How does pressure change as the depth of water changes?
PR O B L E M SO L V E R
Use a milk carton to perform your investigation. Create a procedure, making sure the holes are spaced evenly down the side of the container.
• Predict what you think you will observe.
• Perform your investigation. Compare your results with your prediction.
1 Explain your observations using a sketch and appropriate terms from this unit.
Evenly space the holes down the side of the container.
Why does the water flow out of the can this way?
Pressure and Depth
In the Problem Solver activity, you saw that pressure forced water out of holes in a container. The water was exerting pressure on the walls of the container. The mass of water in the upper part of the container acted as a force pressing down on the water in the lower part of the container. The more water there was above the hole, the greater was the pressure, and the farther the water flowed out of the container.
1 How does Pascal’s law explain your observations in the Problem Solver: What Pascal Saw?
2 If your container had been taller and the holes at the same height as you placed them in the Problem Solver activity, what would you observe? Why?
3 How might the ideas related to pressure and depth be used to create new technologies?
4.5 Hydraulic Systems
The serious work of the Jaws of Life® in helping free victims from the twisted wreackage of automobile crashes depends on a hydraulic system.
Fun activities at an amusement park also depend on hydraulic systems.
Systems that use liquids in a confined space to transfer forces are called hydraulic systems. These systems operate according to Pascal's law. The liquid put into a hydraulic system is called hydraulic fluid. It is moved through the system by a pump.
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The power of a hydraulic system, however, exists when there is a difference in area between two parts of a system. A small input force can be increased by designing an output device that has a larger area than the input device.
For example, look at the diagram of the car lift or hoist on this page. In the diagram, you can see that the input device is a piston with a small area. A piston is similar to the syringes you used in your activities. The output device is another piston, but it has an area 16 times larger than that of the input device.
In a hydraulic system, pressure results from a force applied to one surface of the liquid by a piston, or cylinder. This pressure is then transmitted equally throughout the entire hydraulic system.
Force applied to one part of a system results in the same pressure throughout the system.
small movable piston
A
B
large movable piston
pressure = 8 kPa force: 4000 N area: 0.5 m2
force: 64 000 N area: 8 m2
A car hoist is used in a repair garage so that mechanics can work more easily and safely. Each arrow in the picture represents the same amount of force. What conclusion can you make about the forces acting on the two pistons?
force (N)
A hydraulic system can create a mechanical advantage. This means that the output force is larger than the input force. In the hydraulic system on the previous page, a fluid exerts the same amount of pressure throughout the system, so there is 16 times as much force being exerted by the fluid on the larger piston. Notice that the small piston has to move a larger distance than the large piston to move the car.
The hydraulic fluid in a system is not used up. It continues to circulate throughout the system. A pump, a machine or device that displaces a fluid by physical or mechanical action, is used to force the fluid to flow, allowing it to move against gravity. Valves control the flow of fluid and keep the fluid flowing in the desired direction.
There are many technologies that use hydraulic systems to move large, heavy objects easily. These include backhoes and excavators, and different systems in a vehicle including brakes and steering.
A Biological Hydraulic System: The Heart
Your circulatory system is a hydraulic system. The hydraulic fluid is blood, and there are valves that control the direction of fluid flow.
Imagine a pump no bigger than a clenched fist that pumps 7200 L of fluid through more than 95 000 km of tubing every day for 80 years. This extraordinary pump, your heart, beats over 100 000 times a day, moving blood through your arteries and veins, supplying all living cells with nutrients while carrying away waste products. Blood pressure is a measure of the force being exerted on a given surface within your circulatory system.
Valves are an important part of any system for moving fluids.
They are devices to control or regulate the amount and direction of flow, like the valves in your bathroom taps.
Your circulatory system needs one-way valves to make sure that blood only moves in one direction.
• Find out how valves in veins work.
• Find out how heart valves work.
• What can be used to replace human heart valves when they no longer work properly?
r e SEARCH
Your heart is a pump with valves that pushes hydraulic fluid (blood) through your circulatory system.
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Hypertension is a common heart disease related to the hydraulic nature of the blood system. In hypertension, an increase in pressure inside the blood vessels causes the heart to work harder just to keep the blood circulating.
A model of a hydraulic lift can demonstrate the power of fluids under pressure.
Hydraulics as Transport Systems
So far, you have examined closed hydraulic systems, where the liquid cannot leave the system. In an open hydraulic system, the liquid can leave the system. Open hydraulic systems can be used to transport liquids. The hand pumps used to bring fresh ground water to the surface through pipes in African countries such as Malawi are examples of open hydraulic systems. In Canada, there are many fluids transported through pipes, including water, sewage, oil, and gas. These fluids must be under pressure in order for them to move through the pipes.