Investigating the inverse square law Essay

The intensity of the influence at each given radius, r, is the etymon strength split up by the bea of the sphere. 1 The contrary square police force stop in any case be applied to gravity, galvanic fields, lite and sound. In relation to electric fields, the electric force in Coulombs law follows the inverse square law If da Gamma rays are a contour line of electromagnetic radiation syndrome and undergo trifling absorption in air, then the intensity, I, should quit inversely as the square of the keep surrounded by the bug and the detector.2 aviation acts as an almost transparent strong point to ? -rays, and the intensity ( point of energy arrival per whole area) of ?-rays emanating from a point reservoir varies inversely as the square of the aloofness from the root system. 3 ?-rays fall into many distinct monoenergetic groups because of their shifting energies which emanate from any particular emitter. The to the lowest degree energetic radiation leave alone completely pass through very nice foils, whereas the most energetic give the axe while away up to several cen eratres of lead.4 As ? -rays t rise to produce 10-4 times as many ion-pairs per unit length as ? -particles do, measurements are usually carried out apply a Geiger-Mi ller (G-M) pipe. 5 G-M furnishs are widely utilise for detecting radiation and ionising particles.The anode is unploughed at a positive potential and the cathode is earthed. The underground may also grant a thin mica end windowpane. 6 When radiation enters the resistance, a a couple of(prenominal) electrons and ions are produced in the gas. If the electromotive force is preceding(prenominal) the breakdown potential (The minimum reverse voltage to make the diode wear in reverse)7 of the gas, the number of electrons and ions are greatly multiplied. The electrons are attracted to the anode, and the positive ions move towards the cathode.The circulating(prenominal) flowing in the high oppone nt resistor (R) produces a pd which is amplified and passed to a counter which registers the passage of an ionising particle or radiation through the thermionic thermionic valve. 8 The vacuum tube can non be filled with air as the discharge persists for a short time by and by the radiation is registered. This is due to electrons universe emitted from the cathode by the positive ions which arrive there. Instead, the tube is filled with argon mixed with a halogen vapour which quenches, reduces the intensity, the discharge quickly, ensuring that the registered radiation does non reach the recording of other ionising particles.When the G-M tube is detecting one particle, if a nonher enters the tube it go away not be detected. This is cognise as dead time the second- wander maximum being nigh 90 microseconds. 9 Because this number is so small, it can justifiably be ignored for this experiment. accentuate radiation must be watchn into account when taking understandings fro m the descent. Background radiation primarily comes from cosmic radiation and planetal sources. 10 This radiation will affect the count and must be corrected. The direct of this radiation varies with location and must be measured before conducting the experiment.N0e-? t so ? = gradient/ N0e-? t Safety Precautions To mark off the utmost safety before, during and later this experiment, few guidelines should be followedFood and drink should not be consumed whilst in the same live as the source Food items should not be stored in the same dwell as the source.The source should plainly be handled with long handled source handling tongs, and as little as possible custody should be washed thoroughly afterwards finish up with the source If in touch on with the source for an extended period, it is recommended that a observe badge is worn As the source will radiate in only one direction, it should not be pointed at anyone The source should be locked away in a lead lined rap when not in use unclouded wounds should be covered securely.Protective gloves should be warn when handling potentially colly items Errors To reduce the possible erroneousnesss within the experiment, an visual bench will be used to agree that the G-M tube and the source are properly aligned throughout, as the source radiates in one direction, the alinement must remain standard. Also, for small outmatchs, specifically the infinite d0 which is the distance the source is from the opening of the toter plus the distance of detection from the window in the G-M tube, vernier callipers will be used to hold as such(prenominal) truth as possible.Vernier callipers read to fractions of a milli touchstone, making them much much accu yard than other measuring devices. some other distances, such as distance d, can be measured with a metre rule as the distances are big which decreases the possible error in measuring. thither will also be the error of human reaction times from observant the fi nal count and pressing the stopclock. To ensure accuracy, practise using the stop-clock and count turn until reasonably consistent results can be obtained. Preliminary Work To uncovering out on an appropriate voltage to use, the G-M tube and source set-up should be tested.Place the source approximately 10 cm from the window of the G-M tube and increase the voltage slowly, until the count evaluate stops changing dramatically. Plot a graphical record of the count-rate, C, against EHT voltage, V. Record the voltages V1 and V2 between which the rate of run does not vary in any case much. If the rate of counting begins to rise after remaining much the same for a range of voltage do not raise the voltage any higher(prenominal) or the tube may concentrate damage. 14 The optimum operating voltage will be halfway between the voltage where the plateau begins and the voltage where it ends.To decide on the range of distances used, the source was travel close to the window of the G-M tube and was move covering slowly until the scaler could count qualifiedly (5 cm). This is the smallest distance that will be used. To find the other extreme, the source was moved back until the count rate fell to a low value, but could still hand over adequate results (35 cm).From these approach results I contribute decided to time for 10,000 counts at 5 cm from the source, 5000 counts for 10cm from the source, and 1000 for 15 30cm. This is because any higher values will take considerably longer to measure. I will take three developments from each, as hot decay is a random performance and it would be unlikely for more than three readings to be similar. An average will be calculated from the three values and the reading for the priming coat radiation will be subtracted to find the corrected count rate. Equipment Geiger-Mi ller tube of i sensitive type.Decade scaler with multivariate star EHT supply Sealed cobalt-60 source close to interrupt contact with the source and to prevent isotropic radiation Long handled source handling tongs to prevent contact with the source Optical bench with source carrier to ensure constant alignment Stop-clock, readable to at least twain decimal places Vernier callipers to measure the distance d0 to a higher level of accuracy Metre rule to measure the distance d Diagram Where B is the ocular bench with source holder, H G is the Geiger-Mi ller tube S is the decade scaler with variable EHT supply.R is the sealed radioactive source, cobalt-60 Cobalt-60 will be used as the gamma source as it is easily produced, by exposing natural cobalt to neutrons in a reactor, and therefore easy to acquire. 15 It also produces ? -rays with energies of 1. 17 MeV and 1. 33 MeV. Method 1. hug the G-M tube to one end of the ocular bench and attach it to the input socket of the scaler 2. company the variable EHT voltage on the scaler at a minimum and turn it on, allowing a few minutes for the scaler to warm up 3. Change the variable EHT voltage on the scaler to the value found through preliminary work and set it to count pulses from the G-M tube.4. function the stopclock and measure the scope radiation for an adequate length of time, e. g. 25 minutes, as background radiation is variable 5. Place the holder containing the ? -source at 5. 0 cm from the window of the G-M tube 6. Start the stopclock and stop after 10,000 counts are registered. Record this value and restate twice 7. instill the ? -source to 10. 0 cm from the window of the G-M tube and repeat effect 5, instead only counting 5000 counts 8. Move the ? -source to 15. 0 cm from the window of the G-M tube and repeat procedure 5, instead counting only 1000 counts 9.Repeat procedure 7 for sets of 5. 0 cm until a distance of 30. 0 cm is reached 10. tabulate these results and find the average count rate for each distance 11. Evaluate 1/(d + do)2 12. Using the recorded value for background radiation, evaluate the corrected count rate for each distance 1 3. Plot the graph of corrected count rate against 1/(d + do)2 1 http//hyperphysics. phy-astr. gsu. edu/Hbase/forces/isq. hypertext mark-up language 2 substantial Pre-University physical science Whelan & Hodgson, page 953 3 ingrained Principles of natural philosophy Whelan & Hodgson, page 472 4 inborn Principles of physics Whelan & Hodgson, page 472.5 meaty Principles of Physics Whelan & Hodgson, page 472 6 http//www. imagesco. com/articles/geiger/03. html 7 http//en. wikipedia. org/wiki/Breakdown_voltage 8 Essential Pre-University Physics Whelan & Hodgson, page 406 9 http//www. imagesco. com/articles/geiger/03. html 10 http//en. wikipedia. org/wiki/Background_radiation 11 Advanced Level practical Physics M Nelkon & JM Ogborn, page 218 12 A Laboratory Manual of Physics F. Tyler, page 269 13 http//en. wikipedia. org/wiki/Cobalt 14 Advanced Level Practical Physics M Nelkon & JM Ogborn, page 212 15 http//en. wikipedia. org/wiki/Cobalt. theme http//en.wikipedia. org/wiki/ Geiger-M%C3%BCller_tube The anode is a central thin telegram which is insulated from the surrounding cathode cylinder, which is metal or graphite coated.