Combined Science
Physics
Cambridge IGCSE
Physics
Cambridge IGCSE
TOPIC 2: THERMAL PHYSICS
The science behind steam power was the key to the industrial revolution. Understanding gases became a 'hot' topic. Scientists used particle models to help them understand how gases such as steam behaved.
The animation here shows gas particles moving freely in a box. The gas particles bounce off the sides and collide with each other. They are in constant random motion.
In this section, we will consider how these particles behave and what should happen when we change the temperature of the gas.
Figure 1. Particles in an ideal gas
Greg L | CC BY-SA 3.0
Some particles are coloured red to see their motion more clearly.
Pressure is a measurement of the force acting on a surface area, as covered in section 1.8. The international unit of pressure is the pascal (Pa). The particles in the box shown above (figure 1) hit the sides of the container at very high speeds. These collisions produce a force on the sides of the container. Therefore, the movement of particles make a pressure on the container. The pressure produced is creating a standard force per unit area.
This is a very common question in exams - you should be able to clearly explain the cause of this pressure.
Once scientists realised that these collisions caused the pressure in a trapped gas, they began to consider the effect of changing the conditions, and asking some simple questions using this model. What would happen to the pressure if we change variables such as the volume of the container or the temperature of the gas?
What would happen if the volume of the box (shown above in figure 1) is increased? In this case, there would be fewer collisions on the sides in a certain time, as the particles would have a greater distance to travel between each collision. Therefore, an increase in volume leads to a decrease in pressure.
A bike tyre has air pumped into it, and so it is compressed into a small volume. This makes the pressure in the tyre high as explained above, and keeps them inflated for a smooth ride.
Figure 2. Pressure in bike tyres
If we increase the temperature, the gas particles move faster. Scientists now know that the temperature of a substance is directly related to the average kinetic energy of the particles. If we increase the temperature of a gas, then two things will happen:
An increase in temperature leads to a pressure increase. This assumes that the volume of the container is kept constant.
Now have a go at these questions to check you understand this section:
Questions:
4. A basketball is left outside in strong sunlight. As the ball heats up, it appears to be harder to compress and bounces higher. This is because the pressure inside the ball has increased.
Using the idea of particles in a gas, explain why the gas pressure inside the ball has increased.
As the temperature increases, the particles move faster. This means they hit the sides of the container harder, and also more often. This produces a larger force on the sides and therefore a larger pressure.
5. An aerosol can contains a fluid under pressure. Explain why it is dangerous to expose the can to high temperatures.
If you increase the temperature, the pressure inside the can increases further. This could cause the can to break open or even explode in high temperatures.
