PhysicsLAB: Syringes and Vacuum Pumps

The old fashioned farm-type pump shown in Figure 20.9 in your text is a lift pump that operates by atmospheric pressure. Its operation is similar to that of a common syringe (the device that holds the needle your doctor or dentist uses, or the gadget inside a toy water pistol).

Refer to the following information for the next four questions.

The nozzle of the syringe is held beneath the surface of water in Figure A. When the piston is pulled up, water fills the nozzle and the chamber under the piston.

What forces water into the nozzle and chamber?

When the syringe is removed from the water and held in the air, the water doesn't leak out. Why?

Would water leak out if air leaked past the piston? Explain briefly.

What happens if the piston is pushed down?

A lift pump is like a syringe with two valves.

Valve A is like a little door that lets water come up but will not let water go down again.
Valve B is in the piston; it also lets water come up but not down again.

In Figure B, both valves are closed and there are no regions of reduced air pressure.

Refer to the following information for the next four questions.

When the piston is raised (Figure C), air pressure is reduced in the pipe that extends into the water. This allows water from below to be pushed up into the pipe by

atmospheric pressurea vacuum

Valve A opens as water enters the chamber. When the piston is pushed down (Figure D), valve A closes and valve B

closes alsoopens

This lets water enter the chamber above the piston. When the piston is lifted again (Figure E), Valve B

closesopens

and water is lifted to flow

out of the pumpback into the pipe

Under ideal conditions and a perfect vacuum, what is the maximum height that water at sea level can be lifted in this manner?

Would this maximum height be greater or less for a lift pump high in the mountains? Defend your answer.