A camp stove's flame sits under a sealed metal cylinder of gas. The gas warms, swells, and pushes the piston outward.
Energy is entering this closed system and leaving it at the same time. This lesson counts every door energy can use to cross a closed system's boundary.
Energy enters the gas from the flame. The flame is hotter than the gas, so the entry is heat.
Energy leaves the gas at the piston. The gas's force moves the piston outward through a distance, so the exit is work.
Hunt for a third door on a closed system and the hunt comes up empty. Matter cannot cross, and every energy transfer across the boundary is driven either by a temperature difference or by a force acting through a distance.
The claim is useful when one door is ruled out. If energy is crossing and the transfer is not heat, it must be work.
And the other way around: if energy is crossing and no force acts on the boundary through a distance, the transfer is heat.
Energy is entering a sealed tank of air, and no part of the tank's boundary is moving. Which way is the energy entering?
A transfer by work needs a force acting through a distance, and nothing on the boundary moves through a distance.
A closed system has exactly two doors, so ruling out work leaves one.
The entry is heat.
A sealed foil balloon is losing energy. Its skin is slowly swelling outward, moving the air around it back, and the balloon sits at the same temperature as the room. Which way is the energy leaving?
Equal temperatures rule out heat.
The skin moves outward: the balloon's force acts on the outside air through a distance.
The exit is work.
Energy is leaving a sealed glass bottle of tea, and nothing on the bottle's boundary moves. How is the energy leaving?
Birta says: 'If energy is crossing into a closed system and the transfer is not work, it has to be heat.' Is Birta correct?
Which statement about a closed system is correct?
Which option correctly pairs each transfer with its driver?
Energy is leaving a sealed metal drum of air. The drum and the room are at the same temperature, so the transfer is not heat. What must the transfer be?
Which pair names the only two ways energy crosses a closed system's boundary?