressure. Voltage is measured in volts, V.
- Is anything in the circuit which slows the flow down. Resistance is measured in ohms.
Charge in Circuits
There is a balance:
- The voltage is trying to push the current round the circuit,a nd the resistance is opposing it - the relative size of the voltage and resistance decide how big the current will be.
- If you increase the voltage - then more current will flow. If you increase the resistance - then less current will flow. ( or more voltage will be needed to keep the same current flowing.)
Breaking the Circuit
If you break the circuit, the current stops flowing.
- Current only flows in a circuit as long as there's a complete loop for it to flow around. Break the circuit and the current stops.
- Wire fuses and circuit breakers (resettable fuses) are saftely features that break a circuit if there's a fault.
All the Wires in a Plug are Colour Coded
In plugs, the correct coloured wire is connected to each pin, and firmly srewed in place so no bare wires show.
Wires yoiu need to learn:
- The live wire carries the voltage. It a\lternates between a high +ve and -ve voltage of about 230v.
- The neutral wire completes the ciruit - electricity normally flows in thorugh the live wire and out through the neutral wire. The neutral wire is always at 0v.
- The Earth wire and fuse (or circuit breaker) are for safety and work together.
All applicances with metal cases must be "earthed" to reduce the danger of electric shock.
If the appliance has a casing that's non-conductive then it's said to be double insulated.
Anything with double insulation doesn't need an earth wire as it can't be live.
Earthing and Fuses Prevent Fires and Shocks
- If a fault develops in which the live wire somehow touches the metal case, then because the case is earthed, a big current flows in through the live wire, through the case and out down the earth wire.
- The surge in current 'blows' the fuse and causes the wire inside it to melt. This cuts off the live supply because it breaks the circuit.
- This isolates the whole appliance, making it impossible to get an electric shock from the case.
- It also stops the flex overheating, which could cause a fire, and it prevents further damage to the appliance.
- A circuit breaker works like a fuse but can be reset after it 'trips' and used again. Fuses break when they 'blow' and have to be replaced.
- Fuses should be rated as near as possible but just higher than the normal operating current. If they were a lot higher, they wouldn't blow when the live wire touched the case or when a fault developed.
Electrical Power and Fuse Ratings
- The formula for electrical power is; POWER = VOLTAGE X CURRENT
- You can use this to work out the fuse that should nbe used in an appliance. First, you need to work out the current that the item will use.
- The fuse used should be rated just a little higher than the current.
- Fuses come with fixed ratings. Choose the first one that's just higher than the current the appliance uses.
- A avriable resistor (or rheostat) is a resistor whose resistancecan be changed by twiddling a knob or something.
- They're great for altering the current flowing through a circuit. When the resistance is high the current drops. When the resistnce is low the current is high.
- Longer wires have more resistance, so have less current flowing through them. This is because the longer the wire, the more material electric charge has to flow through, which increases the resistance.
- The thickness of a wire also matters - thinner wires have more resistance and so less current can flow.
- The thinner the wire, the less space electric charge has to move through, which inncreases the resistance.
- The resistance of a (non-variable) resistor is steady (at constant temperature)
- If oyu increase the voltage across a resistor, the current increases as well.
- For the same voltage, current increases as resistance decreases.
- You can calculate the resistance of a resistor using the formula:
- Resistance = Voltage / Current
Test Circuits are Used to Measure Resistance
This is a standard test circuit:
- As you vary the variable resistor it alters the current flowing through the circuit.
- This allows you to take several pairs of readings from the ammeter and voltmeter.
- The ammeter measures the current (in amps) through the component. It's placed in series (in line) with the other component.
- The voltmeter measures the voltage (in volts) across the component. It's placed in parallel around the component being tested.
- The proper name for volatge is potential difference, pd.
Sound is a Longitudinal Wave
You need to know the features of longitudinal waves:
- Sound waves squash up and stretch out the arrangement of particles in materials they pass through, making compression and rarefractions.
- Compressions are the bits under high pressure and rarefractions are the parts under low pressure.
- The wavelength is a full cycle of the wave
- Frequency is how many complete waves there are per second. Frequency is measured in hertz.
- The amplitude tells you how much energy the wave is carrying, or how loud the sound is. You can see the amplitude of a sound on a CRO. CRO displays show sounds as transverse waves so you can see what's going on. YOOu measure the amplitude from the middle line to the crest, not from a trough to crest.
In longitudinal waves the vibrations are along the same direction as the wave is travelling.
In transverse waves the vibrations are at 90 degrees to the direction of travel of the wave
Ultrasound is sound with a higher frequency than we can hear.
Electrical devices can be made which produce electrical oscillations of any frequency . These can easily be converted into mechanical vibrations to produce longitudinal waves beyond the range of human hearing. This is called ultrasound and it has loads of uses in hospitals
- Breaking down accumulations in the body - Getting rid of kidney stones
- An important example is the removal of kindey stones. An ultrasoound beam concentrates high energy waves at the kidney stones and turn it into sand-like particles. These particles then pass out of the body in urine. It's useful because the patient doesn't need surgery and it's relatively painless
- For body scanning
- Ultrasound waves can pass through the body, but whenever they reach a boundary between two different media some of the wave is reflected back and deteched, returning back from different depths at different times.
- The exact timing and distrubution of these echoes are processed by a computer to produce a video image of whatever is being scanned.
Ultrasound Has Advantages over X-Rays
- X-rays pass easily through soft tissues like muscle and skin, so you can usually only use them to make images of hard things like bonne. Ultarsound is great for imaging soft tissue.
- The other advantage is that ultrasound is, we're pretty sure it doesn't damage living cells. X-rays are ionising radiation. They can damage living cells and cause cancer if you're exposed to too high a dose.
Radioactivity Comes From an Unstable Nucleus
- Radioactive materials are made up of atomswith unstable nuclei that naturally decay at random
- As they decay, they can give out three forms of radiations: alpha,beta and gamma. During the decay, the nucleus willl often change into a new element.
- Gamma radiation happensq after alpha and beta emission if nucleus has some extra energy to get rid of. It emits a gamma-ray that has no mass or charge. This means the atomic and mas number don't change.
Alpha Radiation is a Helium Nucleus
- An alpha particle is a helium nucleus, mass 4 and charge of +2, made up of two protons and two neutrons.
- So, when a nucleus emits an alpha particle:
- The mas number decreases by 4 - because it loses two protons and two neutrons
- The atomic number decreases by 2 - because it has two less protons
- It forms a new element - because the number of protons has changed.
Beta Radiation is a Fast-Moving Electron
- A beta particle is a fast-moving electron, with virtually no mass and a charge of -1.
- So, when a nucleus emits a beta particle:
- The mass number doesn't change - because it has lost a neutron but gained a proton.
- The atomic number increased by 1- because the number of protons has changed.
Radioactivity and Half-Life
- Each time an unstable nucleus decays and emits radiation, that means one more radioactive nucleus isn't there to decay later.
- As more unstable nuclei decay, the radioactivity of the source as a whole decreases - so the older a radioactive source is, the less radiation it emits.
- How quickly the activity decreases varies a lot. For some isotopes it takes just a few hours before nearly all the unstable nuclei have decayed. For others it can take millions of years.
- The problem with trying to measure this is that the activity never reaches zero,. which is why we have to use the idea of half-life to measure how quickly the activity decreases.
Definition of half-life: Half-Life is the time taken for half of the radioactive nuclei now present to decay.
- A short half-life means the activity falls quickly, because lots of the nuclei decay in a short time.
- A long half-life means the activity falls more slowly because most of the nuclei don't decay for a long time - they just sit there, basically unstable, but kind of biding their time.
Ionising Radiation Harms Living Cells
- Nuclear radiation and X-rays are ionsing radiation.
- Some materials absorb ionising radiation - it can enter living cells and interact with molecules.
- These interactions cause ionisation- kthry produce charged particlescalled ions.
- Ionisation occurs because the particle gains or loses electrons.
- X-rays and gamma rays can transfer energy to electrons. The electons then have enough energy to escape from the atom, ionising it and leaving it positively charged,
- Beta particles can remove electrons from atoms or molecules they collide with, leaving them positively charged. A beta particle can also stick to an atomo, ionising it and making it negatively charged.
- Alpha particles can remove electrons from atoms and molecules they pass by or hit, making them positive.
- Alpha particles are good ionisers for two reasons:
- They're relatively large - so it's easy for them to collide with atoms or molecules
- They're highly charged - so they can easily remove electrons from the atoms they pass or collide with.
Ionising Radiation Harms Living Cells II
- Lower doses of ionising radiation tend to cause minor damage without killing the cell. This can give rise to mutant cells which divide uncontrollably. This is cancer.
- HIgher doses tend to kill cells completely , which causes radiation sickness if a lot of cells all get blasted at once
The Most Dangerous Radiation Depends on Where it i
- Outside the body, beta and gamma sources are the most dangerous.
- This is because beta and gamma can still get inside to the delicate organs - they can pass through the skin.
- Alpha is much less dangerous because it can't penetrate the skin
- Inside the body, an alpha source is the most dangerous because they do all their damage in a very localised area.
- Beta and gamma sources on the other hand are less dangerous inside the body because they are less ionising and mostly pass straight out without doing much damaged.
X-rays and Gamma Rays are Electromagnetic Waves
- X-rays and gamma rays are both high frequency, short wavelength electromagentic waves.
- They have similar wavelengths, and so have similar properties, but are made in different ways:
- Gamma rays are released from some unstable atomic nuclei when they decay. Nuclear decay is completely random, so there's no way to ccontrol when they're released.
- X-rays can be produced by firing high-speed electrons at a heavy metal like tungsten. These are much easier to control than gamma rays.
- X-rays pass easily through flesh but not so easily through thicker and denser materials like bones or metals.
- The thicker or denser the materials, the more x-ray that's absorbed. So it's the varying amount of radiation that's absorbed that makes an x-ray image.
- Since high doses of gamma rays will kill all living cells, they can be used to treat cancers.
- The gamma rays have to be directed carefully and at just the right dosage, so as to kill the cancer cells without damaging too many normal cells.
- However, a fair bit of damage is inevitably done to normal cells, which makes the patient feel very ill. But if the cancer is successfully killed off in the end, then it's worth it.
- To Treat Cancer:
- The gamma rays are focused on the tumour using a wide beam.
- This beam is rotated round the patient with the tumour at the centre.
- This minimises the exposure of normal cells to radiation, and so reduces the chances of damaging the rest of the body.
Tracers in Medicine
- Certain radioactive isotopes that emit gamma raditaion can be used as tracers in the body.
- They should hvae a short half-life - around a few hours, so that the radioactivity inside the patient quickly disappears.
- They can be injected inside the body, drunk, eaten or ingested.
- They are allowed to spreads through the boyd and their progress can be followed on the outside using a radiation detector.
- All isotopes which are taken into the body must be gamma and beta. This is because gamma and beta radiation can penetrate tissue and so are able to pass out of the body and be detected.
- Alpha radiation can't penetrate tissue, so you couldn't detect the radiation on the outside of the body. Also alpha is more dangerous inside the body.
Sterilisation of Surgical Instruments
- Medical instruments can be sterilised by exposing them to a high dose of gamma rays, which will kill all microbes.
- The great advantage of using radiation instead of boiling is that it doesn't involve high temperatures, so heat-sensitive things like thermometers and plastic instruments can be totally sterilised without damaging them.
Tracers in Industry
This is much the same technique as medical tracers.
- Radioactive isotopes can be used to track the movement of waste materials, find the route of underground pipe systems or detect leaks or blockages in pipes.
- To check a pipe, you just squirt the radioactive isotope in, then go along the outside with a detector. If the radioactivity reduces or stops after a certain point, there must be a leak or blockage there. This is really useful for concealed or underground pipes - no need to dig up the road to find the leak.
- The isotope used must be a gamma emitter, so that the radiation can be detected even through metal or earth which may be around the pipe. Alpha and beta radiation wouldn't be much use because they are easily blocked by any surrounding material.
- It should also have a short half-life so as not to cayse a hazard if it collects somewhere.
- A weak alpha radioactive source is placed in the detector, close to two electrodes
- The source causes ionisation of the air particles which allows a current to flow.
- If there is a fire, then the smoke particles are hit by the alpha particles instead.
- This causes less ionisation of the air particles - so the current is reduced causing the alarm to sound.
The background radiation we receive comes from:
- Radioactivity of naturally occuring unstable isotopes which are all around us - in the air, in food, in building materials and in rocks. A large proportion of background raditation comes from these natural sources.
- Radiation from space, which is known as cosmic rays. These come mostly from the Sun.
- Radiation due to human activity e.g. fallout from nuclear explosions, or waste from industry and hospitals. But this represents a small proportion of the total.
- The amount of backgroound raditaiton can vary depending on where you are and your job. For example, what type of rock your house is built on.
Radioactive Dating of Rocks
- The discovery of radioactivity and the ideas of half-life gave scientists their first opportunity to accurately work out the age of some rocks and archaeologicial specimens.
- By measuring the amount of a radioactive isotopes left in a sample and knowing its half-life, you can work out how long the thing has been around.
Radiocarbon Dating - Carbon 14
- Carbon-14 makes up about one ten-millionth of the carbon in the air.
- The level stays fairly constant in the atmosphere - it hasn't changed for thousands of years.
- The same proportion of carbon-14 is also found in living things.
- But when they die, they stop exchanging gases with the air outside and the carbon-14 is trapped inside, and it gradually decays with a half-life of 5730 years.
- By measuring the proportion of carbon-14 found in some old axe handle, burial shroud etc. you can calculate how long ago the item was living material using the known half-life.
Dating Rocks - Relative Proportions Calculations
- Uranium isotopes have very long half-lives and decay via a series of short-lived particle to produce stable isotopes of lead.
- The relative proportions of uranium and lead isotopes in a sample of rock can therefore be used to date the rock, using the known half-life of uranium.
- Nuclear power stations are powered by nuclear reactors.
- In a nuclear reactor, a controlled chain reaction takes place in which uranium or plutonium atoms split up and release energy in the form of heat - this is nuclear fission.
- This heat is then used to heat water to produce steam.
- The steam turns a turbine which drives a generator that produces electricity.
The Splitting of Uranium-235 Needs Neutrons
Uranium-235 is used in some nuclear reactors and bombs.
- Uranium-235 is actaully stable, so it needs to be made unstable before it'll split.
- Materials can become radioactive when they absorb extra neutrons - so slow-moving neutrons are fired at the U-235.
- A neutron joins the nucleus to create U-236, which is unstable.
- The U-236 then splits into two smaller nuclei, releasing loads of energy and producing radoactive waste.
- The split nucleus also releases 2 or 3 fast-moving neutrons which go onto produce a chain reaction.
- To get a usefull amount of energy, loads of U-235 atoms have to be split. So neutrons released from previous fissions are used to hit other U-235 atoms
- Each split uranium nucleus releases more than one neutron.
- These neutrons cause further nuclei to split, releasing more neutrons, whcih cause more nuclei to split and release more neutrons. This process is called a chain reaction.
- Nuclear bombs are chain reactions that are out of control.
- But in nuclear reactors the chain reaction is controlled using control rods.
Control Rods Control the Chain Reaction
- Free neutrons in the reactor "kick-start" the fission process.
- Neutrons collide with surrounding uranium atoms, causing them to split and the temperature in the reactor to rise.
- Control rods, often made of boron, limit the rate of fission by absorbing excess neutrons.
- This stops the reaction going out of control but allowed enough neutrons to hang around to keep the process going.
- Nuclear fusion is the opposite of nuclear fission.
- In nuclear fusion, two light nuclei combine to create a larger nucleus.
- Fusion releases a lot of energy - all the energy released in stars comes from fusion at extremely high temperatures and pressures. So people are trying to develop fusion reactors to make electricity.
- Fusion doesn't leave behind much radioactive waste and there's pleanty of hydrogen about to use as fuel.
- The big problem is that fusion only happens at really high temperature and pressures.
- No materials can physically withstand that kind of temperature and pressure - so fusion reactors are really hard to build.
- It's also hard to safety control the high temperatures and pressures.
- There are a few experimental reactors around at the moment, the biggest one being JET, but none of them are generating electricity yet. It takes more power to get up to temperature than the reactor can produce.
- Research into fusion power production is carried out bby international groups share the costs, expertise, experience and the benefits.