Pascal's principle: the hydraulic lift
Push the small piston of a hydraulic lift and see how the pressure is passed through the oil to lift a car on the big piston.
Core concept
Pressure applied to an enclosed liquid is passed on equally in all directions, so a big piston gets a big force
Topic: Pressure
Liquids cannot be squashed. Pascal's principle says that a pressure: How much force presses on an area. A sharp knife has high pressure at its edge. All key words applied to a liquid in a closed container is passed on equally to every part of the liquid. In a hydraulic lift, a small force: A push or a pull. Forces can start, stop or turn things. All key words on a small piston makes a pressure P = F₁ ÷ A₁. The same pressure pushes up on the large piston, giving a force F₂ = P × A₂, which is bigger because A₂ is bigger. But the large piston moves a shorter distance, so no energy: What is needed to make things move, heat up, light up or grow. All key words is gained. Car lifts, hydraulic brakes and JCB diggers work this way.
What you will learn
- State Pascal's principle.
- Calculate the output force of a hydraulic machine using F₂ = F₁ × A₂ ÷ A₁.
- Explain why the large piston moves a shorter distance.
Before you start
- Calculate pressure = force ÷ area.
Materials
- Syringes of different sizes joined by a tube filled with water (a model)
Equipment
- Hydraulic lift model with interchangeable pistons
Procedure
- Push with 20 N on the 1 cm² piston with the 100 cm² piston on the other side. Record whether the car lifts.
- Change the areas and the effort until the 4000 N car is lifted, and record it.
- Compare the pressure under each piston and how far each piston moves.
How this simulation is simplified
- The oil cannot be compressed and there is no friction at the pistons.
- The car weighs 4000 N, and the small piston is pushed down 10 cm each time.