An operator in a hot-air balloon activates a burner and the balloon gains altitude. Briefly describe the thermodynamics of this event in a qualitative way, identifying any heat or work done (and their signs), as well as any processes (be sure to state whether a process was done reversibly or irreversibly). If you wish and find it necessary, you may use reasonable, simplifying assumptions provided they are clearly stated.
When a hot air balloon is heated, the air inside the balloon absorbs energy and gets heated up, which causes the temperature to increase and air particles' kinetic energy increases. This causes the balloon to inflate and the density of air inside the balloon is much less than that of the surrounding air ( which is much colder).
When this happens, this balloon gets lifted up because of the buoyant force exerted on it by the air displaced by the balloon, due to Archimedes' principle.
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