Answer:
Doing thermodynamic work on the system
Step-by-step explanation:
The internal energy of a system can be increased by introduction of matter, by heat, or by doing thermodynamic work on the system. ... If the containing walls pass neither matter nor energy, the system is said to be isolated and its internal energy cannot change.
Internal energy is NOT all the energy in the system, it is the kinetic and potential energy associated with the random motion of the molecules of an object. If you heat an object, you will increase its internal energy. As the object cools, its internal energy will decrease.
The internal energy can be changed by changing the temperature or volume of the object without changing the amount of particles in the object. Temperature: If the temperature of a system rises, the molecules will travel quicker, therefore have more kinetic energy and so the Internal Energy will increase.