Physics - P4

These revision cards go through in detail the physics OCR gateway science topic, P4, grade by grade.

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When an object is earthed it loses its charge and is discharged so any charge on it is removed as any electrons from the object are transferred from the object to the earth, if you touch the charged object the charge will be earthed and you shall receive  an electrostatic shock.  Ultrasound is given to the sound that is above the human hearing range of 20 000HZ and greater. Ultrasound has many uses : Ultrasonic waves can be used to make kidney stones vibrate and they will then break down and can safely pass through the urine without any need for any painful surgery : also ultrasound can be used for body scans as it can make images of inside the body it does this by the waves being partially reflected at boundaries - we can use the time at how long it takes to reflect to calculate the depth of the surface- then the reflected waves can be used to regenerate an image on a screen. Ultrasound imaging has to main advantages - it can produce images of soft tissue and also it doesn't damage living cells. Gamma rays also has many uses - they can pass through the skin and damage cells therfore this is a useful prpery as it can damage cancer cells however cation has to be taken not to damage any healthy cells so lots of beams of gamma rays are centred on the tumor and rotated around the body with the beam at the centre so to concentrate the gamma rays onto the tumor and minimize the impact on healthy cells. Also gamma rays can be used to sterilize equipment and as a tracer - a small radioactive amount if material is either injected or swallowed into the body , the material must be able to penetrate out of the body so it must emit beta or gamma rays and a radiation detector outside the body will detect the progress of the place being examined and how well it works.

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Electrical current is a the flow of charge which consists of moving electrons in a conductor such as a wire, Resistance is how much strain is acting on the current whilst it is passing through the wire, for example id there is a higher resistance the current travels slower and there is therefore a lower resistance. Alpha decay/emission and beta decay/emission occur and sometimes emit a nucleus containing surplus energy and this emits as gamma radiation - gamma has no effect on the structure of the nucleus . Alpha Emission involves ejecting an alpha particle from the nucleous  and a new nucleus is formed but the nucleus is different as  it is a different element and has 2 less protons and neutrons and therefore the atomic number decreases by 2 and the mass number decreases by 4. During Beta emission the beta atom decays changing its neutron into a ejected beta particle electron and a proton and a new atom is formed by beta decay, the nucleus is now a different element and has 1 more proton and 1 less proton so the atomic number increases by 1 and mass number remains the same. Every human or animal on earth has a small sample of radioactive carbon-14 and when it dies the object does not exchange gases anymore and therfore the carbon-14 decays and the radioactivity of the object decreses, using the ratio of a living objects radioactivity and the dead one we can estimate the age of the object this is also known as carbon dating - the amount of carbon-14 has not changed in the air for a long period of time now.

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An object can become positively charged if a material loses electrons and the material that gains electrons becomes negatively charged this means each material now has a negative charge. An electrostatic dust precipitator is used to reduce the amount of smoke particles in a chimney, a metal grid that is connected to a high voltage is attached to the chimney and the dust particles are attracted to the metal grid and when the dust is heavy enough it clumps together and falls. A fuse works by stopping the circuit of an appliance when a fault occurs, then the live - brown- wire makes contact with the neutral -blue- then the current increases as less resistance and then the fuse wire will melt breaking the circuit as it is above the current rating, this protects the appliance becoming live and causing injury or death it doesn't allow fires as wires cant overheat and no damage to appliance. Circuit breakers do the same job as fuses but can be reset, the current rating of a object is slightly above normal current that flows. Radioisotopes are useful as can find out what occurs in an object without breaking into it i.e if we get a radioisotope that emits gamma we can put it in a pipe and follow it, a blockage the radioisotope will stop flowing and if  a leakage then the radioisotope shall escape up to the surface. We also use radioisotopes to follow waste dispersal and finding routes of pipes.Nuclear fission make heat energy in a nuclear reactor, an example of nuclear fission process is uranium absorbing a neutron casing it to split into two again etc. this is a chain reaction large scale. Small scale-  bombarding a uranium atom with a neutron causing it to split and energy is given out, a nuclear bomb is a chain reaction out of control, to control nuclear fission we use control rods absorb neutrons to stop fissions these rods can be lowered or raised

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Static electricity can be dangerous as it can lead to an explosion of a static electricity comes into contact with flammable gases as it can ignite them causing an explosion also if large quantities go through your body it can also damge you. We can reduce the chance by wearinf insulating sole shoes,use insulation mats or we could earth things such as vehicles that carry flaamble substances ned to be earthed before as a build up of friction can lead to a charge which could lead to a spark. Antistatic sprays, liquids and clothes transfer of charge from one insulator to another and therfore no accumalation of charge. At times static electricity can be a nuciance as it can make dust stick to tv screens and insulators and also it may cause some cloths to stick to your body. A deffibralater uses two charged paddles to contract the heart by delivering charge  and they have to be put in good electrical to the chest so hair may be shaved off, care taken not to shock operator. Static electricity is also used in car painting, surface is positive ccharge and paint negative charge in negatively charged gun so the particles repel making finer smoother paint and less wasted paint and even back of object sprayed. Also static electricityused in photocopiers and laser image put onto electrically charged plate and light leaks charge away leaving impression of image and it attracts opp charged blck specks of paint and then prints image on paper..AC is used in power stations by rotating an electro magnet arouna a coil of wire wihch produces an alternating current. Ultrasound is a longitudanal wave so each particle movbes back and forth in the same position and place of as direction of wave movement. Tranverse waves each particle moves up and down at right angles to direction of wave movement at normal position.All waves transfer energy without tranferring any particles.X rays are made by firing high speed electrons at a metal target, the electrons lose energy rather quickly and are easier to control than gamma rays, x-rays pass easily through soft tissue but not through bone so create a shadow image, by a radiographer

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The half life of a radioisotope  is the time it takes for half the undecayed nuclei in radioactive substance to decay therfore if it has a long half life it remains radioacitve for a long time. Igneous rocks contain unstable uranium atoms and produce lead stable atoms, uranium has a very long half life. Smoke alarms work by havins amercian-241 -an alpha emitter- the emitted alpha particle ionise the air particles the air ions then attract to the oppisitly charged electrodes producing a current, when smoke enters there is less ionisation betweeen the 2 electrodes so a smaller current flows causing the alarm to sound. Background radiation is all around us in small concentrations and comes from both natural sources, such as released radioactive substances in the soil and rocks and also cosmic rays. Manmade sources include waste producets, industry and hospitals.  Substances are radioactive if they have an unstable nucleous so the nucleus emits out nucleur radation and therfore the substances decay.An alpha particle has a heliumn nucleus which means that it contains two neutrons and two protons, a beta particle is a fast moving electron and a gamma particle is an electromagnetic wave. Radiation is measured by the number of nuclear decays per secind, the decays reduce by time.Radioactivity is used in medicine to steralize equipment, break down kidney stones, as tracers, treating cancer as it kills cells and can penatrate through skin, gamma rays and x-rays have similar wave length so can be used in medicine.In a nuclear power station uranium is used as an energy sorce in a nuclear reactor  so to make heat and then steam driving turbines. and turns generator to generate electric, in a conventional power station its the same but with fossil fuels as an energy source. The process in a power station  that releases energy is nucleor fission.

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The two types of electrical charge are positive and negative. Static electricity is used in dusting brushes to attract the dust to an oppisitly charged brush, and also in photocopiers, laser printers, defibillarators, painting, reducing smoke particles in chimmenys. But it can be dangerous when flammable gases in a lorry cause friction making a spark which could ignite the gases and cause an explosion and also when lightining flows through your body. Also it can be a nuciouse as it can cling clothing to the body, make your hair stick up and alsodust may attract to insulatinf surfaces such as tv and computer screens. A variable resistor is a ressitor that has a sliding metre so that you can change the resistance of the circuit to ncrease or reduce the current in an electricel circuit. To make electrical products safe each appliance has 3 wires, earth - yellow and green- , live-brown- and neutral - blue. The live wire carries the current at a high voltage and the fuse is always part of the live wire. The neutral wire carries the current away from the appliance and the earth wire stops object becoming live and is also known as the safety wire. All things with outer metal cases muct be earthed  so the current short circuits to earth wire which offers less resistance and fuse melts so circuit broken, some appliances are double insulated objects don't need earthing as the cases are insulaters so dont need an earth wire as cannot become live. However they still have a fuse wire just in case, if the live wires in a double insulated appliance touch it still does not matter as the electrical object can still not become live. For a fixed resistor, the current increses as the voltage increses as it has more power, however if the resistance increses the current travels slower and is held back resulting to a reduced amount of current.

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intresting find



thank you so much



Very Helpful :) 

Miss Meera J




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