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ZingPath: Mass, Weight, and Pressure

Pressure in Liquids

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Mass, Weight, and Pressure

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Pressure in Liquids

Physical Science

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You investigate factors that influence pressure in liquids.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that liquid pressure is directly proportional to the depth of the immersed object.
  • Explain that for liquids of different densities, the pressure at the same depth will be different.
  • Explain that when the liquid type and depth remain constant, the shape of a container does not affect pressure in liquids.

Everything You'll Have Covered

All matter is composed of small particles (atoms, molecules, and ions). Scientists often classify matter on Earth in three main states-solids, liquids, and gases-and these states have properties that identify them. Particles of liquids are closely packed but are not fixed in definite places and are freer to move than those of solids. However, since there is little space between particles of liquids, they cannot be compressed easily. Particles in a liquid state have more kinetic energy than particles in solids. This means the particles can move past each other, so that liquids can flow and take the shape of their container.

Liquid pressure can be measured with a U-tube manometer. It can be calculated using the following formula: P = pgh, where P is pressure, p is fluid density (mass/volume), g is acceleration of gravity, and h is depth. The SI unit of measure for pressure in liquids is the pascal (Pa). Pressure in liquid is directly proportional to the depth of an immersed object, and the pressure at the same depth is different in different liquids when their densities are different. However, when the type of liquid and depth remain constant, the shape of a container does not affect pressure in liquids.

Learners experiment with containers of liquids in this Activity Object. The same principals apply to large bodies of water, too. Salt water is slightly denser than fresh water, so salt water exerts slightly more pressure than fresh water. The air pressure at sea level is defined as one atmosphere of pressure (1 ATM). Pressure increases at a rate of 1 ATM for every 33 feet of salt water or 34 feet of fresh water, so a SCUBA diver in an ocean experiences 2 ATM at 33 feet of depth, 3 ATM at 66 feet, and so on. (A diver in a body of water exceeding 1000 feet above sea level must take this into account when calculating water pressure.) The shape of the land under the body of water (which is analgous to the container of the liquid) does not affect pressure.

Pressure in any state of matter is equal to the force exerted per unit area, or P = F/A. The SI unit for pressure is the pascal (Pa) and is equal to one newton per square meter: Pa = N/m2. Blaise Pascal, a French mathematician, physicist, and religious philosopher, lived in the seventeenth century. His earliest work was in science where he made important contributions to the study of fluids (liquids and gases) and expanded on the work of Evangelista Torricelli regarding pressure and vacuums. Pascal discovered a helpful property of fluids. Pascal's principle states that "a change in the pressure of an enclosed incompressible fluid is conveyed undiminished to every part of the fluid and to the surfaces of its container." Pascal's principle is put to good use when mechanics raise a car on the hydraulic lift or when you squeeze mustard out of a serving-size packet.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with pressure.
Course Physical Science
Type of Tutorial Experiment
Key Vocabulary depth, experiment, fluid