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Synoptic question trouble!!!!!!!!!

In the june 03 paper question 4:

Tidal power could make a significant contribution to UK energy requirements. This question is about a tidal power station which traps sea water behind a tidal barrier at high tide and then releases the water through turbines 10.0 m below the high tide mark.
(i) Calculate the mass of sea water covering an area of 120km2 and depth 10.0 m.
density of sea water = 1100 kgm–3
..............................................................................................................................................
(ii) Calculate the maximum loss of potential energy of the sea water in part (i) when it is released through the turbines.
..............................................................................................................................................

I can do part i), but for part ii) the markscheme says
(use of Ep = mgh gives) ΔEp = 1.3 × 10^12 × 9.8 × 5 = 6.4 × 1013 J

WHY is the height not 10 used here????? :confused:

Any help appreciated.
Thanks

Reply 1

discombob

I'm not too sure, but isn't it because the water will flow across only until they're equal, ie. halfway = 5m down?

Before opening the turbine it would be 10m || 0m of water.
After opening the turbine, it can only be 5m || 5m of water, not 0m || 10m of water?

Dunno :redface:

haha

^^^^ correct

The h in the change in pe equations means the change in height of the centre of grvity of the object.

This question looks familiar - see this years OCR A synoptic paper!