[TypicalAzn]

The air in an inflated balloon (defined as the system) is warmed over a toaster and absorbs 130 J of heat. As it expands, it does 78 kJ of work.

What is the change in internal energy for the system? Express your answer using two significant figures.
 Delta E  =     kJ


I am just learning about heat and energy and i am a little confused about the signs (when a system is positive or negative/ when work is positive or negative). Also would i have to convert J to kJ first to use it in the equation :Delta E = q+w

[renge ishyo]

If you are doing work on a system it absorbs energy and the sign should be positive (indicating that this increases the internal energy of the system). If the system on the other hand is doing work on the surroundings, it is releasing energy on the surroundings and the sign should be negative (indicating that this decreases the internal energy of the system).