Protons for Breakfast Are Mobile Phones Safe? Week 5 - PowerPoint PPT Presentation

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Protons for Breakfast Are Mobile Phones Safe? Week 5

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  1. Protons for BreakfastAre Mobile Phones Safe?Week 5 March 2013

  2. In the event of… In the event of… In the event of a new ice age… In the event of…

  3. This evening… 0. The Media! • How do mobile phones work? • What is the hazard? • How do electromagnetic waves interact with matter? • SAR • Microwave Ovens • Mobile Phone • Are mobile phones safe?

  4. The word ‘radiation’ • The word ‘radiation’ means • Anything which ‘radiates’ on ray-like paths • Could be • Sound • Radio • Light • Infra-red light • Particles or waves emitted from the nuclei of atoms • Nuclear Radiation

  5. Media

  6. Media

  7. Sunday, 20 January 2008 It also complements other recent research. A massive study, following 1,656 Belgian teenagers for a year, found most of them used their phones after going to bed. It concluded that those who did this once a week were more than three times – and those who used them more often more than five times – as likely to be "very tired".

  8. 1. How do mobile phones work?

  9. Magic? Any sufficiently advanced technology is indistinguishable from magic Arthur C. Clarke

  10. Not magic • Each colour represents a base station operating at a slightly different frequency • Mobile Phones are ‘radio’ phones • Operating at microwave frequencies • Handset power minimised by having a network of local transmitters and receivers • 52,500 base stations in UK

  11. Mobile Phones (3)Recognise the masts? • Each cell has a transmitter and receiver mast • Notice the typical three way structure

  12. 3G Masts Photo Credit Brighton and Hove Green Party

  13. Linking to base stations • Mastpower between 60 watts and 120 watts • Handsetpower less than 1 watt • Handsetsends signals every few minutes to establish which is the nearest mast • A central computer • keeps track of all the telephones that are switched on • remembers which cell they are in

  14. Making a mobile-to-mobile call Network Control Knows in which cell every telephone is Receiving Telephone Originating Telephone Its complicated!

  15. 2G or GSM phone Encoding to Radio Signal Peak Power 2 W Maximum Average power is 0.25 W Maximum Hello! How are you? No sound: No signal 217 pulses of encoded sound every second Global System for Mobile Communications Hello! How are you? Sound

  16. 2G or GSM phone

  17. Anatomy of a GSM Signal

  18. 2G or GSM phone 217 Pulses per second per telephone call Each frequency channel can carry 7 calls 217 Pulses per second

  19. Safety • The safety issues surrounding mobile phones concern… • the interactionsof the microwaves emitted by • mobile phones • base stations with human tissue

  20. And its not just mobile phones! • ‘WiFi’ Wireless Networking • Bluetooth devices • Wireless keyboards and mice • DECT cordless phones • Baby Monitors • ‘Walkie Talkie’ • All involve electromagnetic waves in the radio and microwave part of the spectrum

  21. 2. How do electromagnetic waves interact with matter? ?

  22. Electromagnetic spectrum Radio & TV Infra Red Ultra Violet Gamma-Rays • Microwaves • 0.8 GHzto • 1000 GHz Microwaves X-Rays Non-ionising Radiation (generally not so bad) Ionising Radiation (generally bad) 1011 109 1010 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 102 103 104 105 106 107 108 1 101 Frequency (Hertz)

  23. Waves & Matter • When an electromagnetic wave interacts with matter… • Some absorption • Some reflection • Some transmission

  24. Summary • Electromagnetic waves interacting with matter can be • Reflected, absorbed or transmitted • What happens depends on • the frequency of the electric field • the natural frequencies of the atoms and molecules • Microwaves emitted by mobile phone systems • Are absorbed by human tissue

  25. Waves Radio & TV Infra Red Ultra Violet Gamma Rays Microwaves X-rays What vibrates? Atoms in solids or molecules Electrons in Atoms Entire molecules or parts of molecules Protons in nucleus Lighter particles Stronger forces 1011 109 1010 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 102 103 104 105 106 107 108 1 101 Frequency (Hertz)

  26. Absorption demo • Near the natural frequency • Absorption • Well below the natural frequency • Not much absorption • Well above the natural frequency • Not much absorption Frequency

  27. Waves & Matter Electricity Atoms Heat Electromagnetic waves • When electromagnetic waves are ‘absorbed’ … • Atoms jiggle faster: • i.e. increase their temperature.

  28. 3. How muchenergy gets absorbed? S.A.R. Specific Energy Absorption Rate

  29. Terminology • Power • watts • Intensity • watts per square metre • Specific energy Absorption Rate (SAR) • watts per kilogram

  30. SAR: Example using light • rather than microwaves • 1 cm from 20 W source 20 W absorbed in hand Intensity 8000 watts per square metre SAR 200 watts per kilogram • 10 cm from 20 W source • 2 W absorbed in hand • Intensity 200 watts per square metre • SAR 20 watts per kilogram • 1 metre from 20 W source • 0.02 W absorbed in hand • Intensity 5 watts per square metre • SAR 0.2 watts per kilogram

  31. Microwave Hazard The potential hazard from mobile phones and other wireless devices arises from the absorption of microwave radiation • Mobile phones • only emit a watt or two • so little power it makes experiments difficult • Microwave ovens • emit a few hundred watts • makes experiments easy

  32. Microwave Ovens 3a. Microwaveovens

  33. Electromagneticspectrum Microwave Oven 2.45 GHz Conventional Oven200 °C Gamma-Rays Radio & TV Infra Red Ultra Violet • Microwaves • From • 0.8 GHzto • 1000 GHz Microwaves Microwaves X-Rays 1011 109 1010 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1 102 103 104 105 106 107 108 101

  34. Absorption of microwaves by liquid water • Near the natural frequency • Absorption • Well above the natural frequency • Not much absorption • Well below the natural frequency • Not much absorption 2.45 GHz

  35. Absorption (1)Too strong • If power were absorbed too strongly, • Microwaves would only penetrate a short distance • Surface would be heated • Inside would remain uncooked

  36. Absorption (2)Too weak • If power were absorbed too weakly, • Microwaves would go right through • No cooking

  37. Absorption (3) • If power is absorbed just right, • Microwaves penetrate about 5 cm (2 inches) • Cooks the outer 5 cm of the food • Good enough for most cases

  38. Microwave OvensSummary • A microwave oven cooks food by heating it • The heating comes from • intense waves at 2.45 GHz • (Instead of a wide spectrum of waves at infra red frequencies) • Frequency chosen because of absorption properties of water molecules at that frequency.

  39. Microwave Ovens Inside a microwave oven

  40. Microwave Power • Power • This is a 700 watt oven • Think of 7 x 100 watt light bulbs

  41. Microwave Electric field • 700 watts • Around 140 000 volts per metre • Look what happens to a CD

  42. Microwave Intensity • Between 104to 105watts per square metre • (Most intense sunlight around 103 watts per square metre) • Very Dangerous • Could I have a stupid volunteer please?

  43. Microwave OvenSAR inside oven • 700 watts absorbed in 1 kg of water: • SAR = 700 watts per kg • Question: After1 minute, how hot would your brain become if subject to an SAR of 700 watts per kg?

  44. Experiment

  45. Microwave OvensComparison with handsets • Expect 1/700 of microwave temperature rise • The effects of blood flow reduce this further 0.01 °C (ish)

  46. Back to Mobile Phones 3b. Mobile Phone SAR

  47. Mobile Phones (14) SAR and Safety New Style Old Style

  48. Mobile Phones (8) Your telephone • Look in the small print!

  49. Typical SAR with phone near the head • SAR Averaged over 10 grams • By law must be less than 2 watts per kilogram • Typical figures

  50. Base Station SAR