How much power is there in a teacup of gasoline?

The Tweet: “A single cup of gasoline, when ignited, has the same explosive power as five sticks of dynamite.”

In the case of an explosion or any other reaction, power is the amount of energy released and the speed at which the energy is released. So, ‘explosive power’ has quite a specific meaning from a scientific point of view. The energy stored in fuel is usually a less important factor in measuring the power of an explosion than the rate at which energy is released.

The difference between an explosive detonation and a loud, hot bang is the speed at which the detonation wave moves. If the detonation wave moves faster than the speed of sound it is considered an explosion. Usually in high explosives this speed is approximately 4500 metres per second (m/s).

So to understand if a cup of petrol releases energy five times as explosively as five sticks of dynamite we need to know how much energy is in that volume of petrol and how long it takes to burn. In 170g of petrol there is 8,000,000 Joules (or 8MJ) of energy. This is how much energy is released when all 170g are burnt. To burn, petrol must be mixed with air, or vaporised. Once vaporised, it releases this energy in 2 to 3 milliseconds at a rate of 3 Giga Watts (3GW). A Watt, is a measurement of power and is equivalent to a joule per second.

Sticks of dynamite are all roughly 20cm in length but vary in width from 3 to 5cm and weight from 0.5 to 4 pounds. Assuming that a stick of dynamite contains 0.5 pounds of explosive there is 1MJ of energy to be released. Dynamite burns much faster than petrol, however, in about 4 microseconds and at a rate of 250GW. This is 83 times more powerful than a cup of petrol.

So even assuming we are using a small stick of dynamite, the energy released is far more explosive than a cup of petrol. The essential fact is how fast the energy is released, not the amount of energy stored in it. For comparison, there are around 8MJ in 1.2 pounds of wood, but wood burns much more slowly than petrol. In total the power in wood is about 30kW, or 1/1,000,000 of the power of petrol.

Image: Death to Stock Photos
This post was first published on The Untweetable Truth (04/12/2014)

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