A body has 15J of heat energy in a particular state. At the same state it has an internal energy of 30J. Assume no work is done, what will happen to the internal energy if 5J of heat is added to the system?

Heat is the energy in transit whenever temperature differences exist. The internal energy is equal to the sum of internal kinetic energy due to molecular motion and internal potential energy due to molecular attractive forces. A hot body has more internal energy than an identical cold body.