thermodynamics -

[wanted90]

Hi

How are you

I have a question how we can know the work if - or +

also heat energy

please could see this question why in answer

take work as -

and heat as +

[studiot]

OK that's a worry that trips many people up when they start thermo.

 

The First Law is basically the Law of Conservation of Energy flows across the system boundary.

Energy in adds to the internal energy. Energy out decreases it.

 

Since energy can flow in either direction we need a sign convention.

 

Unfortunately there are two sign conventions.

 

The original convention, used mainy by engineers and physicists is

 

Work done by the system is positive (= work out = energy out)

Heat added to the ssytem is positive (= heat in = energy in)

 

 

This leads to the First Law in the form

 

dU = q - w

 

Which is the form at the bottom of your example.

 

The second convention, used by chemists and many scientists counts all types of energy in as positive and all types of energy out as negative.

Heat added to the system is still positive

but

Work done by the system is now negative.

 

This leads to the First Law in the form

 

dU = q + w

 

You should always check the sign convention in use.

 

Note this advice applies in other areas of physics that also have different possible sign conventions.

 

Does this help?

[wanted90]

thanks but I face problem before this semester teacher said work done by system is always positive

 

and now in this semester he said that work done by the system is negative

 

that why I was confusing !!

thanks so much

thanks so much