The First Law of Thermodynamics is essentially a statement of conservation of energy. It involves changes in internal energy, the heat flowing into the system and the work done by the system. When heat flows into the system, the internal energy of the system increases. When positive work is done bythe system, the system loses internal energy.
As an example, if a balloon (our system) expands it does positive work on its surroundings (the air surrounding the balloon and the rest of the universe). Suppose in the process of expanding the balloon does +4010 J of work on its surroundings. The work is positive since the balloon exerts a force outward and the displacement is also outward (force and displacement in the same direction).
1) What is the work done by the surroundings on the balloon? (Hint: Which way does the force of the surroundings act on the balloon? How does the force of the surroundings on the balloon compare to the force of the balloon on the surroundings?)
2) At the same time that the balloon does +4010 J of work on its surroundings, 14300 J of heat flows into the system. What is the change in internal energy of the system?