20th November: Answers to School Kids’ Questions – Pressure and Polystyrene

A few weeks ago, a class from a school in London got together to think up some questions to ask the scientists on board. Topics range from how a polystyrene cup shrinks when sent to the bottom of the ocean to how corals can affect the climate. Allison and I have done our best to give these some answers which you can find below.

1)   Pressure and Polystyrene

• How deep are you going to make the cup go and how long are you going to keep the cup under water? (p.s. can you shout my name out when you drop my cup?)

So far the deepest we’ve sent cups down is 5 thousand meters. Those cups went down when we lowered an instrument called a CTD which we use to measure the properties of sea water like temperature and salt and nutrient content. The CTD also takes water samples from various depths.  Because we want to be very careful with our equipment we lower everything to the bottom at a gradual pace instead of just dropping it straight down. Our last CTD went down to the bottom at 4am and reached the surface four hours later at 8am.

Ps. The speed of sound in water is about 1400 m/s which is much faster (about 15x) than the speed of sound in air.  Unfortunately, sounds made in the air largely bounce off the surface of the water so your cup probably didn’t hear us.

Allison Jacobel

• How does the polystyrene cup get crushed and how do you pick up the cups from the bottom of the sea?

We send the cups down very deep; some of them have been more than four kilometres to the bottom of the ocean. The pressure created by all that water is very high. Imagine sitting on the sea bed with 5km of water on your head! You may have experienced the pressure caused by the weight of water if you’ve ever dived to the bottom of a deep swimming pool and felt an uncomfortable feeling in your nose and ears.  That’s because the pressure is squeezing the air behind your eardrums and your nose. If you can feel the effects of pressure at the bottom of a swimming pool imagine how much greater the effect is at the bottom of the ocean! As you can imagine that’s not a very good environment for people, so we send the cups down in nets attached to the wires used to lower scientific equipment.  The air in the cups all gets squeezed out by the pressure of the water and so the cups get smaller. When we bring the equipment back up to the ship the cups come up too.

Allison Jacobel and Pete Spooner.

• How long does it take for water pressure to crush something big?

No matter how big the object the pressure at the bottom of the ocean will crush it instantly (assuming it can be crushed).  The ROV ISIS isn’t crushed because it’s intentionally made of materials that are pressure resistant.

Allison Jacobel

• Is the pressure of the ocean different in some areas and what is the different to the mass of the ocean and the weight of the pressure?

An excellent, thoughtful question. Yes, the pressure of the ocean is different in different areas. The pressure is dependent on the amount of water, the temperature of the water and the salinity (salt content) of that water. Mostly we think of depth as the primary control on pressure and say that the deeper the water at a given location the greater the pressure.  However, if you have a column of warm, fresh water and a column of cold, salty water of an equal height the column of cold salty water will have a greater pressure at its base because it is heavier! If you keep the temperature and salinity of the water constant (let’s say fresh water at ~4^oC), and if the column of water has an area of 1m², for every additional cubic meter of water you add to your column the pressure at the base of the column will increase by ~10,000 Pa!

Allison Jacobel

• Is the pressure in the Atlantic stronger than other seas and oceans?

From the answer to the previous question we know that the pressure of a column of water will vary with temperature and salinity.  On average the Atlantic Ocean is saltier and colder than the Pacific so if you were to take a package of water of 4000m from the Atlantic and one from the Pacific, you would expect the pressure at the bottom of the one from the Atlantic to be higher.

Allison Jacobel