Bottom Pressure GOCE. Gravity field & steady state Ocean Circulation Explorer. Background Information:

Transcription

1 GOCE Gravity field & steady state Ocean Circulation Explorer Bottom Pressure Background Information: During the last Ice Age, the pressure created by the heavy load of ice caused the Earth s crust to depress. As the thick ice sheets melted, the crust was relieved of some of this pressure and it has risen in places. This process is called post-glacial rebound. It affects countries such as Scotland, Denmark, Canada and the US. In fact Scandinavia and Canada are currently rising by a few cm per year. The melting ice caps have resulted in a global redistribution of water in the oceans. Sea levels have fallen close to the locations of the original ice caps but they have risen further away. The rise in sea level at some sites is significantly more than at others. This is due to the gravitational attraction between the mass of the melted water and other masses, such as remaining ice sheets, glaciers, water masses and mantle rocks. A better understanding of post glacial rebound is needed to help assess the potential dangers of sea level change. To achieve this, scientists working on the Arctic project have been monitoring changes in the bottom pressure i.e. the pressure at the bottom of the water column. This lets them work out how much mass has been added to the oceans from the melting ice. Ice and water have mass, which means that as they move around, they exert a gravitational pull on other masses. The redistribution of ice/melted water therefore affects the Earth s gravity field. This means the results of GOCE, the Gravity field and Ocean Circulation Explorer satellite, can be compared to the findings from the Arctic study. GOCE will also be compared to measurements made by bottom pressure sensors in oceans throughout the rest of the world. GOCE will also build on the previous US-German GRACE (Gravity Recovery and Climate Experiment) Mission. Dr Watkins a project scientist at the US space agency s Jet Propulsion Lab says: The two missions use different ways to measure the gravity field, so they have different errors that are great to cross calibrate. Increasingly science is about pulling together a broad range of datasets; the full picture only emerges when different perspectives and disciplines interweave. Gravity data collected from pressure sensors deployed from ships in various places around the world will be used to calibrate and validate the GOCE mission. It is common for scientists to repeat others work, but in slightly different ways, in order to check the reliability.

2 Learning Objectives: Pupils will appreciate that in order to ensure the validity of GOCE s results, scientists need to determine the movement of ocean currents by several different methods. They will know how pressure varies with depth in a fluid. Curriculum Links: Edexcel GCSE in Physics (2109) Show understanding of how scientific knowledge and ideas change over time and how these changes are validated. P3 5.5: Describe and explain the pressure of a gas in terms of the motion of its particles The Twenty First Century Science suite GCSE Physics A (J635) P7.4.5: Recall that when the volume of a gas is reduced its pressure increases and be able to explain this using a molecular model AO1: Show understanding of how scientific knowledge and ideas change over time and how these changes are validated. AQA Physics 2009 (4451) 10.1 The data to be used as evidence must be reliable and valid, as only then can appropriate conclusions be made. The reliability of evidence refers to how much we trust the data. The validity of evidence depends on the reliability of the data, as well as whether the research answers the question. If the data is not reliable the research cannot be valid Candidates should know and understand that evidence must be looked at carefully to make sure that it is: reliable, i.e. it can be reproduced by others and valid, i.e. it is reliable and answers the original question Using data to draw conclusions - Candidates should know and understand that in evaluating a whole investigation the reliability and validity of the data obtained must be considered. The reliability of an investigation can be increased by looking at data obtained from secondary sources, through using an alternative method as a check and by requiring that the results are reproducible by others. Suggested activities: Bottle Fountain: Insert a straw into a drinks container and blow into it. Move your face away from the bottle. What happens? Water rushes out of the straw like a fountain. How did this happen? When you blew air through the straw, you increased the pressure of the air inside the bottle. As the pressure inside the bottle increases it exerts this pressure on the water, pushing it out through the straw. Expanding Marshmallow: Place a marshmallow into an empty alcopop bottle. Use a wine bottle pump to remove air from the bottle and observe the marshmallow expand.

3 Magdeburg Hemispheres and Plumbing Magic - Are You Stronger Than The Air? In order to demonstrate the operation of the vacuum pump in the 1600s, a pair of hemispheres in a town called Magdeburg were evacuated. A team of wild horses were incapable of separating them. This experiment can be replicated using plungers by wetting them and sticking them together. When you rammed them together, air was forced out of the cavity. This makes the pressure inside the plungers less than outside. The pressure acting in is more than that acting out, keeping them together. These experiments show the powerful effect of air pressure. The pressure at the bottom of the ocean is significantly higher than atmospheric pressure. Bottom pressure recorders make use of this. It is made of two glass hemispheres which are held together by the pressure of the surrounding water. Egg in a Bottle: Select a glass bottle with a neck large enough that an egg can be squeezed through. Remove the shell from a hard boiled egg. Light some cardboard and toss it in. Place the egg on top. Once the oxygen inside the bottle is used up, the flame goes out. The gas cools and the air pressure inside the bottle decreases. The higher pressure pushes the egg in. Collapsing Can: Place some water in an empty drinks can and use tongs to hold it in a Bunsen flame. When steam emerges, quickly invert the can into a dish filled with water. The can collapses. The air in the can expanded as it was heated. Cooling it quickly caused the pressure inside to drop. Since the air outside the can is at a higher pressure, it pushes the can in and crushes it. Alternatively a vacuum pump can be used to remove air from inside the can. Both experiments show the effect of atmospheric pressure. At the bottom of the ocean, the pressure is hundreds of times more than that involved in the collapsing can experiment. These polystyrene cups were squashed by water pressure, demonstrating the need for the equipment used by oceanographers to be incredibly robust. The instrument on the right was squashed by the pressure of the ocean.

4 Inverted Water Glass Trick: Pour water into a glass until it is about a third full. Place a piece of cardboard on top of the glass invert the glass, holding the cardboard in place as you do so. Once the glass is upside down, remove the hand which is holding the cardboard. The water and the cardboard piece do not fall because they are held in place by atmospheric pressure. Jumping Coin: Select a coin which is bigger than a glass coke bottle s opening. Fill a bowl with some cold water and place the neck of the bottle and coin in it. Then place the coin on top of the bottle. Chilling them in the water beforehand helps ensure that an airtight seal is formed. Ask pupils to predict what will happen if you place your hands around the bottle. The air inside the bottle heats up, pushing harder on the coin than the cool air. This forces the coin up, making it jump. When you remove your hands it stops because the air inside the bottle cools down. Spearing Spuds: Ask a volunteer, someone who thinks they are stronger than you to place a potato on the table top and try to stab a straw into it. Hold your thumb over the hole in the top of another straw and stab this straw into the potato. The student s open-ended straw bends and only a little bit of the straw cuts the potato, but the closed straw penetrates deeply into the potato. As the straw enters the potato, the air trapped inside the straw is compressed. This increases the pressure and allows the straw to cut further. Cartesian Diver: Instructions for making a Cartesian diver are available from Squeezing the bottle increases the pressure, forcing additional water to enter the small hole in the diver. This increases his density and makes him sink. Spouting Can: What do you notice when you dive to the bottom of the pool? Discuss dam walls and qualitatively demonstrate the relationship between pressure and depth using a spouting can. If students have already seen the traditional spouting can, you could remind them of the key points with this alternative demonstration. Cut the base off two bottles and attach a tube to the neck of each one. Add the same volume of water to each. The water flows at a higher rate from the longer tube, confirming that pressure increases with depth. Pressure sensors: A Bourdon tube pressure gauge works on much the same principle as a party blower, in that it inflates and unrolls when you blow in its tube. The amount of uncoiling that occurs is proportional to the pressure inside the Bourdon tube. As the tube uncoils, it moves the needle, indicating the new pressure.

5 Experiment to find relationship between pressure and depth: Use a digital pressure sensor to measure atmospheric pressure. Measure the pressure under different depths of fluids. As the graph is a straight line through the origin, the pressure is directly proportional to the depth. Use the gradient of the graph to determine the density of the fluids. 2.5 P = ρgh Pressure (kpa) gradient = ρg Diving bells: Depth (cm) The diving bell was one of the earliest types of equipment used for underwater exploration. Its use was described by Aristotle in the 4th century BC. The pressure of the water keeps air trapped inside the bell. The diving bell is lowered by a chain from a ship. The whole diving bell is underwater, but the water rises no higher than its floor. The pressure of the air keeps it out. They are still used for underwater rescue and salvage. The diver goes down to the bottom of the harbour and fastens ropes to whatever he wants to hoist to the surface. The principles behind the diving bell can be demonstrated using a viewcam and two glass beakers as shown. References/Resources: A video showing the Magdeburg hemispheres demonstration: A marshmallow man in a vacuum: Shaving cream in a vacuum: More videos of air pressure experiments: