…is an exercise on the efficiency of a wind turbine, check it out first.
How is the power (or energy per unit time) from the wind actually used? Or, putting another way, some energy is wasted, obviously. But how, and where?
1. If the initial speed of the wind is (say) 7m/s, the wind speed won’t fall to zero after the air passes through the blades. It might fall to (say) 2m/s. It’s the difference in speed before and after that is used in a KE calculation, i.e, 5m/s.
2. The turbine will have down times when there is either too little or too much wind.
3. Thermal energy is generated in the form of friction between the moving parts in the turbine. This energy can’t be used.
Here’s the original post.
A lot of exam boards are asking about the Greenhouse Effect these days, I’m going to write separately about this.
My original post can be found here – this was a favourite practical test.
Additions for 2013. To paraphrase George Orwell, curves are good, straight lines are better. This means that we can learn more about the data with a good straight line than we can with a curve, simply because curves are notoriously difficult to draw well. In exams, they’ll penalise you a mark if your curve is too thick, has a feathery look to the line or it doesn’t go through all the error bars.
How can we determine if a decay is exponential or not? The right way to do this is to plot the ln of the y value against the x value and if the points fall on a straight line within the error bars, the function is an exponential. The other way is less elegant but you’re also expected to know that if a curve is exponential the slope of the y value is proportional to y.
Here’s the trick. Draw tangents to the curve at 3 separate y values. Make the triangles as big as you can. If it’s exponential, the slope divided by the y value will give the same answer, k