Failing light
Sitting in my garden last night with just candles for light, I noticed that each one began to flicker and make a popping sound just before it finally fizzled out. Why does a candle do this, and what factors control the frequency of its dying flickers?
鈥� Candles are made out of paraffin wax, which is a mixture of hydrocarbons and is solid at room temperature. For a flame to form you need three things: fuel, oxygen and an ignition source. You also require the ratio of fuel to air to be within the flammable range.
For example, fire will not occur in an environment of 100 per cent methane, but a mixture of one part methane to one part oxygen will burn quite readily. When a candle is burning, it is not the solid wax that is burning, but the wax that is vaporising from the heat of the flame. These vapours are burnt off in the flame, so more heat is generated and more wax is vaporised.
Advertisement
To answer the question, when there is too much wind the flame may be extinguished, but it may reignite itself depending on a couple of factors.
The first is how hot the wick of the candle is, because this provides the ignition source. The second is how much vapour is present around the ignition source 鈥� the fuel-to-oxygen ratio 鈥� because when the flame goes out, parts of the candle are still vaporising.
If the oxygen and the temperature of the ignition source are at the required levels, the candle will reignite, and because there is a higher concentration of wax vapour than in a normal burning candle, it will cause the 鈥減op鈥� you hear. Even though the candle鈥檚 flame is out for less than a second, this is long enough for the wax vapour to increase in concentration.
Incidentally, this sound can be used to detect the presence of hydrogen, and is called, not surprisingly, the 鈥減op test鈥�.
Jason Kuzmanovski
Brisbane, Queensland, Australia
Long shot
My 5-year-old daughter wants to know if you spit a cherry stone while swinging on a swing, would it go farthest if you spit it while you are at the lowest point of the swing (because you are moving so fast), or would it be better to spit it at the highest point?
鈥� If we assume that in both cases the cherry stone is aimed horizontally then the distance it travels will depend on two things: its forward velocity and the distance that it has to fall before hitting the ground, which determines the amount of time available for the stone to travel forwards.
If the child鈥檚 mouth is 2 metres above the ground at the highest point of the swing and 0.75 metres above the ground at the lowest, then, taking acceleration due to gravity as 10 metres per second per second, we can calculate that the falling time is approximately 0.63 seconds and 0.38 seconds respectively, a ratio of 1.66:1.
At the lowest point of the swing the forward velocity of the cherry stone will be equal to the 鈥渟pit鈥� velocity plus the swing velocity. At the highest point the swing is stationary, so the stone will have the 鈥渟pit鈥� velocity only. In this example, the 鈥渟pit plus swing鈥� velocity will need to be at least 1.66 times the 鈥渟pit鈥� velocity alone if the stone is to travel further from the lowest point of the swing.
I reckon that a good spit would result in a stone velocity of about 7 metres per second, while the swing might achieve a peak of about 12 metres per second. In this case the combined spit plus swing velocity of 19 metres per second is easily greater than 1.66 times spit velocity alone, and the stone spat from the lowest point would achieve a distance of 7.2 metres against only 4.41 from the high point of the swing.
Obviously, as the relative contribution of spit velocity to swing velocity increases, the outcome changes. For example, if your daughter could do a superspit of 20 metres per second then, with the other variables remaining the same, she would achieve 12.6 metres distance from the high point but only 12.16 metres from the low point.
And, in the unlikely event that she were to then fire a bullet at 300 metres per second, it would travel about 70 metres further from the high point of the swing.
Jonathan Wallace
Newcastle upon Tyne, UK
鈥� The distance that the cherry stone travels is dependent upon the time it is in the air and its velocity. I have derived various equations for the distance the stone travels. This distance also depends upon the speed the stone is projected and the angle the swing makes with the vertical at its highest point.
Assuming the child spits the stone out at about 1 metre per second, which is a fairly conservative estimate, then the stone travels farthest when spat at the lowest point of the swing. However, if the child could spit at a velocity of higher than 6.5 metres per second, and was able to swing at angles between 38掳 and 48掳 then, while spitting parallel to the ground, the stone would go about 10 centimetres farther than it would at the bottom of the swing. Spitting even faster increases this extra distance and the range of valid angles.
The derivation of the equations and an interactive display are available on my website at .
Michael Morley
No address supplied
This week鈥檚 questions
Leave stones unturned
I eat too many avocados and I have noticed that half an avocado with the stone left in does not go brown as quickly as a halved avocado with no stone. Why is this?
Imogen Holmes
London, UK
Seasonal shift
I was always under the impression that the equinoxes fell on 21 March and 21 September, dividing the year into four equal parts along with the solstices. However, I often read that the equinox will fall on a day other than the 21st. Surely there has to be an equal division of the seasons, relying on the Earth鈥檚 orbit around the sun? What could possibly change this?
Kingsly Richard
Toulouse, France