Current SI Standards in Science

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain why standard units are used in science, and list and describe the seven standard SI units scientists use. A short quiz will follow.

Why are Standard Units Needed in Science?

Science is the systematic study of the physical and natural world through repeated observation, and experimentation. It's a method we use to make sense of the world in which we live, and to really explain it in a fundamental and detailed way.

Science is done all over the world, and that's great. But it also causes some issues. Science in one place isn't always done identically to science in another place. We need to share ideas, to standardize, and make sure things are being done properly and carefully.

One thing that compounds this issue is units. If Europeans are working in meters, and Americans are working in feet, people have to spend a lot of time converting. Imperial units are not only problematic when comparing notes, but they also aren't as user friendly. When you multiply a number by ten, you add a zero. This makes tens a really great way of grouping numbers. 10 millimeters in a centimeter, 100 centimeters in a meter.

Compare that to imperial units, where 12 inches are in a foot, and 16 pounds are in a stone, and 1760 yards are in a mile, and it soon gets confusing.

For this reason, scientists have agreed on standard, user-friendly units that everyone can use to make things easier. We call them SI units.

SI Units

SI units, otherwise known as System International units (the words are backwards, because they're supposed to be in French), are a standard set of basic units from which scientists operate. Today we're going to go through and define each of them.

There are seven current standard units in all: meters, kilograms, seconds, amperes, kelvins, moles and candelas. It should be possible for every other unit in science to be derived from these basic ones.

A meter, with the symbol m, is the standard unit of length. It's defined as the distance travelled by light in a vacuum in approximately one 300 millionth of a second.

Standard Unit of Length
Standard Unit of Length

A kilogram, with the symbol kg, is the standard unit of mass. It's defined by a standard 1 kilogram block that is kept in France.

Standard Unit of Mass
Standard Unit of Mass

A second, with the symbol s, is the standard unit of time. It's defined as the 'duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.' Yes, that might be specific. But that's what a second is considered to be by scientists, because it's a time that's always the same.

Standard Unit of Time
Standard Unit of Time

An ampere (or amp), with the symbol A, is the standard unit of electric current. It's defined as the current which if run through two parallel, infinitely thin, infinitely long wires, exactly 1 meter apart, in a vacuum, would produce a force of 0.0000007 newtons per meter of length.

A kelvin, with the symbol K, is the standard unit of temperature. It's defined as 1 273rd of the thermodynamic temperature of the triple point of water - the point where solid, liquid and gas phases can all exist together.

Standard Unit of Temperature
Standard Unit of Temperature

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