P4 Key facts sheet: Key Fact

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1 A: Sparks P4 Key facts sheet: Insulating materials can become charged when rubbed with another insulating material due to friction. This causes a transfer of electrons, resulting in both materials becoming charged. There are 2 kinds of charge; positive and negative. Like charges repel and unlike charges attract You can get an electrostatic chock if you become charged and then become earthed (i.e. touch a conductor): touching metal water pipes after walking on a floor covered with an insulating material, e.g. synthetic carpet static electricity can be dangerous: In atmospheres where explosions could occur, e.g. inflammable gases or vapours or with high concentrations of oxygen (fuelling aircraft) In situations where large quantities of charge could flow through the body to earth static electricity can be a nuisance: Dirt and dust attracted to insulators (plastic containers, TV monitors etc) Causing clothing to cling The chance of receiving an electric shock in some of the above situation can be reduced by: using correct earthing; use of insulating mats; using shoes with insulating soles; bonding a fuel tanker to aircraft (H) Anti-static sprays, liquids and cloths help reduce the problems of static electricity. Antistatic sprays work by making the surface of the charged surface conductive, to make an easy path for the charges to move away. Cloths are conductive, so they carry away any charges from the objects they are used to wipe (H) Static electricity is created by the movement of electrons: a positive charge due to the lack of electrons; a negative charge due to an excess of electrons (H) Atoms or molecules that have become charged are ions (H)

2 B: Uses of Electrostatics Static electricity can be useful for electrostatic dust precipitators to remove smoke particles etc from chimneys: The dust passes through a charged metal grid or past charged rods inside the chimney. The dust particles become charged and stick to plates which are earthed or charged opposite to the grid. The plates are then struck and the dust falls to collector where it can be removed Static electricity can be useful for paint spraying: The spray gun is charged; paint particles are charged the same so repel giving a fine spray and coat. The object being painted is charged oppositely to the paint so it attracts paint into the shadows of the object giving an even coat and less waste Static electricity can be useful for restarting the heart when it has stopped (defibrillator): The paddles are charged. There needs to be good electrical contact with patient s chest. The charge is passed through the patient to make the heart contract. Care must be taken not to shock operator, and other people should stand away to avoid a shock. Static electricity is used in electrostatic dust precipitators to remove smoke particles etc from chimneys: high voltage metal grids are put into chimneys to produce a charge on the dust, so the dust particles gain or lose electrons. The dust particles induce a charge on the earthed metal plate which means the dust particles are attracted to the plates (H) In addition to Fact 2 (above): static electricity is used in paint spraying, in terms of the paint and the car both gaining and/or losing electrons. The car and paint are oppositely charged to ensure a smooth finish and so that shadow areas are also evenly painted (H) questions below...

3 C: Safe Electricals Electric charges (electrons) flow around a circuit A wire can be used as a variable resistor in a circuit: longer wires give less current (more resistance); thinner wires give less current (more resistance) Resistance is measured in Ohms (Ω) The relationships between current, voltage (pd) and resistance are as follows: - For a given resistor, current increases as voltage increases and vice versa - For a fixed voltage, current decreases as resistance increases and vice versa Resistance = Voltage Current (you need to rearrange this on the Higher paper) The colour coding for live, neutral and earth electrical wires are: live brown (carries high voltage); neutral blue (completes the circuit); earth green/yellow (a safety wire to stop the appliance becoming live) An earthed conductor cannot become live and you will not get a shock Fuses and circuit breakers (as re-settable fuses) are used to reduce the risk of fire: If the appliance develops a fault and too large a current flow, the fuse will melt. This prevents flow of current, prevents the flex overheating and causing a fire, and prevents further damage to appliance Power = Voltage x Current (Watts) Double insulated appliances do not need earthing: the appliance is a non conductor and cannot become live. This is because it has a casing made of an insulating material, so if the wire inside becomes loose and touches the inner casing, the electricity cannot conduct through it. The combination of a wire fuse and earthing protects people; the fuse wire to protect the appliance (as described above) and the earth wire to prevent people from getting a shock if there is a fault with the live wire (H) Power = Voltage x Current can be used to work out the current flowing through an appliance (rearrange the equation). The fuse should be chosen based on this value of current; the fuse chosen should always be the lowest value above the usual current used by the appliance (H)

4 D: Ultrasound Ultrasound is a longitudinal wave. Longitudinal waves have: wavelength; frequency; compression (higher pressure); rarefaction (lower pressure) find a diagram to label these The frequency of ultrasound is higher than the upper threshold of human hearing (20,000 Hz) because the ear cannot detect these very high frequencies Ultrasound can be used in medicine for diagnostic purposes (to diagnose problems) by: looking inside people by scanning the body (e.g. pregnancy scans, or organ scans), or measuring the speed of blood flow in the body to check whether arteries are constricted The high frequency vibration of ultrasound can be used in medicine for non-invasive (nonsurgical) therapeutic purposes such as to break down kidney stones and other stones Ultrasound is used for: body scans (the waves reflect from different layers of body tissue, returning at different times from different depths); and breaking down accumulations in the body such as kidney stones (H) Reasons for using ultrasound rather than X-rays for certain scans are: Ultrasound is able to produce images of soft tissue, unlike X-rays which only image hard tissue; Ultrasound does not damage living cells whereas X-rays can (H) In longitudinal waves, the particle vibrations are parallel to the motion of the wave energy. In transverse waves, the particle vibrations are perpendicular to the motion of the wave energy. The more energy, the higher the amplitude (H)

5 E: What is Radioactivity The radioactivity or activity of an object is measured by the number of nuclear decays emitted per second The radioactivity of a source decreases with time Radioactive substances are unstable sources that decay naturally, giving out nuclear radiation in the form of alpha, beta and gamma Half life is the time it takes for half of the unstable nuclei in a source to decay (i.e. the time for the activity of a sample to halve). It can be calculated from a graph (learn to do this) Nuclear radiation comes from the nucleus of an atom and can ionise materials (the removal or gain of electrons) An alpha particle is a helium nucleus; a beta particle is a fast moving electron When an alpha particle is emitted from a nucleus: the mass number decreases by four (nucleus has two fewer neutrons and two fewer protons); the atomic number decreases by two so a new element is formed (H) When a beta particle is emitted from a nucleus: the mass number is unchanged because the nucleus has one less neutron but also has one more proton (the neutron turns into a proton, emitting a beta particle as it does so). Because the atomic number increases by one, a new element formed (H) Alpha particles are good ionisers because they are large, so it is easy for them to collide with other atoms/molecules. They are also highly charged, so they can easily remove electrons from the atoms they pass by (H) You must be able to construct and balance nuclear equations in terms of mass numbers and atomic numbers to represent alpha and beta decay. Use your revision guide to practice this. (H)

6 F: Uses of Radioisotopes Background radiation is the radiation that is all around us. It mainly comes naturally from rocks and cosmic rays Some background radiation comes from waste products and man-made sources, e.g. waste from: industry and hospitals Radioactive tracers are used in industry: The radioactive material is put into an underground pipe; its progress is tracked with detector above the ground/outside of the pipe. The leak/blockage shown by a reduction (or no radioactivity detected) after the point of blockage Smoke detectors use alpha sources to work: Without smoke, the alpha radiation ionises the air particles, which creates a current in the alarm. When smoke particles get in the way there is less ionisation of air particles and the current is reduced. This causes the alarm to sound. Radioactive dating of rocks depends on the calculation of the uranium/lead ratio. The Uranium decays into lead, so after 1 half life, the ratio of uranium to lead will be 1: After 2 half lives it will be 1:3 (¼ uranium : ¾ lead) Measurements from radioactive carbon can be used to find the date of old (once living) materials such as plants and animals (or materials made from these, such as paper) Measurements of the activity of radioactive carbon can lead to an approximate age for different materials: The amount of carbon-14 in the air has not changed for thousands of years and this amount is found in all living things. When an object dies (e.g. wood) gaseous exchange with the air stops and the carbon-14 is not replaced. As the carbon- 14 in the wood decays the activity of the sample decreases, so the ratio of current activity from living matter to the activity of the dead sample is used to calculate the age within known limits (H) The background radiation you experience will depend on where you live or where you work. If you work in an industry that uses radiation, or if you live in an area where the rocks are particularly radioactive, or even if you are an airline pilot (more cosmic rays) the relative significance of the sources of background radiation will change (H) Gamma radiation is used as an industrial tracer because it can pass through objects such as underground pipes to be detected, whereas beta or alpha would be absorbed (H)

7 G: Treatment X-rays and gamma rays are similar because they are both ionising electromagnetic waves and have similar wavelengths, but they are produced in different ways Radiographers may take X-rays (diagnostic) or treat people with cancer (therapeutic) in hospital. Because they use high energy radiation, they must wear protective clothing, such as lead lined aprons, to reduce their exposure by absorbing some of the radiation. With X-rays, the image is produced by the absorption of X-rays into the bones. The amount of absorption depends on the thickness and density of the absorbing materials Medical radioisotopes are produced by placing materials into a nuclear reactor where they absorb extra neutrons, making them unstable Nuclear radiation is used in medicine. This includes: diagnosis and treatment of cancer using gamma rays. It can also be used to sterilise equipment as it can kill any bacteria on it Only beta and gamma radiation can pass through skin. Alpha is always absorbed Medical tracers should not remain active in the body for long periods in case they cause damage to the cells (increasing risks of cancer). It is important therefore to use a tracer with a short half life. Gamma rays are given out from the nucleus of certain radioactive materials. Some radioisotopes are created for use in medicine. X-rays are made differently; by firing high speed electrons at metal targets which creates X-rays when they collide. This makes X-rays easier to control than gamma rays (H) Radioactive sources are used in medicine: 1) to treat cancer: gamma rays are focused on tumour; a wide beam used and rotated round the patient with tumour at centre. This limits damage to non-cancerous tissues. 2) as a tracer: beta or gamma emitters with a short half life are used; they are drunk/eaten/injected into the body and allowed to spread through the body. This is followed on the outside by a radiation detector (H)

8 H: Fission and Fusion Nuclear power stations use Uranium as their fuel. Heat from the Uranium is used to used to produce steam by heating water. This is then used to produce electricity The process that gives out energy in a nuclear reactor is called nuclear fission and it produces radioactive waste Fission is the splitting of nuclei; Fusion is the joining of nuclei Nuclear fusion releases energy: Fusion happens when two nuclei join together and it produces large amounts of heat energy. Fusion happens at extremely high temperatures and is not the process which is used in nuclear power stations (which is Fission) Fusion for power generation is difficult: It requires extremely high temperatures and these have to be safely managed. Fusion research and power generation is a joint international venture, to ensure that scientists are sharing ideas, expertise and resources (e.g. money). Domestic electricity is generated at a power stations using nuclear fuel where a nuclear reaction produces heat; this heats water to produce steam; the steam spins a turbine and this drives a generator Uranium decays in a chain reaction and this needs to be controlled. A nuclear bomb is just a chain reaction that has gone out of control. One group of scientists have claimed to successfully achieve cold fusion (nuclear fusion, but at room temperature), but their findings were discovered to be unreliable (nobody who repeated the experiment got the same answers) The cold fusion experiments and data were shared between scientists to make sure that the experiment was tested multiple times Cold fusion is still not accepted as a realistic method of energy production because it is just so difficult to do! (H) Different isotopes of hydrogen can undergo fusion to form helium, e.g. (H) In stars, fusion happens under extremely high temperatures and pressures. This is what we want to replicate on Earth because it releases so much energy. Fusion bombs are started with a fission reaction which creates exceptionally high temperatures and enables fusion to occur. For power generation, exceptionally high temperatures and/or pressures are required and this combination offers (to date) safety and practical challenges which means it is not a viable option (H) Uranium releases energy when the uranium nucleus is hit by a neutron; The collision causes the uranium nucleus to split, releasing energy. 3 more neutrons are also released in this process which go on to split further uranium nuclei (H) The term chain reaction means that when each uranium nucleus splits, more than one neutron is given out (3 in this case); these neutrons can cause further uranium nuclei to split (H) Scientists stop nuclear reactions going out of control by placing Boron rods in the reactor. These absorb some of the neutrons, allowing just enough neutrons to remain to keep the process operating (H) questions below...