[Dan B]

[xx(] spoil sport []

[neilep]

Soz Dan B....forgive ?

'Men are the same as women...just inside out !'

[Dan B]

[] maybe.

[NakedScientist (OP)]

Sorry for the delay in the appearance of the latest QOTW, but here it is :

"WHEN YOU WALK AROUND AT HOME BAREFOOT, WHY DO SOME SURFACES FEEL WARM, YET OTHERS FEEL COLD, EVEN THOUGH BOTH SURFACES ARE ACTUALLY AT THE SAME TEMPERATURE ?"

TNS

[cuso4]

It's all to do with the thermal conductivity of materials. For example if you stand on a piece of metal bare foot(let's say that the temperature of the foot is higher than that of the metal) you feel cold because heat is transferred to the metal. However, if you stand on a piece of wood it doesn't feel as cold because wood is a poor thermal conductor and the heat from your foot doesn't transfer to the wood as efficently.

Angel

[neilep]

Well, as an experiment for 'The Naked Scientists' I just instructed my family...(well..not wifey...she's on the phone BLAH BLAH BLAH')...erhhmm...as I was saying me and the kids just went barefeet and walked ariound the house !!...we discovered that the kitchen tiles were cold, the lounge carpets were warm, and the parquet flooring in the bathroom was warm.....then we all walked over burning hot coals and they were hot !!....then we stuck our feet in the freezer and that was cold.....hope my experiment helps.



'Men are the same as women...just inside out !'  

[NakedScientist (OP)]

Angel was absolutely right with her explanation of why some surfaces  feel warm to the touch, whilst other surfaces, despite being at the same temperature, feel cold.

Now here's this week's QOTW

HOW DOES SUNSCREEN PROTECT YOU FROM THE SUN ?

TNS

[Rokitansky]

It filters sunrays in a specific part of a spectrum, part of which is hazard to humans  ?

[Trang]

HOW DOES SUNSCREEN PROTECT YOU FROM THE SUN
just becuse of the layer (O3.)
But more details...
sorry [:I]

Chemistry is my love

[chris]

No one seems to have been particularly keen on tackling this one...so I will

Sunburn occurs when the skin is damaged by ultraviolet (UV) radiation in sunlight. This is short-wavelength radiation lying just outside the visible spectrum and is mostly mopped up by the ozone layer (O3) in the upper atmosphere. UV radiation is dangerous because it can damage DNA in skin cells, potentially triggering skin cancer. The skin responds to UV exposure by increasing the production of the dark pigment melanin, which is secreted into the skin by a population of cells called melanocytes. Melanin absorbs UV very efficiently, preventing further UV radiation from reaching the nucleated (DNA-containing) cells deeper in the skin, and hence preventing DNA damage.

Sunscreen essentially does the same job as melanin. It contains molecules which absorb ultraviolet radiation and change it into less harmful forms of energy, mainly heat (infrared). Having said that, reports have claimed that some sunscreens might not be as safe as others. Indeed some may be re-emitting radiation that is almost as harmful as UV...

We discussed this issue on the radio in January - here's the link :

http://www.thenakedscientists.com/html/shows/2004.01.25.htm

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

[NakedScientist (OP)]

Here is this week's QOTW

WHAT IS LIGHTNING, WHY DOES IT OCCUR, AND HOW IS IT PRODUCED ?

TNS

[Donnah]

It's light caused by the discharge of electricity between two electrified cloud masses or a cloud and the earth.  I have no idea how it's produced, but now I'm curious.

[nilmot]

Is it the charge is so strong that it ionise the surrounding air? I think it's something along those lines.

Tom

[OmnipotentOne]

Visible discharge of electricity when part of the atmosphere acquires enough electrical charge to overcome the resistance of the air.


To see the world through a grain of sand.

[tweener]

Lightning is the discharge of electric charge between clouds or a cloud and the ground.  It is not clearly understood what causes the migration of the charge in the first place.  Positive charge collects at the top of the cloud, while negative charge build up at the bottom.  When the charge builds to a high enough level, possibly many millions of volts, an ionized channel is formed and a current flows to equalize the charge.

----
John - The Eternal Pessimist.

[neilep]

So clouds become big balls of fluffy batteries then eh ?[]

'Men are the same as women...just inside out !'

[tweener]

Yeah!  If you could figure out how to capture and store the energy, you could power a fair sized city for several days on the power generated in a single thunderstorm.  And you might just become relatively wealthy too!

----
John - The Eternal Pessimist.

[neilep]

I've got myself a very tall ladder, some empty batteries and some rubber gloves.....hang on !!..if i get all the clouds interested in playing musical instruments then perhaps I can be a Lightning Conductor !!!! *oh god...someone ban me from this site please *

'Men are the same as women...just inside out !'

[NakedScientist (OP)]

ANSWER TO "WHAT IS LIGHTNING, WHY DOES IT OCCUR, AND HOW IS IT PRODUCED ?"

At any given instant there are 2,000 thunderstorms happening around the world, producing a 100 lightning strikes a second, or 8 million lightning strikes a day !

Lightning is an electrical discharge between one cloud and another, or between a cloud and the Earth, accompanied by the emission of light.

This electricity originates in clouds which behave like giant capacitors or accumulators in the sky. Collisions between the water particles (called hydrometeors) which make up the clouds lead to the smaller particles acquiring a positive charge and the larger particles acquiring a negative charge. The precise mechanism by which this happens is not fully understood.

Under the influence of updrafts, the small particles (carrying a net positive charge) are carried to the top of the cloud, leaving a net negative charge concentrated at the bottom of the cloud. This can lead to the accumulation of a large potential difference exceeding millions of volts.

This electrical potential creates a strong electric field between the clouds and the ground (earth) which is sufficient to cause the earth's surface to become positively charged as electrons are repelled away.

When the field becomes sufficiently strong to overcome the resistance of the air, the cloud discharges to earth, producing a lightning bolt.

Each lightning flash is about 3 miles long but only about a centimetre wide. It discharges about 1-10 billion joules of energy, produces a current of some 20,000 amps, heats the surrounding air to a temperature 3 - 5 times hotter (20,000 degrees C) than the surface of the sun (6000 degrees C), and is actually made up of 3 or so 'strokes' lasting only a matter of milliseconds or less.

So with all that energy, could lightning be collected to power a town ?

No - simple maths shows that this is not feasible :

100 joules of energy keeps a 100 watt lightbulb burning for 1 second. So 1 billion joules of energy (the amount in a modest lightning strike) would keep the same single lightbulb burning for just under 120 days.

The average household uses about 500-1000 kilowatt hours (kWh) per month. 1 kilowatt hour is 1000 Joules per second x 3600 seconds (the number of seconds in an hour); i.e. 3,600000 Joules. So the average household consumes about 500 x 3,600000 = 1.8 billion joules per month.

So if you could collect all of the energy contained in one lightning strike it would run one home for a month.

Sounds like good news, but not all of the energy in lightning is available as electricity - in fact probably less than 1% of the energy (10 million joules or so) could be effectively harnessed as electricity.

Then you have to take into consideration the 'strike frequency' for any given area, the cost involved in building a tall tower to work as a lightning collector, and then tackle the problem of how to construct a sufficiently big capacitor to store all of the charge you collect...

And what about thunder ?

The intense heat of the lightning disharge (over 20,000 degrees C) superheats the surrounding air causing it to expand explosively. This creates a compression or 'shock' wave - the thunder - which spreads out through the air in all directions, travelling at about a fifth of a mile per second.

The flash and the thunder clap are produced simultaneously - as anyone unlucky enough to have ever got very close to a lightning strike can tell you - but the light from the flash travels much more rapidly (186,000 miles per second) than sound (0.2 miles per second approximately). The light therefore reaches you first, then a short while later (depending upon how far away the storm is), the thunder rolls in.

TNS

[NakedScientist (OP)]

Here's this week's QOTW :

"WHAT MAKES THE HANDS GLOW ON A CLOCK OR WATCH FACE ?"

TNS