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2.3

## University of Liverpool

Skip to 0 minutes and 4 seconds So I’d like to talk about simple systems. A system is the part of the universe that is the centre of our attention. And the surroundings is everything else around it. So more formally, we can talk about a thermodynamic system as a well-defined quantity of matter, which can exchange energy with its environment. So we need to define the boundary to our system. And we can draw the boundary anywhere we like. So for instance, for the water in this cup, we can define the boundary as the interface with the cup and with the atmosphere. That would be an open system because we can transfer matter across our system boundary.

Skip to 0 minutes and 41 seconds We can add more water to the cup, or we could take a drink and remove the water. So that’s an open system. But the water inside this bottle is a closed system. Because the boundary here is impervious to matter. And so the water can’t escape and no extra water can get in. We can also talk about an isolated system. For instance, the coffee inside this flask is in an isolated system because the boundaries are impervious to everything. Nothing that happens in the surroundings causes any changes in the system. No heat can get in or out. And no matter can get in or out. So that’s a special type of system that we call an isolated system.

Skip to 1 minute and 22 seconds Another special type of system is a control volume. So if we look at something like this pipe here then we can have flow going in one side and out the other. So this is an open system. But because we have flow in and out we call it a controlled volume. System boundaries can be flexible and they can move. So if we wanted to analyse the flow as we pour water into the glass here, then we could define a boundary around the flowing water here. And that will be a flexible boundary of a control volume because water will be flowing in and out of our system.

Skip to 1 minute and 56 seconds Or if we are interested in the air inside a bicycle pump, then we could define our system boundary at the boundaries of the air inside here. And that system boundary would move as we push the piston up inside the pump.

# Simple systems for complex processes

Eann explains different types of thermodynamic systems and the terminology used to describe them with some everyday objects.