textbook 4b.7

[Maia V 1L]

In a combustion chamber, the total internal energy change produced from the burning of a fuel is -2573 kJ. The cooling system that surrounds the chamber absorbs 947 kJ as heat. How much work can be done by the fuel in the chamber?

Why would it be -947 and not positive 947 when calculating work?

[Andrew Nguyen 1E]

The question is asking us to consider the fuel. Additionally, it states that the surrounding chamber absorbs 947kJ of heat from the combustion. As a result, q is -947 since the fuel (the system) has lost energy.

[William Huang 1K]

For the fuel's frame of reference, the fuel has lost internal energy in the form of heat, which means that the sign is negative, -947 kJ.

[Allison Li 2F]

Hi! In this case, the system is the chamber and the surrounding is the cooling system that surrounds the chamber. The cooling system "absorbs 947kj", which means the surroundings gain the heat energy whereas the chamber loses it. 947 is negative because we are looking in relation to the chamber, which loses it.