 # Gas Pressure

In this worksheet, students will learn about the concept of gas pressure in relation to the Particle Model. Key stage:  KS 4

Curriculum topic:  Physics: The Structure of Matter

Curriculum subtopic:  Relating Models of Arrangements and Motions of Molecules

Difficulty level:   ### QUESTION 1 of 10

Think what would happen if you were to push a pin on its pointy end. Yes, you would hurt yourself, but why? The pin would push on your finger so much that it would break the bonds between the cells and become physically inserted into your finger. What would happen if you did the same, but this time with the flat end? The pin would go into the wall, this time breaking the bonds of the atoms in the wall and making a nice snug home for itself. Assuming you applied the same force in both cases, the end result would be different, because of the difference in the pressure applied on your thumb. In the first case, the force is concentrated on a very small area, so the pressure is very high. In the second, the force is spread out over a larger area, so the pressure is lower.

The formula used to calculate pressure is:

pressure = force / area

When the force is in Newton (N) and the area in m2, the unit for pressure is Pascal (Pa) or N/m2. If the area is in cm2, then the pressure is measured in N/cm2.

Let’s think about this now, this means that if you increase the force then the pressure goes up. What will happen if you increase the area though? Well because pressure = force divided by area; this means that pressure is inversely proportional to pressure. This means that as area increases, the pressure will decrease.

Let’s not think about this in terms of gases and individual particles. Use your imagination to visualise how gas particles collide (crash) onto the walls of the containers in the diagram below. Each time they hit the walls of the container; they push it a little bit. Just like if you were to throw a ball at a book, when they collide the book will move back a little bit – it has been pushed. This is where our force comes from in the equation pressure = force/area.

In this example, the second container has a smaller volume, but the number of particles is the same, so the particles collide more, because there is not much space. Remember, every time there is a collision, there is a force small force – so if there are more collisions, what will happen to the total force? Boyle's Law is used to calculate how much the pressure changes when the volume of a gas changes, provided the temperature and mass of the gas stay the same:

P1 × V1 = P2 × V2

P1 and V1 is the pressure and volume at the start, and P2 and V2 are the final pressure and volume of the gas. If you rapidly change the volume of the container, then you will change the pressure as well as there will be more collisions in a smaller area – both of these are conditions for a higher pressure.

Why would you hurt yourself if you pushed a pin on its pointy end?

The force would spread out.

The force would concentrate on the tip of the pointy part of the pin.

The pressure on the pointy end is smaller.

Calculate the pressure if a force of 238.0 N is applied on an area of 2.3 m2. Give your answer to an acceptable number of decimal places.

What happens to gas pressure when the volume is reduced?

Increases

Decreases

Stays the same

If you were to increase the temperature of a gas, what do you think would happen to the pressure?

If a system has an initial pressure of 5Pa in a volume of 3m3 and it is then compressed to a pressure of 8Pa, what would the new volume be? Give your answer to an appropriate number of decimal places.

Choose the appropriate word to complete the sentence.

Gases are ___________.

Compressible

In-compressible

What is the atmospheric pressure on the surface of the Earth (at sea level)?

1000 Pa

10,000 Pa

100,000 Pa

What happens to atmospheric pressure at high altitudes?

Increases

Decreases

Stays the same

What causes pressure in gases?

Volume

Temperature

Collisions of particles

Use Boyle's Law to calculate the pressure inside a 200 cm3 balloon, if the initial pressure is 7 N/cm3 and you squash it to 140 cm3.

Your answer must be in N/cm3.

1540

1000

10

• Question 1

Why would you hurt yourself if you pushed a pin on its pointy end?

CORRECT ANSWER
The force would concentrate on the tip of the pointy part of the pin.
EDDIE SAYS
The force would be concentrated on a very small area so the pressure would be very high. They are inversely proportional.
• Question 2

Calculate the pressure if a force of 238.0 N is applied on an area of 2.3 m2. Give your answer to an acceptable number of decimal places.

CORRECT ANSWER
103.5
103.5 Pa
EDDIE SAYS
Force = 238.0N Area = 2.3 m2 Force = 238.0 / 2.3 Force = 103.4782 The question is given to 1dp so your answer should also be given to 1dp. Force = 103.5Pa - remember the unit!
• Question 3

What happens to gas pressure when the volume is reduced?

CORRECT ANSWER
Increases
EDDIE SAYS
In a gas, the more collisions you have per unit area, the more pressure you have. By reducing the volume, you are increasing the number of collisions per unit area.
• Question 4

If you were to increase the temperature of a gas, what do you think would happen to the pressure?

CORRECT ANSWER
Increase
Goes up
EDDIE SAYS
The temperature increase would mean that the particles are moving faster. This would, in turn, mean that there would be more collisions with a higher force - this is all pointing to the pressure going up!
• Question 5

If a system has an initial pressure of 5Pa in a volume of 3m3 and it is then compressed to a pressure of 8Pa, what would the new volume be? Give your answer to an appropriate number of decimal places.

CORRECT ANSWER
2
EDDIE SAYS
V1xP1 = V2xP2 Work out the left hand side first. V1 = 3m3 P1 = 5Pa V1xP1 = 15. Rearrange the equation 15 = V2xP2 15/P2 = V2 Put in the final numbers P2 = 8 15/8 = V2 V2 = 1.875‬m3 Don't forget to round!
• Question 6

Choose the appropriate word to complete the sentence.

Gases are ___________.

CORRECT ANSWER
Compressible
EDDIE SAYS
Gases can be squashed; they are compressible - this is why we need to consider Boyle's law when looking at gases and pressure.
• Question 7

What is the atmospheric pressure on the surface of the Earth (at sea level)?

CORRECT ANSWER
100,000 Pa
EDDIE SAYS
The atmospheric pressure on the surface of the Earth is 100,000 Pa.
• Question 8

What happens to atmospheric pressure at high altitudes?

CORRECT ANSWER
Decreases
EDDIE SAYS
If we were to think about it - there is a massive column of air 15 miles high pressing down on you, so the air near the sea is going to be squished by the column. We know that when air is squished it increases in pressure, so the air at sea level must be at a higher pressure.
• Question 9

What causes pressure in gases?

CORRECT ANSWER
Collisions of particles
EDDIE SAYS
Gas pressure is caused by collisions of the gas particles with the surfaces around them. Whilst volume and temperature can affect the pressure - it is only the collisions of the particles with the walls that will create the pressure.
• Question 10

Use Boyle's Law to calculate the pressure inside a 200 cm3 balloon, if the initial pressure is 7 N/cm3 and you squash it to 140 cm3.

Your answer must be in N/cm3.

CORRECT ANSWER
10
EDDIE SAYS
P1 x V1 = P2 x V2
200 x 7 = P2 x 140
P2 = 1400 / 140 = 10 N/cm³
---- OR ----

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