Question

calculate the change internal energy for a system that is giving off 45.0kJ of heat and is performing 855J of work on the surroundings

Answer 1

According to first law of thermodynamics energy is conserved.

Heat and work are the means by which energy is released or absorbed.

For better understanding you can take example of bank.

In bank you deposit money through cheque or through cash ,so you can say cheque is heat and cash is work .now internal energy is change of both of them If you deposit 10$ by cash and withdraw 5$ by cheque then the remaing amount is with the bank ,which is equivalent to internal energy.

Q+W = ∆U

it is giving off 45.0kJ energy = 45000J

and performing 855J work

in both case it is loosing energy or maybe you can say withdrawing from a credit card.

-45000J + (-855J) = ∆U

∆U = -45855J