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Patent Pending In the physical sciences, an energy transfer or 'energy exchange' from one system to another is said to occur when an amount of energy crosses the boundary between them, thus increasing the energy content of one system while decreasing the energy content of the other system by the same amount. The transfer is characterised by the quantity of energy transferred, which can be specified in energy units such as the joule (J), in combination with the direction of the transfer, which can be specified as in (to) or out of (from) one system or the other.