6 — pressure

Question 1

Pressure

Answer 1

Pressure is defined as the force acting per unit area

SI: pressure
1N/m^2

Question 2

Density

Answer 2

Density is defined as mass per unit volume of a substance

Question 3

Atmospheric pressure

Answer 3

Weight of a layer of air that acts on the Earth per unit area

Question 4

Density of mercury

Answer 4

13600kg/m^3

Question 5

Approximate density of air

Answer 5

1.28kg/m^3

Question 6

Atmospheric pressure

Answer 6

P0 = 1.0 x 10^5 N/m^2
= 1.0 x 10^5 Pa
= 76cmHg
= 1 bar
= 1 atmosphere

Question 7

Characteristics of liquid pressure

Answer 7

1. Pressure in liquid acts in all directions
2. Pressure at the same depth is the same
3. Pressure depends on depth

Question 8

Properties of liquid relevant to Pascal’s Principle

Answer 8

1. Liquids r incompressible
2. If pressure is applied to a trapped liquid, the same pressure is transmitted to all parts of the liquid.

Question 9

Barometer

Answer 9

1. Instrument to measure atmospheric pressure
   - mercury barometer
   - aneroid barometer
2. Length of a mercury column is used to obtain readings to measure atmospheric pressure
3. Shape of tube/angle of tilt and size does not affect the height of the mercury column. Pressure only depends on the vertical height of the liquid.

Question 10

Manometer

Answer 10

1. Instrument to measure gas pressure
2. U-shaped tube containing a liquid, can be mercury, water or oil
3. Pressure of gas>P0 = h cmHg + 76cmHg
4. Pressure of gas <P0 = 76cm Hg - h cmHg
5. Constant atmospheric pressure -> constant difference in height between 2 points of the manometer

Question 11

Lifting heavy weights w small pressure

Answer 11

Liquid is incompressible, pressure exerted on oil by small piston is transmitted uniformly throughout the oil to the larger pistons. Due to larger area, a greater force is exerted on the larger piston.

Question 12

How leak in system affects pressure to lift weight.

Answer 12

The force exerted on object will be smaller due to loss in pressure in the oil by the leak. Hence, pressure transmitted from smaller piston to larger piston is decreased.

Question 13

The smaller piston moves 25cm to the left. State and explain whether large piston moves >25cm, = 25cm or <25cm.

Answer 13

Smaller than 25cm. Since work done is equal, a larger force will result in a smaller distance moved by the larger piston.

Question 14

State one modification to the system to lift a heavier load for the same applied force.

Answer 14

Decrease area of small piston/ increase area of larger piston

Question 15

If A and B are mid-points of liquid C and water column respectively, would pressure at both points be identical?

Answer 15

Yes pressure will be identical. Since difference in height representing atmospheric pressure is constant and heights are halved equally, pressure between the points will be identical.

Question 16

When the U-tube is brought up a high mountain, state and explain what would happen to levels of X and Y.

Answer 16

Difference between the levels remain the same. Decrease in atmospheric pressure will be the same on both arms.

Question 17

If liquid L in point X of the U-tube is replaced by another liquid of the same volume twice as dense, state what would happen to levels of X and Y.

Answer 17

Level at X will be lower than that at Y. New liquid will exert greater pressure and push Y up.
(Take reference from when gas w pressure >P0)

Question 18

Consideration of designing U-tube for set-up.

Answer 18

Length must be long enough for water measuring atmospheric pressure.

Question 19

Crushed can by atmospheric pressure

Answer 19

1. Some water was heated in an open can until the water boils.
2. The can is removed from the heat and the cap is screwed on.
3. The can is now placed under running water.
4. The metal can suddenly crumples.

a. Steam produced remove some air from the can.
b. Prevents air from entering back to the can
c. To condense the steam in the can, removing more gas from can.
d. Force due to atmospheric pressure acting on can > inner pressure exerted on the walls of the can thus can is crushed.

Question 20

Why are we not crushed by atmospheric pressure.

Answer 20

Pressure inside our body balances atmospheric pressure.

Question 21

Uses of atmospheric pressure

Answer 21

Rubber sucker sticking onto kitchen tiles.
Drinking water from straw.

Question 22

Uses of liquid pressure

Answer 22

Hydraulic jack
- 2 cylinders of different diameters joined tgt by a pipe. Liquid used is usually oil.
Hydraulic car brake

Question 23

Why can’t air be present in the mercury barometer. (Why vacuum?)

Answer 23

If air is present, there is air pressure is acting on mercury -> presenting inaccurate value of atmospheric pressure.

Question 24

Suggest why the hydraulic jack cannot work properly if it contains bubbles of air. [1]

Answer 24

Pressure is not able to transmit uniformly as air is compressible and some of the work done is used to compress the air bubble, resulting in a loss in pressure from the reduction in the force exerted by piston Q.