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Tides. What Causes Tides?. Tides are the daily rise and fall of Earth’s waters on its coastlines. As the tide comes in, the level of the water on the beach rises gradually . When the water reaches its highest point, it is high tide .
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What Causes Tides? • Tides are the daily rise and fall of Earth’s waters on its coastlines. • As the tide comes in, the level of the water on the beach rises gradually. • When the water reaches its highest point, it is high tide. • Then the tide goes out, flowing back toward the sea. • When the water reaches its lowest point, it is low tide.
What Causes Tides? Which is picture A? Which is picture B?
Tides occur in ALL bodies of water (even bathtubs!!); however, they are only noticeable on oceans and large lakes.
Tides are caused by the interaction of Earth, the moon, and the sun….their gravitational pull on each other! Every ‘body’ exerts a gravitational force of attraction (pulling together) on every other ‘body!’ Sometimes that force is huge and sometimes it is small.
What Causes Tides? • Gravity is the force exerted by an object that pulls other objects toward it. • Gravity keeps you and everything around you on Earth's surface. • As the distance between objects increases, however, gravity's pull grows weaker.
The further away two objects are, the less their gravitational force on each other. Thus, the moon has a bigger impact on our tides than the sun!
As a result of the moon pulling on Earth and its waters, two big bulges of water form: one directly under the moon and another on the exact opposite side of the moon As the Earth spins, the bulges follow the moon.
The diagram shows the effect of the moon's gravity on the water on Earth's surface. The moon pulls on the water on the side closest to it (point A) more strongly than it pulls on the center of the Earth. This pull creates a bulge of water, called a tidal bulge, on the side of Earth facing the moon.
The water at point C is pulled toward the moon less strongly than is Earth as a whole. This water is “left behind,” forming a second bulge. Where the water bulges = High tide; where the water is slack = low tide.
High tides occur about 12 hours and 25 minutes apart in each location But in other places, the range between the water levels is less dramatic. In some places, the two high tides and two low tides are easy to observe each day.
Range (bore) vertical distance between high and low tide; can be big or almost unnoticeable (Sapelo vs San Sal)
Spring tide:occur at each new moon and each full moon. • Because the sun and moon are lined up at these points, the gravitational attraction is greater than usual, thus producing high than usual high tides and lower than usual low tides. • It produces the greatest difference between high and low tide or tidal bore.
Neap tides occur at the first and third quarter of the moon (exactly between full and new moon) where the moon and sun are pulling at right angles. • These tides have the least difference between high and low tide or the smallest bore.
Monthly Tide Cycle • Despite the complex factors affecting the tides, scientists can predict tides quite accurately for various locations. • They combine knowledge of the movements of the moon and Earth with information about the shape of the coastline and other local conditions. • If you live near the coast, your local newspaper probably publishes a tide table. • Knowing the times and heights of tides is important to sailors, marine scientists, people who fish, and others who live along a coast.
Energy From Tides • The movement of huge amounts of water between high and low tide are a source of potential energy. • Potential energy is energy that is stored and waiting to be used.
Energy From Tides • Engineers have designed tidal power plants that capture some of this energy as the tide moves in and out. • As high tide swirls up the river, the plant's gates open so that the water flows into a basin. As the tide retreats, the gates shut to trap the water. • Gravity pulls the water back to sea through tunnels.
Energy From Tides • The energy of the water moving through the tunnels powers generators that produce electricity, just as in a hydroelectric dam on a river. • Although tidal energy is a clean, renewable source of energy, it has several limitations.
Energy From Tides • Harnessing tidal power is practical only where there is a large difference between high and low tides—at least 4 or 5 meters. • There are very few places in the world where such a large difference occurs. • Daily tides also may not occur at the time when there is a demand for electricity.
Comprehension Questions 1. Explain how the moon causes a tidal bulge to form on the side of Earth closest to it. 2. How can tides be used to generate electricity? 3. Describe the positions of the sun and the moon in relation to Earth when spring tides occur.
