Flashcards in P4 Deck (106):
How can an insulating material become electrically charged?
If it's rubbed with another insulating material. Electrons transfer from one material to another, leaving one positively charged, and the other negatively charged.
How can someone generate static electricity?
By rubbing a balloon, comb or strip of plastic against an insulating material such as a jumper.
How can synthetic clothing become charged?
Due to friction between the person's body and the clothing when the clothes are put on. When the clothing is removed from the body, static sparks are sometimes produced.
How can a charged object be discharged?
By earthing it. When an object discharges, electrons are transferred from the charged object to earth.
If you became charged and then earthed, what could happen?
You could get an electrostatic shock, for example, you become charged by friction between the soles of your feet and the floor if you're walking on an insulator such as carpet or vinyl, and if you then touch a water pipe, the charge is earthed and discharge occurs, giving a shock.
How can static electricity be a nuisance?
It can cause dirt and dust to be attracted to insulating materials such as television screens and computer monitors, and it can cause some materials to cling to your skin.
How can static electricity be very dangerous?
Flour mills and petrochemical factories have atmospheres that can contain extremely flammable gases or vapours, or high concentrations of oxygen. A discharge of static energy, i.e. a spark, can lead to an explosion. As well as this, static is dangerous in any situation where large amounts could flow through your body to earth, for example, lightning.
What will two insulating materials with the same charge do?
They will repel, for example, if a positively charged Perspex rod is held near to a suspended positively charged Perspex rod, the suspended rod will be repelled,. The same would happen if both had a negative charge.
What happens when electrons are rubbed off one material onto another?
Electric or static charge builds up. The material that receives the electrons becomes negatively charged due to an excess of electrons, and the material giving up the electrons becomes positively charged due to a loss of electrons.
What happens to both when a Perspex rod is rubbed with cloth?
The perspex rod gives up electrons and becomes positively charged. The cloth receives the electrons and becomes negatively charged
What happens to both when an Ebonite rod is rubbed with fur?
The ebonite rod receives receives electrons and becomes negatively charged. The fur gives up electrons and becomes positively charged.
What are atoms and molecules that have become charged called?
What are atoms and molecules that have an excess of electrons?
What are atoms and molecules which have lost electrons?
How can the chances of receiving an electric shock be reduced?
By making sure appliances are correctly earthed, by using insulation mats effectively, and by wearing shoes with insulating soles.
Why do lorries that contain inflammable gases, liquids or powders need to be earthed before unloading?
Because friction can cause a build-up of charge. This charge could lead to a spark, which could then ignite the flammable substance.
How do anti-static sprays, liquids and cloths help to reduce the problems of static electricity?
They prevent the transfer of charge from one insulator to another. If there is no build-up of charge, there can't be any discharge.
How is static electricity used in spray painting cars?
The paint gains electrons as it passes through the negatively charged nozzle of the gun, making the paint particles negatively charged so that they repel each other, forming a fine spray. This ensures that the paint is applied evenly. The car panel has lost electrons and so is positively charged, attracting the oppositely charged paint. As the paint sticks to the car the charges cancel so that the ar becomes neutrally charged and no more paint is attracted.
How can static energy remove smoke particles from chimneys?
With Electrostatic dust precipitators. Metal grids are installed in the chimney and are connected to a high potential difference (voltage). The dust becomes positively charged as it passes the grid, inducing a positive charge on the dust particles when they lose electrons. The dust particles are attracted to the negative charged plates, where they form large particles that fall back down the chimney when they're heavy enough, or if the plates are stuck.
How can electricity be used to start the heart when it has stopped?
With a defibrillator. Two paddles are charged and are put in good electrical contact with the patient's chest using gel. Taking care not to shock the operator, charge is then passed through the patient to make the heart contract.
What is a circuit?
A completed loop that allows an electrical current to flow. Electrons flow around the circuit from the negative electrode of the power source to the positive electrode. But this was only discovered recently, so circuit diagrams are drawn showing the current flowing from the positive to the negative electrode.
What is resistance, and how is it measured?
Resistance is a measure of how hard it is to get a current through a component in a circuit at a particular voltage (potential difference). Resistance is measured in ohms (Ω)
How can the current through a circuit be controlled?
By varying the resistance.
There are two types of resistor. What is a fixed resistor?
A fixed resistor has constant resistance. The bigger the resistance, the smaller the current that flows for a particular voltage.
There are two types of resistor. What is a variable resistor (or rheostat)?
A variable resistor/rheostat has a changeable resistance.
How can the resistance of the rheostat/variable resistor be changed?
By moving the slider, changing the length of wire between the contacts. Long wire= high resistance, low current. Short wire = low resistance, large current.
What does a long length of wire between the contacts in a rheostat/variable resistor mean?
high resistance, low current.
What does a short length of wire between the contacts in a rheostat/variable resistor mean?
low resistance, large current.
Complete this sentence in terms of current and voltage: For a given resistor....
For a given resistor, current increases as voltage increases, and vice versa.
Complete this sentence in terms of current and resistance: For a fixed voltage....
For a fixed voltage, current decreases as resistance increases, and vice versa.
What is the formula to calculate resistance?
Resistance (Ω) = Voltage (V) / Current (A)
What colour is a Live wire, and what does it do?
A live wire is brown, and it carries current to the appliance at a high voltage (230V)
What colour is a Neutral wire, and what does it do?
A neutral wire is blue, and it completes the circuit and carries current away from the appliance
What colour is an Earth wire, and what does it do?
An Earth wire is green and yellow, and it is a safety wire that stops the appliance becoming live.
Which appliances do/don't have earth wires?
All appliances with outer metal cases (conductors) have an earth wire, so they are earthed. An earthed conductor can't become live. Appliances with cases made of insulators don't have an earth wire, though still have a fuse. They're double insulated so they can't become live.
Describe how electrical appliances with outer metal cases are earthed in order to protect the appliance and the user, with the earth wire and fuse working together.
A fault in the appliance causes the casing to become live. The circuit short-circuits (the path of the flow of charge changes) because the earth wire offers less resistance. The fuse wire melts, and the circuit is broken. The appliance and the user are protected.
What is a fuse?
A fuse is a short, thin piece of metal with a low melting point. It's used to prevent cables or appliances from overheating. To work properly, the current rating of the fuse must be just above the normal current that runs through the appliance.
How does a fuse work?
If the current flowing through an appliance is below the current rating of the fuse, the appliance will work properly. However, if a fault occurs inside the appliance, the live wire will make contact with the neutral wire. The current flowing would then be higher than the current rating of the fuse due to lower resistance. This causes the fuse wire to get hotter and hotter until it melts and breaks the circuit. The current is then unable to flow, so there's no danger of the flex overheating, which would result in a fire.
What does the power rating of a device tell you?
How quickly electrical energy is being charged (transferred) into another form within that device.
What is Power measured in?
What do fuses and circuit breakers prevent?
Injury and death, as they stop appliances from becoming 'live'. Fires, as they stop cables and flexes from overheating. Damage to the components of an appliance as they break the circuit if a higher than normal current flows through the appliance.
What is ultrasound?
Sound waves with frequencies above the upper limit of the human hearing range [i.e. above about 20,000 hertz (Hz)]
Which type of wave does ultrasound travel in?
A longitudinal wave
What are the five key features of waves?
Rarefaction, compression, wavelength, frequency and amplitude.
With waves, what is rarefaction?
The area of low pressure
With waves, what is compression?
The area of high pressure
With waves, what is the wavelength?
The distance between corresponding points on two successive disturbances.
With waves, what is the frequency?
The number of waves produced, or that pass a particular point, in one second.
With waves, what is the amplitude?
The maximum disturbance caused by a wave
What can be achieved by scanning the body with ultrasound waves?
A picture can be built up of the body's organs
How do ultrasound waves break down kidney stones so that they can be removed from the body naturally, which avoids the need for painful surgery?
Ultrasound waves cause the kidney stones to vibrate. The stones break up, are dispersed, and can then be passed out of the body in urine.
Other than scanning the body to build up a picture of the body's organs, and breaking up kidney stones, how is ultrasound used in medicine?
To measure the speed of blood flow, to detect gallstones and tumours, and for prenatal screening because there is less risk to mother or baby than using X-rays
How can ultrasound waves be used to calculate the depth of a reflecting surface?
Ultrasound waves are partially reflected at a boundary as they pass from one medium or substance into another. The time taken for these reflections to be detected can be used to calculate the depth of the reflecting surface. The reflected waves are usually processed to produce a visual image on a screen.
What are the two types of wave?
longitudinal and transverse
What are the two main advantages of Ultrasound over X-ray imaging?
It's able to produce images of soft tissue, and it doesn't damage living cells.
Do waves transfer any particles of matter when they transfer energy from one point to another?
How do longitudinal waves move?
Each particle moves backwards and forwards about its normal position in the same plane as the direction of wave movement
How do transverse waves move?
Each particle moves up and down about its normal position at 90° to the direction of the wave movement
What do radioactive materials give out?
Nuclear radiation from the nucleus of each of their atoms. These atoms are unstable and decay naturally.
What is an alpha particle?
A helium nucleus
What is a beta particle?
A fast-moving electron
What is a gamma ray?
An electromagnetic wave (energy)
How is radiation measured?
By the number of nuclear decays emitted per second. This number decreases with time.
When does ionisation occur?
When an uncharged (neutral) atom gains or loses electrons
Why is alpha radiation highly ionising?
Because it's missing two electrons (it has a 2+ electric charge). It attracts electrons away from atoms it passes, leaving them positively charged.
During alpha emission, how does the atom decay?
By ejecting a alpha particle (a helium nucleus made up of two protons and two neutrons) from the nucleus
How does the nucleus of the new atom formed after an atom ejects an alpha particle differ from the original atom?
It is a different element, it has two fewer protons and two fewer neutrons, the atomic number has decreased by two, and the mass number has decreased by four.
During beta emission, how does the atom decay?
By changing a neutron into a proton and an electron. The high energy electron ejected from the nucleus is a beta particle.
How does the nucleus of the new atom formed after an atom ejects a beta particle differ from the original atom?
It has one more proton and one less neutron, the atomic number has increased by one, and the mass number remains the same.
What is half-life?
Half-life is the time it takes for half the undecayed nuclei in a radioactive substance to decay.
What does it mean if a substance has a very long half-life?
It remains active for a very long time
How is it possible to date rocks?
By measuring the proportion of uranium and lead in the rock, and by knowing the half life of uranium
How do you calculate half life?
amount remaining = (initial amount) x (1/2)^(number of half-lives)
How do you calculate the number of half lives?
number = time elapsed / length of half life
What is most background radiation released by?
Radioactive substances in soil and rocks, and cosmic rays from outer space.
Radioisotopes are used as tracers in industry and hospitals. In which ways are tracers used in industry?
To track the dispersal of waste, to find leaks and blockages in underground pipes, and to find the routes of underground pipes.
How do tracers work in industry to find leaks and blockages in underground pipes?
A radioactive material that emits gamma rays is put into the pipe. The progress of the material is then tracked by a detector above ground. If there's a: leak- the radioactive material will escape and be detected at the surface, blockage- the radioactive material
What do most smoke detectors contain that is an alpha emitter?
How do smoke detectors containing Americium-241 work?
Emitted particles cause air particles to ionise, and the ions formed are attracted to the oppositely charged electrodes. This results in a flow of electric current. When smoke enters the space between the two electrodes, the alpha particles are absorbed by the smoke particles, and less ionisation takes place. A smaller current than normal flows, and the alarm sounds
How does carbon dating work?
The activity of radioactive carbon can be used to find the approximate age of a once-living material. The amount of radioactive Carbon-14 in the atmosphere has remained unchanged for thousands of years. A dead object doesn't exchange gases with the air as living matter does. As the Carbon-14 in the dead object decays, it is not replaced so the radioactivity of the sample decreases. This means that the dead object will have a different radioactivity to living matter. The ratio of these two activities can be used to find a fairly accurate approximate age for the object within known limits (approximately 50yrs)
How are medical radioisotopes produced?
By placing materials in a nuclear reactor. They become radioactive when they absorb extra neutrons.
What is a radiographer?
The person in a hospital who takes X-rays and uses radiation.
How are X-rays made?
By firing high-speed electrons at metal targets.
After alpha or beta decay, what does a nucleus sometimes do?
When it, sometimes, contains surplus energy, it emits this as gamma radiation, which is very high frequency electromagnetic radiation.
How are gamma rays used to treat cancer?
A wide beam of gamma rays from a source outside the body is focused on the tumour. The beam is rotated around the outside of the body with the tumour at the centre, which concentrates the gamma rays on the tumour, but minimises damage to the rest of the body.
What is the main advantage of treating cancer with gamma rays, and what is the main disadvantage of it?
It can destroy cancer cells without the need for surgery, but it may damage healthy cells and cause sickness
What are medical tracers?
Small amounts of radioactive materials (with a short half-life) which are swallowed or injected into a patient. The tracer spreads through the body, whilst its progress is followed using an external radiation detector (a gamma camera)
For a medical tracer, which types of radiation must be emitted, and why?
Either gamma or beta, because they both pass through skin, and so they can be detected outside the body.
Why can gamma rays be used to sterilise medical equipment?
Because they kill germs and bacteria
How do conventional power stations produce energy?
They burn fossil fuels (coal, oil and gas). This produces heat which boils water and creates steam. They then use the steam to drive turbines, which turn generators and produce electricity.
How do nuclear power stations produce energy?
They use uranium. A nuclear reaction, called 'fission', takes place, which produces the heat required to make steam.They then use the steam to drive turbines, which turn generators and produce electricity.
What is nuclear fission?
The process by which heat energy is released when a radioactive nucleus (i.e. uranium) splits. This heat energy can be used in a nuclear reactor.
How does nuclear fission occur?
When a uranium nucleus absorbs an extra neutron it splits, releasing energy and more neutrons. These neutrons can then cause further uranium nuclei to split. This is called a chain reaction.
What is a dangerous result of nuclear fission?
What is nuclear fusion?
The process by which heat energy is released when nuclei join (fuse) together/ Fusion happens easily in stars, but is not yet a practical energy resource on earth.
How does small scale nuclear fusion occur?
The uranium atom is hit with a neutron, and the nucleus splits into two smaller nuclei, such as barium and krypton. Energy and new neutrons are released, and the new atoms formed, are themselves radioactive.
What happens if more than one neutron is released when the uranium nucleus splits in small scale nuclear fusion?
These neutrons can cause further uranium nuclei to split.
How do scientists stop nuclear reactions from getting out of control?
By placing control rods in the reactor. The rods absorb some of the neutrons, preventing further fissions. They can be lowered or raised to control the number of neutrons available for fission, which allows the process to keep operating safely.
Why is nuclear fusion not yet a possible energy source on earth?
Because when two nuclei fuse together, a large amount of heat energy is released. This only occurs at extremely high temperatures, and it is very difficult to manage these high temperatures.
What is an example of nuclear fusion?
When two hydrogen nuclei join to for a helium nucleus. This takes place in both stars and fusion bombs (also called H-bombs or hydrogen bombs)
In a fusion bomb, what are the initial high temperatures needed produced by?
A fission reaction
Why, for decades, have scientists been attempting to reproduce fusion reactions under controlled conditions in laboratories?
Because if the reaction can be controlled then the energy released could be harnessed and used as an alternative energy source, replacing fossil fuels.
What does the term 'cold fusion' refer to?
A fusion reaction that occurs at room temperature.
Which two scientists claimed to have achieved a cold fusion reaction, and when?
Martin Fleischmann and Stanley Pons, in 1989
Why, when a new discovery is made, is it important to share data with other scientists?
Because then Scientists from across the world are then able to try to replicate the experiment. They must check that the experiment can be repeated and that the same data can be produced. This shows that the discovery is genuine, and it doesn't show anomalous results. Only when experiments are repeated, and matching data continually collected, can a new theory be accepted.