P1.3 Pressure

Question 1

Describe the motion of molecules in a gas

Answer 1

They are in constant random motion

Question 2

What is the unit for pressure?

Answer 2

Pascal (Pa)

Question 3

How can the pressure of a gas be increased whilst kept at constant temperature?

Answer 3

The volume can be decreased.

Question 4

How can the pressure of gas be decreased whilst kept at constant temperature?

Answer 4

The volume can be increased

Question 5

What effect does increasing temperature have on the pressure of a gas when held at constant volume?

Answer 5

Pressure of the gas will increase as the temperature increases.

Question 6

Why does pressure increases as temperature increases (at a constant volume)?

Answer 6

• Kinetic energy of molecules increases
• Collisions between molecules becomes more frequent
• Greater rate of change of momentum
• Greater force and therefore pressure

Question 7

If gas A is at a low pressure, and gas B is at high pressure, what can be said about the rate of collisions in each gas?

Answer 7

• There are more collisions per second in gas B than in gas A
• The rate of collision is higher in B

Question 8

Describe the force that the pressure of a gas exerts on the walls of its container.

Answer 8

• The net force acts at right-angles to the container’s surface
• The magnitude of the force increases as pressure increases

Question 9

Explain how increasing the volume of a gas results in a decrease of pressure

Answer 9

• Molecules become more spread out so collisions become less frequent
• This reduces the rate of collisions
• Rate of change of momentum decreases, and so force exerted on container decreases, resulting in a lower pressure

Question 10

What can be said about the product of pressure and volume for a fixed mass of gas at a constant temperature?

Answer 10

It is constant.

p V = constant

Question 11

What increases when you do work on a gas?

Answer 11

• The internal energy of gas

- This can also lead to an increase of temperature

Question 12

Why does the temperature of air inside a bike pump increase when it is pumped?

Answer 12

• Work done on a gas when it is compressed
• Doing work on a gas increases its internal energy, so also increases the average kinetic energy of the molecules
• Temperature increases with an increase of average kinetic energy

Question 13

Why does atmospheric pressure decrease as you move higher above the earth’s surface?

Answer 13

The atmospheric pressure at a point is equal to the weight of the air in the column above that point, as you move higher above the earth the amount of air in the said column decreases - resulting in less weight above and as a result less pressure at the point.

Question 14

What does an object in a fluid experience as a result of pressure?

Answer 14

Forces at a right angles to all of its surfaces.

Question 15

Pressure in a fluid ______ with depth.

Why?

Answer 15

• Increases, because it is caused by the gravitational force on the fluid above the point.
• The deeper you go, the greater the weight of water above you, and so a higher force and pressure.

Question 16

What equation is used to calculate the pressure at a given depth of liquid?

Answer 16

Pressure = depth x liquid density x gravitational field strength

Question 17

What is upthrust?

Answer 17

The upwards force that an object experiences in a fluid as a result of a greater pressure below the object than above it.

Question 18

Under what circumstances will an object float?

Answer 18

When the object’s upthrust equals its weight.

Question 19

Under what circumstance will an object sink?

Answer 19

When the object’s upthrust is less its weight.

Question 20

When you submerge on object in a liquid, what is the upthrust acting on the object equal to?

Answer 20

The weight of the volume of liquid displaced as it is submerged

Question 21

What can be said about the density of an object that sinks in water?

Answer 21

The object’s density is greater than the density of water.