2.14. Tide http://www.ck12.org
crest up our paddling pool. Similarly each Atlantic trough sends a trough up the paddling pool. Consecutive crests
and troughs are separated by six hours. Or to be more precise, by six and a quarter hours, since the time between
moon-rises is about 25, not 24 hours.
Figure 14.2:Woodbridge tide-pool and tide-mill. Photos kindly provided by Ted Evans.
The speed at which the crests and troughs travel varies with the depth of the paddling pool. The shallower the
paddling pool gets, the slower the crests and troughs travel and the larger they get. Out in the ocean, the tides are just
a foot or two in height. Arriving in European estuaries, the tidal range is often as big as four metres. In the northern
hemisphere, the Coriolis force (a force, associated with the rotation of the earth, that acts only on moving objects)
makes all tidal crests and troughs tend to hug the right-hand bank as they go. For example, the tides in the English
channel are bigger on the French side. Similarly, the crests and troughs entering the North Sea around the Orkneys
hug the British side, travelling down to the Thames Estuary then turning left at the Netherlands to pay their respects
to Denmark.
Figure 14.3:An artificial tide-pool. The pool was filled at high tide, and now it’s low tide. We let the water out
through the electricity generator to turn the water’s potential energy into electricity.
Tidal energy is sometimes called lunar energy, since it’s mainly thanks to the moon that the water sloshes around so.
Much of the tidal energy, however, is really coming from the rotational energy of the spinning earth. The earth is
very gradually slowing down.
So, how can we put tidal energy to use, and how much power could we extract?