What is the unit of viscosity? Please explain/

1 Answer
Aug 31, 2017

The SI unit of viscosity is the pascal-second (Pa·s).

Explanation:

The viscosity of a fluid is a measure of its resistance to flow.

We usually think of the viscosity of a liquid as its "thickness".

For example, we say that honey is "thicker" (has a higher viscosity) than water.

One way to measure viscosity is to place the fluid between two flat parallel plates and slide one plate past the other.

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In terms of the above diagram, the formula for viscosity #eta# is

#color(blue)(barul|stackrel(" ")(eta = (Fδ)/(Av))|)#

Thus, the units of viscosity are #("N" × color(red)(cancel(color(black)("m"))))/("m"^2 × color(red)(cancel(color(black)("m")))·"s"^"-1") = "N·s·m"^"-2" = "Pa·s"#.

If a fluid is placed between two #"1 m"^2# plates that are 1 m apart and one plate is pushed sideways with a force of 1 N and it moves at a velocity of #1color(white)(l) "m·s"^"-1"#, then the fluid has a viscosity #eta = "1 Pa·s"#.

Many chemists still use an old unit of viscosity, the poise (P), named after the French physician Jean Poiseuille.

It is a convenient unit because #"1 mPa·s" = "1 cP"#.

For example, the viscosity of water at 20 °C is #"1.002 mPa·s"# or #"1.002 cP"#.

Honey has a viscosity of about #"10 Pa·s"# or #"10 000 cP"#.