Q: During an isothermal expansion, a confined ideal gas does -150 J of work against its surroundings. which of the following describes the heat transfer during this process? (a) 150 J of heat was added to the gas; (b) 150 J of heat was removed from the gas; (c) 300 J of heat was added to the gas; (d) 300 J of heat was removed from the gas.
The answer is (a). The explanation on the book is that "… since ΔU = Q + W by the first law of thermodynamics, and since ΔU = 0 due to isothermal process, therefore Q = -W. Since W = -150 J, it must be true that Q = +150 J."
My question is that all textbooks write the law as ΔQ = ΔU + W, why does this book writes it differently? Any help would be very much appreciated. Thanks for your time.