Physics – Pressure (P1.3)
Pressure in a fluid
Key words
Learning objectives:
pressure pascal (Pa) density upthrust
• explain how pressure acts in a fluid • calculate pressure at different depths in a liquid • explain what causes upthrust.
Submarines can only dive safely to a certain depth. If they go deeper than that, the water pressure is too great for the hull to withstand and it gets crushed. plastic bag
Pressure in fluids
small holes
The pressure in fluids causes a force normal to (at right angles to) any surface. This means the pressure in a fluid acts in all directions (Figure 1.32). A fluid can be either a liquid or a gas. To calculate pressure we need to work out how much force is being applied over a certain area. We use an area such a square metre, or a square millimetre. We refer to this as the unit area. Pressure is the force acting normal to a unit area of a surface. force normal to a surface Pressure, p = area of that surface F p= A Pressure is measured in pascals (Pa) when the force is in N and the area is in m2. 1 Pa = 1 N/m2. A pascal is a small unit so we often use kilopascals (kPa).
water comes out in all directions
Figure 1.32 Pressure in a fluid acts in all directions
Higher tier ONLY
Pressure at different depths in a liquid The deeper you go in water, the greater the pressure becomes. This is because the deeper you are, the greater the weight of the water above you. If you dive into deep water, your ears may hurt because the pressure outside your ears is much greater than the pressure inside your ears.
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1
If you make holes at different heights on the side of a tall container and fill it with water (Figure 1.33), why does water come out further from the lower holes?
2
Imagine taking a sealed tin can on a dive. As you take it down, deeper and deeper, think about the pressure acting on it. Explain what will happen to the can if you take it deep enough.
Figure 1.33 Pressure increases with depth
OCR Gateway GCSE Physics: Student Book
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8/11/16 7:34 AM