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118

A 40W incandescent light bulb has a luminous efficiency of 1.9%. That means only 1.9%, or 0.76W, of the energy consumed by the bulb ends up as visible light. LED bulbs have an efficiency of around 10% - the efficiency depends on the design and can be as high as 15% or as low as 8%. So a 6W LED bulb will produce between 0.9 and 0.48W of visible light. The ...


60

The advertising suggests that the new 6W bulb generates as much light as a 40W incandescent light used to. Most of the energy in an incandescent light bulb is converted into infrared radiation that we can't see or it's at the red end of the spectrum where the human eye is not very sensitive. This is a typical incandescent spectrum and you can see how ...


41

Candle wax expands considerably when hot and molten. So while burning the candle the level in the glass rises. But when the candle is extinguished the outer region (nearest the glass) cools down quicker (candle wax doesn't conduct heat very well) and solidifies first, becoming immobile. The molten remainder then shrinks before solidifying. So it's the ...


38

You feel cold when heat is flowing from you to the surroundings, your body tries to burn more energy to keep up your temperature, so you shiver. Water conducts heat much more effectively than air (more than 100x as well) so even with water at the same temperature as air you will lose a lot more heat and feel cold. When your body is too hot it losses energy ...


33

You're mixing power needs with luminous effects. According to the advert you posted, that LED bulb consumes 6W (power) to get a luminous flux of 500 lumens (lumen is a photometric unit, like candela or lux): https://en.wikipedia.org/wiki/Luminous_flux On the other hand, an incandescent bulb would need to consume 40W (power) to get the same luminous flux. ...


21

The tides are a result of the response of the Earth's oceans to the tidal forces exerted on the water by the Moon and the Sun. The responses are vastly complicated by the Earth's rotation about its axis, by the physical geography of the Earth, and by the nature of the orbits of these bodies. Of key interest with regard to this question are the inclination of ...


12

Yeah, I would guess that the alternate heating/cooling of the wax in the sun pushes it up the side of the glass. Presumably the surface tension between the wax and the glass is quite strong and holds the wax up once it's been pushed up. Subsequent cycles cause wax to "backfill" the wax that's been pushed up. It would be interesting to design an experiment ...


9

OK, here's a theory: It's all (when I say all I mean mostly) got to do with the fact that the moon's plane of revolution is inclined about 20 degrees or so relative to the earth's equator. That causes differences in the frequency of tides in various latitudes. In order to see how that happens, here's a simplified example: Draw a cross, so that you have 4 ...


7

The energy comes from the sun, that much is certain. One possible mechanism is capillary action resulting in a meniscus, assuming the sun heats the wax to a (near) liquid state. The other possible mechanism is a vaporization/redeposit cycle. During the day, heat from the sun creates wax vapor, which is heavier than air [citation needed] and therefore ...


6

It is entirely possible that the entire candle has melted in the sun. If this happens the molten wax has a significantly greater volume than the solid wax candle and so the whole level will rise up the glass ie liquid wax takes up more volume than solid wax. As it cools again the level will fall as it solidifies. Typically it will cool from the top and ...


5

What you are hearing is mains hum: mains electricity is alternating current (ie the voltage is approximately sinusoidal and symmetric about zero), with a frequency of 50Hz or 60Hz. things like kettles and heaters use a lot of power and parts of them will mechanically change shape at this frequency, which is audible. This kind of physical noise from things ...


5

As @Tweej suggests, it's because of water solubility. Because water molecules are quite polar, most things that are charged or polar are soluble in it (i.e.~"hydrophilic"). When a coffee stain dries up, the residue sticks to the surface. But when water is applied, it will readily mix with the water, and more easily be removed. Fats and oils are ...


4

In summertime, the ceiling fan blows air downwards and cools down your body using the wind chill effect. In wintertime, if you have an active heating system at home, it will heat up the air in the room. Hotter air moves up and accumulate near the ceiling, colder air being down. A ceiling fan in reverse direction moves cold air up pushes hot air downwards to ...


4

An average person uses approx. 1500-2500kcal/day. Since one kcal equals 4148J in SI units, that's between 6.2-10.4MJ per day. A day has 86400 seconds, which brings us to an average power consumption of 72-120W... about as much as a light bulb. :-) Physical exercise varies between light (300kcal/h) at an additional 350W to very strenuous at probably six ...


4

When you feel hot, you perspire so as to benefit by evaporative cooling. As the relative humidity gets closer to 100%, the sweat cannot evaporate and evaporative cooling becomes less effective. Liquid water is a much better conductor of heat than air (even humid air) is, so if the water is even a few degrees cooler than your body, you feel cold because the ...


4

My simple answer to this is that high and low tides are a bit arbitrary; they're the turning points of a continuous function over time. At the places you've mentioned, the (usually most significant) bi-daily tidal harmonic is "swamped out" by the (usually less significant) daily harmonic. It's still there – it's almost certainly non zero, but it's just not ...


3

The air flow around the car creates a vortex. There is an image of the air flow around a car: which was used by this earlier answer - that rotating air behind the car is what spins the wheels of the bike.


3

We're talking about the Coanda Effect, right? Then I think this article could provide some useful insights. Quoting from the article: When the fluid flows over the heated curved surface in proximity of the curved surface as the temperature of the curved surface increases, dynamic viscosity of the fluid at the vicinity of the wall is increasing with ...


2

When you work "fairly hard", your body can produce about 200 W of power - enough for two incandescent bulbs. Top athletes can produce more - in short bursts. Your body is roughly 25% efficient in converting "calories" (which are actually kilo calories) to Joules - meaning that if you work out hard enough to burn 600 kcal per hour, then you actually produced ...


2

A (kilo)calorie is a unit of energy, while a watt is a unit of power, which describes the rate at which energy is expended. So a 100W bulb is using 100 joules a second. A kcal is about 4184 joules, so a 100W bulb takes about 42 seconds to consume (really: convert into light and heat) a kcal. The joule is the SI (derived) unit of energy. Units of energy ...


2

In the case of inorganic matter, such as metal sheets, folding/creasing produces a substantial bulk stress in the material which can modify the molecular structure in a large number of complex (and not fully understood) ways. For example, it can break bonds, cause amorphisation, and propagate dislocations. These same mechanisms are at play when you cut a ...


2

What happens during folding is that the material undergoes plastic deformation. When a sheet of material is bent slightly that deformation is usually elastic, meaning it will return to its original shape when the deforming stress is withdrawn. But when the deformation is larger we enter the plastic zone: the material will no longer fully recover its ...


2

This is a surprisingly difficult question to answer properly, but if you read the report from the Ricardo group (thanks @pentane for the reference!) it includes the following table: They do a lot of analysis to show that if you reduce the weight of a car, you can get away with a smaller (more efficient) engine; but I am assuming that you just want the ...


2

From what I've learnt, this relative strengthening,of cloth when wet, and weakening of paper is due to two factors. First up:The cloth - google fibre structures of cloth, and you will notice that cloth fibers are uniformly but relatively less interlocked, and addition of water causes further attraction via hydrogen bonding. For the paper, it has relatively ...


2

Either way will cool you in Summer: Either way, keeps the air circulating in the room, and the moving air increases the rate at which perspiration can evaporate from your sweaty body. Either way will warm you in Winter. Either way keeps the air circulating in the room, thereby preventing thermal stratification (i.e., it prevents all the heat from collecting ...


2

There are several kinds of "UV Light." Only kind I know of that "looks purple" is a so-called black light. Black lights emit UV that is very close to the top-end of the visible spectrum. The designers try to minimise the visible radiation so that it won't wash out the light emitted by fluorescent substances in the field, but it's hard to filter all of the ...


2

The uv light often originates from excited Mercury atoms which are in the bulb. Excited Mercury atoms also produces violet and blue visible light.


1

You are describing two different mechanisms of cooling the human body: 1) When we sweat our body produces fluids that tranfer heat from us through their evaporation (fluids gets the heat from our body and evaporate). One can easily understand that the more the surrounding enviroment has a highly humidity value , the more this heat- transfer gets ...


1

There is a hint of it here (http://travel.stackexchange.com/questions/8861/do-you-have-a-gps-signal-on-board-of-a-plane?newreg=bc0d5655b2a14ee5a21e77fe52978b3e) Unfortunately the plane body does an excellent job of blocking GPS signal. In general if you're in a window seat you will be able to get a signal by holding the GPS near the window (or, for ...


1

There's an important difference between liquid water and water vapor. The difference is 539 calories per gram to be exact. That's why being in humid air makes you hotter - the water vapor has all this latent heat it can release upon you by condensing. Equivalently, it prevents your sweat from evaporating and absorbing this energy. Liquid water doesn't have ...



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