Physics · Ch 8 — Heat and Thermodynamics
First Law of Thermodynamics
First Law of Thermodynamics
The first law of thermodynamics extends the law of conservation of energy to explicitly include heat: the change in a system's internal energy equals the heat supplied to it minus the work done by it on the surroundings, (Some books instead write , treating work done ON the system as positive -- either convention works provided it is applied consistently.) Figure 8.20 fixes the sign convention used throughout the unit: when the system gains heat, when it loses heat; when work is done BY the system, when work is done ON it. Heat flowing in, or work done on the system, both increase ; heat flowing out, or work done by the system, both decrease it. Examples 8.12 and 8.13 apply the first law to a person stirring water with a paddle (mechanical work in, heat lost through the container) and to jogging (wo …
What this figure shows. A 2x2 grid of four small diagrams, one for each combination of the signs of Q and W: top-left shows Q>0, W>0 (heat flows into the system while the system also does work outward); top-right shows Q>0, W<0 (heat flows in while work is done on the system); bottom-left shows Q<0, W>0 (heat flows out while the system still does work outward); bottom-right shows Q<0, W<0 (heat flows out while work is also done on the system). Each panel uses arrows crossing a system boundary to show the direction of heat flow and the direction of the piston/boundary motion for work, fixing the sign convention used throughout the rest of the unit for e …