1st Law of Thermodynamics

The first law of thermodynamics states that energy in a system cannot be created or destroyed only transferred from one state to another. The formula for this is DeltaU=Q-W. The DeltaU stands for the change of energy in a system. The Q is heat transferred into the system and W is the amount of work done by the system. So, if lets say, a runner is running on a track on a hot day then the change of energy in a system will equal kinetic energy that was made by potential energy stored inside him (kinetic energy is the energy of moving which was created by energy that was inside him from food, water etc.) minus the work done so minus the energy that he releases as thermal energy (energy from heat) All of this would be mesured in joules. Another example is lightning. The change of energy is equal to the electric energy minus the thermal energy it gives to its surroundings . Energy is a very interesting thing and cannot be destroyed or created. That is why perpetual motion machines are theoretically impossible as energy cannot be created infinitely or created at all.