1. Starter: What do you understand already about… Density Pressure
Pressure in liquids
𝐷𝑒𝑛𝑠𝑖𝑡𝑦 ρ= 𝑚𝑎𝑠𝑠 𝑚 𝑣𝑜𝑙𝑢𝑚𝑒 𝑣 = 𝑘𝑔 𝑚 3
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑃= 𝐹𝑜𝑟𝑐𝑒 𝐹 𝐴𝑟𝑒𝑎 𝐴 = 𝑁 𝑚 2 =𝑃𝑎
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑃= ρ𝑔ℎ= 𝑁 𝑚 2 =𝑃𝑎

2. Lesson goals… To explain and use a manometer to measure gas pressure.
To explain why pressure and forces are transmitted through liquids and gases.
To explain the principles behind hydraulic lifts/ brakes.

4. Manometer
We can measure the air pressure
using a manometer

5. Manometer
When the pressure acting on the left equals the pressure on the right the height either side of the manometer is the same

6. Manometer
The pressure on the left (the air pressure) is the same as before
BUT the pressure on the right is now bigger (different gas)
So for it to be even…
The air pressure AND the pressure caused by the difference in height from the liquid can then balance.
And luckily…
we have an equation for that
BRILLIANT
+ =

7. Pressure = g ρ h REALLY IMPORTANT
The manometer measures the difference in the pressure between two gases
Therefore if you want the actual pressure of the gas you need to add on the air pressure to your result from using the equation P= ρ g h

8. Manometer
A manometer uses the fact that liquids transmit pressure to measure the pressure of a gas.
The difference in pressure of the gas to atmospheric pressure is found using the difference in height h.
ΔP = ρgh

9. Using a manometer
Use the manometer to measure the pressure of the gas supply.
Turn on the gas supply GENTLY!
Measure the difference in height between the two halves of the manometer.
In your books.
Draw a diagram of the manometer & explain how it measures gas pressure. What were your results?

10. Liquids cannot be compressed because the particles are already close together10

11. This means that liquids can be used to send forces from one place to another in hydraulic systems
Liquids cannot be compressed because the particles are already close together 11

12. Hydraulics
Hydraulic systems use the principle that pressure is transmitted throughout a liquid to allow a force applied in one part of a system to be transferred to another.
This allows us to avoid linking the parts mechanically. All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid.
Force
applied
here
Force
transferred
here
Pressure inside all parts of the
hydraulic system is the same

13. Hydraulic machines
If you push on the top syringe plunger what will happen?
Would there be a difference if the syringe system was filled with air not water?

14. Hydraulics Pressure is transmitted through a fluid.
A hydraulic machine uses this to multiply a force e.g. car brakes.
A hydraulic system will not work if any air is trapped within it. The air will not transmit pressure, but will be compressed.
The particles in the liquid are much closer together and so can not be compressed.

15. How do pistons work?
A small force F1 on a small area A1 transmits a pressure through the liquid which over a larger area A2 creates a much larger force F2
The pressure at the small piston = pressure at the large piston. F2

16. Hydraulic Example
Stick example into your book and complete.

17. Force
Area
a) Pressure =
10N
2cm2
= 5N/cm2
b) Force =
Pressure  Area
= 5N/cm2 x 20cm2
= 100N

18. Homework Using hydraulic systems worksheet
Answer all questions showing all working clearly & drawing out a diagram (not complicated) for each question.
Due in Monday.