I am confused on U = W+Q types of problems, and do not know whento use +/- for W

Question

I am confused on U = W+Q types of problems, and do not know whento use +/- for W and Q. I think that if something is added to thesystem or surroundings it is positive, and likewise - if it isremoved it is negative. Here are situations where I getconflicting answers when I look at it differently:

If a jogger releases 800 kJ of heat and does 200kJ of work- what is the change in internal energy equal to?

Similarly - is it true that an increase in internal energyof a system is when

work is done on the surroundings and heat is transferred intothe system

OR

work is done on the system and heat is transferred into thesurroundings?

Can you please explain so I get to undersyand this clearly?

Thanks

Explanation / Answer

If the system absorbs heat then Q is taken as positive If system emits heat then Q is taken as negative If work is done by the system then W is taken aspositive If work is done on the system then W is taken asnegative If the internal energy is increases then U is taken asPositive If the internal energy is decreases then U is taken asnegative Given Q = -800 kJ            W= 200 kJ So, the change in internal energy U = Q - W                                                      = -1000 kJ Internal energy is increases when work is done on thesurroundings and heat is transferred into the system

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