Section 5: Density and Pressure

Question 1

What’s the equation relating density, mass and volume

Answer 1

Density = Mass/Volume

Question 2

What is density measured in

Answer 2

g/cm3 or kg/m3

Question 3

Finding the density of air

Answer 3

1. Fill a round bottom flask with air.
   Stopper flask with bung with glass and rubber tubing with clip
2. Using a top pan balance measure the mass
3. Attach the flask to a vacuum pump to remove as much air as possible
4. Making sure the flask is sealed, remove the pump
5. Measure the mass of the flask, difference between the 2 readings is the mass of air removed
6. Fill a trough with a water
7. Place flask upside down, put end of tube into the water and unclip
8. Water rushes up into flask. The volume of water is the volume of air removed
9. Density = Mass / Volume

Safety:
Safety screen, plastic bag around flask)

Question 4

What’s the equation relating pressure, force and area

Answer 4

Pressure = Force / Area

Question 5

High pressure

Answer 5

Examples: drawing pin, high heels

• The force applied (e.g weight) over a very small area
• The force is very concentrated
• P = F/A
• This means that pressure exerted is very large

Question 6

Low pressure

Answer 6

Examples: snow shoes, camel’s feet

• The force applied over a large area
• The force is spread out
• P = F/A
• This means that pressure exerted is very small

Question 7

Pressure in fluids

The 3 holed can

Answer 7

3 holed can:
The water comes out faster and travels further across the deeper you go in the liquid

Conclusion:
Pressure increases as you go deeper in fluid

Question 8

Magdebug Hemisphere

Answer 8

1. Air inside removed using a pump so pressure inside is very low
2. The pressure outside is high (normal atmospheric pressure)
3. Students/ horses unable to pull hemisphere apart
4. Forced together by pressure difference

Question 9

What is a manometer

Answer 9

A U-shaped tube used to measure difference in pressure.

It contains liquid and one end of the tube is attached to a gas tap. The difference in liquid height on either side shows the diff in pressure between air and gas.

Question 10

What is normal air pressure

Answer 10

Normal air pressure : 101,000 Pa

Question 11

Difference in pressure equation

Answer 11

Diff in pressure = △ h x p x g

= △ height x density of liquid x gravity

diff in pressure = pressure of gas supply - atmospheric pressure

Question 12

What is pressure?

Answer 12

Pressure = how concentrated a force is

Question 13

What is the unit for pressure

Answer 13

Pa(N/m2) or N/cm2

Question 14

How would you measure the volume of an irregular shaped solid?

Answer 14

measure its displacement in water

Question 15

A manometer contains water of density 1000 kg/m3, when attached to a gas tap there is a 8 cm difference in water levels. Given atmospheric pressure is 101 000 Pa, calculate pressure of the gas.

Answer 15

Diff in pressure = △ h x p x g

= 0.08 x 1000 x 10
= 800 Pa

101 000 + 800 = 101 800 Pa

Question 16

How does pressure act at a point in gas or liquid which is at rest?

Answer 16

Equally in all directions