Why does a sausage become warm when placed in an oven

Why does a sausage become warm when placed in an oven  

We can express this truth by saying the sausage gets warmer as the magnitude of its internal energy increases so, from Equation 

But we can now be more specific. The internal energy U changes in response to two variables, work w and heat energy q, as defined by 

We have already met the first law of thermodynamics. Equation (3.5) here is the definitive statement of this law, and is expressed in terms of the transfer of energy between a system and its environment. In other words, the magnitude of AU is the sum of the changes in the heat q added (or extracted) from a system, and the work w performed by (or done to) it. 

The internal energy can increase or decrease even if one or other of the two variables, q and w, remains fixed. Although the sausage does no work w in the oven, the magnitude of AU increases because the food receives heat energy q from the oven. 

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