Are mobile phones safe? Protons for Breakfast Week 5
How it all fits together… Electricity Heat Electromagnetic waves Atoms
What this evening is about… 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?
Warning…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
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".
Mobile Phones 1. How do mobile phones work?
Mobile Phones (1) Magic? Any sufficiently advanced technology is indistinguishable from magic Arthur C. Clarke
Mobile Phones (2)Not magic • Each colour represents a base station operating at a slightly different frequency • Mobile Phones are ‘radio’ phones • Operating frequency is in the microwave region of the spectrum • Handset power is kept to a minimum by having a network of local transmitters and receivers • 50,000 base stations in UK
Mobile Phones (3)Recognise the masts? • Each cell has a transmitter and receiver mast • Notice the typical three way structure
Mobile Phones (4)New 3G Masts Photo Credit Brighton and Hove Green Party
Mobile Phones (4)Linking to base stations • Masts broadcast with a power of between 60 watts to 120 watts • Handsets broadcast with a power of Maximum Power of 2 watt • Handsets send 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
Mobile Phones (5)Making a mobile-to-mobile call Network Control Knows in which cell every telephone is Receiving Telephone Originating Telephone Its complicated!
Mobile Phones (6)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 Hello! How are you? Sound
Mobile phone 2G or GSM phone 217 Pulses per second per telephone call Each frequency channel can carry 7 calls 217 Pulses per second
Mobile phone Safety • The safety issues surrounding mobile phones concern… • the interactionsof the microwaves emitted by • mobile phones • base stations with human tissue
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
Mobile Phones 2. How do electromagnetic waves interact with matter? ?
Electromagneticspectrum 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)
Waves & Matter (5) • When an electromagnetic wave interacts with matter… • Some absorption • Some reflection • Some transmission
Waves & Matter (6)Summary • Electromagnetic waves interacting with matter can be • Reflected • Absorbed • Transmitted • Exactly 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
What ‘absorbs’ the energy?‘Natural’ FrequencyResonance • If electrons (very light) jiggle • natural frequency is in the visible or ultra violet range • If atoms (10000 times heavier) jiggle • natural frequency is in the infra-red/microwave range • If whole molecules (Many times heavier) jiggle • natural frequency is in the RF/Microwave range
Atoms, Molecules & Materials‘Natural’ FrequencyResonance • Near the natural frequency • Absorption • Well above the natural frequency • Not much absorption • Well below the natural frequency • Not much absorption
Waves & Matter (3) Electricity Atoms Heat Electromagnetic waves When electromagnetic waves are ‘absorbed’ by matter … Atoms jiggle faster: i.e. increase their temperature.
How much energy gets absorbed? S.A.R. 3. How much energy is absorbed? Specific Energy Absorption Rate
Terminology (6)Summary • Power • watts • Intensity • watts per square metre • Specific energy Absorption Rate (SAR) • watts per kilogram
SAR:Example using light • rather than microwaves Terminology (5)Specific energy AbsorptionRate (SAR) watts per kilogram • 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
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
Microwave Ovens 3a. Microwaveovens
Electromagneticspectrum Microwave Oven Conventional Oven 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
Microwave OvensThe absorption of microwave energy by liquid water • Near the natural frequency • Absorption • Well above the natural frequency • Not much absorption • Well below the natural frequency • Not much absorption 2.4 GHz
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
Absorption (2)Too weak • If power were absorbed too weakly, • Microwaves would go right through • No cooking
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
Microwave OvensSummary • A microwave oven cooks food by heating it • The heating comes from • intense waves at 2.45 GHz • rather than • a wide spectrum of waves at infra red frequencies. • Frequency chosen because of absorption properties of water molecules at that frequency.
Microwave Ovens Inside a microwave oven
Microwave Power • Power • This is a 700 watt oven • Think of 7 x 100 watt light bulbs
MicrowaveElectric field • 700 watts • Around 140 000 volts per metre • Look what happens to a CD
Microwave Intensity • 700 watts • transmitted into an area of around 1/25th square metre • Between 104to 105watts per square metre • (Most intense sunlight around 103 watts per square metre) • Very Dangerous • Could I have a stupid volunteer please?
A Cautionary Tale August 14, 2002 I don't want to sound like I know everything in the world or even like I know quite a lot. But you had a question regarding“If a microwave oven door were to open while it was still on, what would happen? Could it hurt you?- JP”Well ..Having the thought process that I have, kinda how should I put it? ...Stupid? or inventive or even in-between. Well, my microwave door did happen to come off. Magic Chef 900-watt microwave. Well, I did my best to try to fix it but the hinge on one side did not attach properly, therefore having a gap between the door and the appliance. Being me (stupid) I wondered if it would burn fast or would it gradually warm up. I slid my finger between...You probably dying to hear what happened... But it didn't gradually warm up at all. It was instant heat! It didn't scar me or anything like that, but sure scared the H*** out of me to find out it got so hot so quick. I didn't get any blisters either. But it just burned like touching something hot on the tip of my finger being that is the only thing I put in. Well you know the old adage, "You learn from your mistakes", stands true. lol - Don’t try this at home!
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?
Microwave Ovens (13) Comparison with handsets • Expect 1/700 of microwave temperature rise • The effects of blood flow reduce this further 0.01 °C (ish)
Mobile Phones Back to Mobile Phones 3b. Mobile Phone SAR
Mobile Phones (14) SAR and Safety New Style Old Style
Mobile Phones (8) Your telephone • Look in the small print!