Using the density of air in a hot air balloon, explain why it rises.
A graph has temperature in K on the x-axis and volume in cm^3 on the y-axis. The points marked on the graph are 273 on x axis and 108 on y axis and other readings such as (278,110), (283, 112), (288, 114), (293,116), (298, 118), (303, 119), etc. Use this information to explain why a hot air balloon rises.
A hot air balloon rises when the air filled in the balloon it is able to displace a volume of air that has a mass greater than the mass of the balloon. This follows from Archimedes' Principle. The balloon continues to rise till the condition mentioned earlier is satisfied. When the mass of the air displaced by the balloon is equal to the mass of the balloon, it floats.
As you can see from the values in the graph, air expands when it is heated. The expansion means a decrease in density as density is given by mass/volume. You can see by the points on the graph (278,110), (283, 112), (288, 114), (293,116), (298, 118), (303, 119) that a point with a higher x-coordinate also has a higher y-coordinate.
Atmospheric air cools as the altitude increases. The balloon will rise till a certain altitude based on the temperature of the gas filled in it. If the temperature of the gas is increased, it rises higher; on the other hand, if the temperature of the gas filled in it is lowered, it drops to a lower altitude.