viernes, 30 de agosto de 2013

How fireworks work

Watch this short National Geographic video explaining how fireworks work and answer the questions about it.

The activity is suitable for intermediate 2 students.

Watch the video through in the first place to get a full understanding of what it is about. Watch it again and say what the following figures refer to.
20 -
10,000 -
£30 -
100 -
7 -
80 -
2 -
12 -
120 -

To check your answers, you can read the transcript below.

We are at one of the UK’s leading firework manufacturers. And this is their 125 millimetre aerial starburst shell. A 20-minute display will launch up to 10,000 fireworks, and 60% of these will be these bad boys. Each one costs around £30 and contains half a kilo of explosive, enough to launch it to a height over 100 metres. As you can see, it’s much more than a ball of loose powder, and this is how they test the component parts.
The basic firework recipe is flash powder, a potent form of gunpowder. It’s a mixture of potassium perchlorate and aluminium. These ingredients are mixed together in 7-kilo batches and a small sample is ignited to make sure it has the requisite brightness. This sample has been mixed with strontium to make it glow red, and it’s clearly a pass, while this sample has barium added to glow green. Again, it does the trick nicely.
If the firework was packed with this loose powder, it would explode in one instant flash. So the gunpowder mixture is turned into pellet, to prolong the burn time. As the burning pellets fall through the sky, they create the spreading patterns we’re all familiar with. This test checks whether this particular pellet size will burn for sufficient time to create the burst diameter of 80 metres.
“There are two perfectly formed rings of stars, a central ring and an outer ring. The outer ring is made of stars of a slightly larger size. Therefore, they’ll be heavier and when the burst in the middle ignites the stars and blows them in the outer circle formation, the inertia of the heavier, larger stars will travel further. Thus in the sky you’ll see one small ring and then a larger ring around it.”
The next test checks the fireworks two fuses. Firstly a length of fuse is measured out, which will slowly burn inside the firework to ensure it ignites at the correct altitude. A fuse this length should burn in about 12 seconds. It has, so it’s a pass.
A second external fuse is needed to launch the firework. Wrapping the fuse in paper should contain the hot gases and make the fuse burn much quicker. 
“I would expect this length of fuse to burn with a 100th of a second. Looks like a pass to me.”
The final test is to ensure the firework is no louder than the European regulation of 120 decibels, the same noise level as a pneumatic drill at close range. 
“That’s a pass.”