Charle's Law

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The Darth Vader hot air balloon is using Charles's law by increasing the temperature, and thus increasing the mass.
Charle's law states that at constant pressure, temperature is directly related to volume.  This is shown in the equation T/V=C, or temperature times volume equals constant.  This also means the colder the temperature the smaller the volume, and the larger the volume, the greater the temperature. The equation can also be expressed as CT=V, or VC=T.