Energy entering a system

If energy enters a system as heat, then q is positive if energy leaves the system as heat, then q is negative. Thus, if 10 J of energy enters the system as a result of a temperature difference, we write q = +10 J, whereas if 10 J leaves the system, then we write q = —10 J. In each case, A U = q but in the former case the internal energy increases by 10 J (AU = +10 J), whereas in the latter case the internal energy decreases by 10 J (AU = -10 J). 

If mechanical energy enters a system at a shaft of radius R, which rotates at an angular velocity, or, exerting a torque, i, what is the mechanical energy flux added to the system What is the force conjugate to this flux ... 

As energy enters a system a substance can change from a solid to a liquid to a gas. 

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