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What do electrically charged objects attract?
Small neutral objects which are placed near them
What is a classic example of electrically charged objects attracting small neutral objects?
Polythene and acetate rods being rubbed with a cloth duster
With a polythene rod and a duster, what happens to the electrons?
They are moved from the duster to the rod
With an acetate rod and a duster, what happens to the electrons?
They are moved from the rod to the duster
What can happen if enough static charge builds up?
- Sudden movement
- Movement can cause sparks or shocks that can be dangerous
Give three ways that static electricity can be a nuisance.
- Attracting dust
- Clothing clings and crackles
- Shocks from door handles
How can static electricity attract dust?
- Dust particles are charged so will be attracted to the opposite charge
- Most household items are insulators
- The dust becomes attracted to them
- Makes cleaning a nuisance
How does static electricity make clothing cling and crackle?
- When synthetic clothing is rubbed together, electrons are rubbed off
- This leaves static charges on both parts
- This leads to attraction
- Causing little sparks or shocks as the charges rearrange themselves
How does static electricity cause shocks from door handles?
- Walking on a nylon carpet with shoes with insulating soles builds up a charge in your body
- Touching a metal door handle makes the charge flow via the conductor and creates a shock
Give two ways static electricity can be dangerous.
- A lot of charge can build up on clothes
- Grain chutes, paper rollers and the fuel filling nightmare
How can a build up of charge on clothes be dangerous?
- Synthetic clothing rubbing against each other creates a build up of charge
- The charge can get enough to create a spark
- If this happens near inflammable gases of fuel fumes, there can be a fire/explosion
How can grain chutes, paper rollers and the fuel filling nightmare be dangerous?
- As fuel flows through a filler pipe, paper drags over rollers or grain shoots out of pipes the static energy can build up
- This can lead to a spark which can lead to an explosion
- These problems can be solved by earthing charged objects
How can sparks be prevented?
- Connecting a charged object to the ground using a conductor (earthing)
- Earthing means no charge can build up to give a shock or a spark
- Insulating mats and shoes with insulating soles prevent static electricity from moving through them, stopping you from getting a shock
Give three uses of static electricity.
- Paint sprayers
- Dust precipitators
Explain how paint spraying uses static electricity.
- Used to get an even coat of paint
- The spray gun is charged which charges small drops of paint
- Each paint drop repels the others so you get a very fine spray
- The object to be painted is given the opposite charge of the gun so it attracts the paint
- Hardly any paint is wasted
Explain how dust precipitators use static electricity.
- Used to clean up emissions
- As smoke particles reach the bottom of the chimney they meet a negatively charged wire grid or rod
- The dust particles gain electrons and become negatively charged
- The dust particles are attracted to the positively charged plates and stick together on them to form larger particles
- When they are heavy enough they fall off the plates to the bottom of the chimney
- This means the gases coming out of the chimney have very few smoke particles in them
Explain how defibrillators use static electricity.
- Used to restart a heart
- Consists of two paddles connected to a power supply
- The paddles give the patient an electric shock, making the heart contract and restarting it
What is current?
- The flow of electrical charge around a circuit
- Basically the flow of electrons
- Measured in amps (A)
- More charge passes around a circuit when a higher current flows
- Current only flows through a component if there is a voltage across that componant
What is voltage?
- A driving force that pushes the current around
- Measured in volts, V
What is resistance?
- Anything in the circuit which slows the flow down
- Measured in ohms, Ω
What is the relationship between current, voltage and resistance?
- Relative sizes 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).
What are used to break the circuit?
- Wire fuses
- Circuit breakers
What colour is the live wire?
What colour is the neutral wire?
What colour is the earth wire?
What is the live wire?
- Carries the voltage
- Alternates between high positive and high negative voltage of about 230V
- Electricity normally flows in through it
What is the neutral wire?
- Completes the circuit
- Electricity normally flows out through it
- Always at 0V
What is the earth wire?
- For safety
- Works with the fuse
What does it mean if an appliance is 'earthed'?
- The case must be attached to and earth wire
- An earthed conductor can never become live
When is earthing not needed?
- If the appliance has a casing that's non-conductive (e.g. plastic)
- Said to be double insulated
- Can't become live anyway
How do fuses work?
- Earthing causes a big current to flow through the circuit, if there is a problem
- The surge blows the fuse and causes the wire inside to melt
- Cuts off the live supply because the circuit breaks
What is a circuit breaker?
- Similar to a fuse
- Can be reset after it trips
How should fuses 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
How can we work out power?
power = voltage x current
What are three fixed ratings of a fuse?
3A, 5A and 13A
What is another name for a variable resistor?
What is a variable resistor?
A resistor whose resistance can be changed by twiddling a knob (for example)
What happens to the current if you turn the resistance up?
What happens to the current if you turn the resistance down?
It goes up
What do old fashioned variable resistors look like?
Huge coils of wire with a slider on them
How do the old fashioned variable resistors work?
As you move the slider, the length of wire changes
What resistance do longer wires have?
- More than shorter wires
- Less current flows through them
- The longer the wire, the more material electric charge has to flow through
How does the thickness of the wire effect the resistance?
- Thinner wires have more resistance
- The less space electric charge has to go through increases the resistance
How can we calculate resistance?
resistance = voltage / current
What is the proper name for voltage?
What is sound?
A longitudinal wave
What happens to sound waves?
They squash up and stretch out the arrangement of particles in material they pass through, making compressions and rarefactions
What are compressions?
The bits under high pressure (lots of particles)
What are rarefactions?
The bits under low pressure (fewer particles)
What is a wavelength?
- The full cycle of the wave
- E.g. from crest to crest
- E.g. from compression to compression
What is frequency?
- How many complete waves there are per second
- Measured in hertz (Hz)
- 1 Hz is one complete wave per second
- For sound, high frequency = high pitch
What is amplitude?
Tells you how much energy the wave is carrying, or how loud the sound is
How can you see the amplitude of a sound?
- On a oscilloscope (CRO)
- Show sounds as transverse waves
- Measure amplitude from middle line to crest
Describe the vibrations in longitudinal waves.
In longitudinal waves the vibrations are along the same direction as the wave is travelling
Describe the vibrations in transverse waves.
In transverse waves the vibrations are at 90oC to the direction of travel of the wave
What is ultrasound?
- Sound with a higher frequency than we can hear
- Electrical oscillations of any frequency can be converted into mechanical vibrations to produce longitudinal waves beyond the range of human hearing
What is the frequency beyond the range of human hearing?
20 000 Hz
How can ultrasounds be used in hospitals?
- Breaking down kidney stones
- Body scanning
How can ultrasound be used to break down kidney stones?
- Ultrasound beam concentrates high energy waves at the kidney stone and turns it into sand-like particles
- These pass out of the body in urine
- Means the patient doesn't need surgery and relatively painless
How can ultrasound be used for body scanning?
- They can pass through the body but whenever they reach a boundary between two different media some of the wave is reflected backwards and detected
- The exact timing and distribution of these echoes are processed by a computer to produce a video image of whatever is being scanned
What are the advantages of ultrasound over xrays?
- Xrays pass easily through soft tissues like muscle and skin so you can only use them to make images of hard things, like bone
- Ultrasound can image soft tissue
- Ultrasound is safe
- Xrays are ionising radiation so they can damage the living cells and cause cancer if you are exposed to too high a dose
Where does radioactivity come from?
An unstable nucleus
What are radioactive materials made up of?
Unstable nuclei which decay at random
What are the three forms of radiation given out when nuclei decay?
- Alpha (α)
- Beta (β)
- Gamma (γ)
When does gamma radiation occur?
After alpha or beta radiation if the nucleus has some energy left over
What happens during the decay?
The nucleus will often change into a new element
What mass and charge does gamma radiation have?
- If its emitted, the atomic mass and numbers don't change
What type of nucleus is alpha radiation?
What is the charge and mass of an alpha particle?
What is an alpha particle made up of?
What happens when a nucleus emits and alpha particle?
- The mass number decreases by 4
- The atomic number decreases by 2
What is beta radiation?
A fast moving electron
What is the charge and mass of a beta particle?
- Mass - virtually no
- Charge - -1
What happens when a nucleus emits beta radiation?
- Mass number doesn't change
- Atomic number increase by 1
What is half-life?
Half-life is the time taken for half of the radioactive nuclei now present to decay
What is a short half-life?
The activity falls quickly because lots of the nuclei decay in a short time
What is a long half-life?
The activity falls more slowly because most of the nuclei don't decay for a long time, they just sit there basically unstable
What type of radiation are nuclear and xray?
What is ionisation?
When charged particles are produced (ions)
Why does ionisation occur?
Because the particle gains or loses electrons
How do xrays and gamma radiation ionise?
- They transfer energy to electrons
- The electrons then have enough energy to escape from the atom, ionising it and leaving it positively charged
How do beta particles ionise?
- They can remove electrons from atoms or molecules they collide with, leaving them positively charged
- They can also stick onto an atom, ionising it and making it positive
How do alpha particles ionise?
They can remove electrons from atoms and molecules they pass by or hit, making them positive
Why are alpha particles good ionisers?
- They're relatively large so it's easy for them to collide wit atoms or molecules
- They're highly charged so they can easily remove electrons from the atoms they pass or collide with
What do lower doses of ionising radiation cause?
- Minor damage without killing the cell
- Can give rise to mutant cells which divide uncontrollably
- This is cancer
What do high doses of ionising radiation cause?
- Tend to kill cells completely
- Causes radiation sickness if a lot of cells get blasted all at once
Which radiation is most dangerous outside the body?
- Beta and gamma
- They can still get inside to the delicate organs as they can pass through the skin
- Alpha can't penetrate the skin
Which radiation is most dangerous inside the body?
- They do all their damage in a localised area
- Beta and gamma are less dangerous because they are less ionising and mostly pass straight out
What type of waves are gamma and xray?
How are gamma waves made?
- Released from some unstable atomic nuclei when they decay
- Nuclear decay is completely random so there's no way to control when they're released
How are xray waves made?
- Can be produced by firing high speed electrons at a heavy metal like tungsten
- Much easier to control than gamma rays
What can/can't xrays pass through?
- Easily through flesh
- Not so easily through thicker and denser materials like bones or metal
- Thicker or denser a material is, the more xray will be absorbed
How is ionising radiation used in hospitals?
- Tracers in medicine
- Sterilisation of surgical instruments
What is radiotherapy?
The treatment of cancer using gamma rays
How are gamma rays used to treat cancer?
- They are focused on the tumour using a wide beam
- The beam is rotated around the patient with the tumour at the centre
Why is the beam rotated?
To minimise the exposure of normal cells to radiation and so reduces the chances of damaging the rest of the body
What half-life do the radiation particles need for tracers?
- Preferably a few hours
- So that the radioactivity inside the patient quickly disappears
What type of radiation is needed for tracers?
How does radiation for tracers get into the body?
Where is iodine-123 absorbed?
The thyroid gland
What type of steralisation is used for cleaning surgical instruments?
Why are the instruments exposed to gamma radiation?
Because it kills all the microbes on it
Why is it good to use radiation rather than boiling?
- Doesn't involve high temperatures
- Heat sensitive things (e.g. thermometers) can be cleaned without damage
What are some uses of radiation (non-medical)?
- Tracers in industry
- Smoke detectors
How do tracers in industry work?
Radioactive isotopes are used to track the movement of waste materials, find the route of underground pipe systems or detect leaks or blockages
How do you check pipes?
- Squirt the radioactive isotope in
- 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
- Really useful for concealed or underground pipes
What type of isotope is needed for tracers in industry?
- A gamma emitter
- In order to be detected
- Short half-life so as not to cause hazards
What radiation is needed for smoke detectors?
How do smoke detectors work?
- 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 smoke particles hit the alpha particles instead, it causes less ionisation of the air particles so the current is reduced causing the alarm to sound
What are the percentages for sources of background radiation?
- Radon gas - 51%
- Cosmic rays - 10%
- Rocks and building materials - 14%7
- Nuclear industry - 1%
- Medical xrays - 12%
- Food - 12%
What are cosmic rays?
- Radiation from space
- Mostly comes from the sun
What does radiation due to human activity include?
- Fallout from nuclear explosions
- Waste from industry and hospitals
What can vary the amount of background radiation?
How can we work out how long a radioactive thing has been around?
By measuring the amount of a radioactive isotope left in a sample and knowing it's half-life
How much of the carbon in the air is carbon-14?
- About 1/10 000 000
- One ten millionth
What is the half-life of carbon-14?
How can we date rocks?
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 the uranium
What is nuclear fission?
Where uranium or plutonium atoms split up and release energy in the form of heat
How can electricity be produced using nuclear fission?
- The heat from the fission is used to heat the water to produce steam
- The steam turns a turbine which drives a generator that produces electricity
What is needed for the splitting of uranium-235?
- It needs to be made unstable
What is uranium-235 used for?
- Some nuclear reactors
When can materials become radioactive?
When they absorb extra neutrons
What controls the chain reaction?
What do control rods do?
- Limit the rate of fission by absorbing excess neutrons
- This stops the reaction going out of control but allows enough neutrons to hang around to keep the process going
What are control rods normally made of?
What is nuclear fusion?
- The opposite of nuclear fission
- Two light nuclei combine to create a larger nucleus
- Releases a lot of energy
- Doesn't leave behind much radioactive waste and there's plenty of hydrogen about to use as fuel
What is a problem with nuclear fusion?
- It needs really high pressures and temperatures (10 000 000oC)
- No material can withstand this temperature so fusion reactions are really hard to build
- Hard to safely control the temperature and pressure
What are fusion bombs?
- Fusion reactions also happen in fusion bombs
- Also known as hydrogen or H bombs
- In fusion bombs, a fission reaction is used first to create the really high temperature needed for fusion
What is cold fusion?
Nuclear fusion which occurs at around room temperature rather than at millions of degrees Celcius
What happened in 1989?
- Two scientists reported that they had succeeded in releasing energy from cold fusion, using a simple experiment
- This caused a lot of excitement - cold fusion would make it possible to generate lots of electricity, easily and cheaply
- After the press conference the data was shared
- Few managed to reproduce the results reliably