The equation of the first law is âˆ†U = Q - W. Delta U refers to the change in internal energy of a closed, defined system. Q is the heat added to the system, and W is the work done by the system. The equation reads, the change in internal energy of any defined, closed system is the head added to the system subtracted by the work done by the system onto the surroundings. Adding heat to the system gives it a positive change in internal energy, while work done by the system on something else gives it a negative change in internal energy.
U is used in thermodynamics as the internal energy of a defined system. It is the total energy contained within a closed system.
Q in thermodynamics refers to heat. In this case, it is the heat added to the system (positive). If heat was taken away from the system, the vaue for Q would be negative.
W is used for Work in physics. The unit for Work is joules (like energy). Work done on the surroundings has a positive value of Q. Thus, some value of Q is subtracted from the internal energy of the system (because work was done by the system). Work done to the system by the surroundings has a negative vaue of Q. Some value of Q is added to the internal energy of the system (because work was done on the system by something outside of the system).
One of the principles of the first law is that energy is not created. This means that there is a finite amount of energy in the universe, and there is no creation of "new" energy.
Energy cannot be destroyed such that it disappears. This is a counterpart to the principle that energy cannot be created. The total energy of the universe is constant.
Because energy cannot be created or destroyed, it must be transferred from one object to the next or transformed into a different type of energy. For example, the kinetic energy from moving my hand with the pen up into the air is converted to the pen's potential energy. We can then convert the potential energy of a pen hanging in the air to kinetic energy as it falls, but we do not destroy or create energy.
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