A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, in a thermodynamic system, the working fluid is a liquid or gas that absorbs or transmits energy.

The working fluid properties are essential for the full description of thermodynamic systems. Although working fluids have a very large number of physical properties which can be defined, the thermodynamic properties which are often required in engineering design and analysis are few. Pressure, temperature, enthalpy, entropy, specific volume and internal energy are the most common.

If at least two thermodynamic properties are known, the state of the working fluid can be defined. This is usually done on a property diagram which is simply a plot of one property versus another.

When the working fluid passes through engineering components such as turbines and compressors, the point on a property diagram moves due to the possible changes of certain properties. In theory therefore it is possible to draw a line/curve which fully describes the thermodynamic properties of the fluid. In reality however this can only be done if the process is reversible. If not, the changes in property are represented as a dotted line on a property diagram. This issue does not really affect thermodynamic analysis since in most cases it is the end states of a process which are sought after.

Christopher John "Topher" Grace (pronounced /ˈtoʊfər/; born July 12, 1978) is an American actor. He is best known for his portrayal of Eric Forman on the Fox sitcom That '70s Show, Eddie Brock/Venom in the Sam Raimi film Spider-Man 3, and Edwin in the 2010 film Predators.

Grace was born in New York City, the son of Pat, an assistant to the schoolmaster of the New Canaan Country School, and John, a Madison Avenue executive.

Grace grew up in Darien, Connecticut, where he knew actress Kate Bosworth, as well as Opie & Anthony producer Nathaniel Bryan, and was sometimes babysat by actress Chloë Sevigny, who later appeared with him in high school stage plays. Grace chose the name Topher because he did not want his full name abbreviated to Chris at school.

Grace was cast as Eric Forman on Fox's That '70s Show, which debuted in 1998. The sitcom was successful and Grace became well known among television watchers; he played the role until the seventh season. The series was renewed for another season despite his absence before its ending in 2006. His character was written out and was replaced with a new character named Randy Pearson who was portrayed by Josh Meyers. Grace makes a very brief guest appearance in the final episode.

Michael Kirk Douglas (born September 25, 1944) is an American actor and producer, primarily in movies and television. He has won three Golden Globes and two Academy Awards; as producer of 1975's Best Picture, One Flew Over the Cuckoo's Nest, and as Best Actor in 1987 for his role in Wall Street. Douglas received the AFI Life Achievement Award in 2009. He is the eldest of actor Kirk Douglas's four sons.

Douglas was born in New Brunswick, New Jersey, the first child of actor Kirk Douglas and Bermudian-born actress Diana Dill. His paternal grandparents were Jewish immigrants from Gomel in Belarus (at that time a part of the Russian Empire). His mother was from Devonshire Parish, Bermuda; Douglas's maternal grandfather, Lieutenant Colonel Thomas Melville Dill, served as Attorney General of Bermuda and was commanding officer of the Bermuda Militia Artillery. Douglas has a younger brother, Joel Douglas (born 1947), and two paternal half-brothers, Peter Douglas (born 1955) and Eric Douglas (1958–2004), from stepmother Anne Buydens.