Second

Definition

A second is the base unit of time in the International System of Units (SI). Defined as 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

5 Must Know Facts For Your Next Test

1. The second is one of the seven base units in the SI system.
2. The current definition of a second was adopted in 1967 by the International Committee for Weights and Measures (CIPM).
3. Atomic clocks are used to maintain and measure seconds with extreme precision.
4. The second can be subdivided into milliseconds, microseconds, and nanoseconds for more precise measurements.
5. In physics equations, time is often represented by $t$ with seconds as its unit.

Review Questions

• What is the SI unit for time?
• How is a second currently defined?
• Why are atomic clocks important for measuring seconds?

Related terms

Meter: The base unit of length in the SI system, defined as the distance light travels in a vacuum in $\frac{1}{299792458}$ seconds.

Kilogram: The base unit of mass in the SI system, currently defined by setting Planck's constant exactly equal to $6.62607015 \times 10^{-34}$ joule-seconds.

Ampere: $The base unit of electric current in the SI system, defined by taking the fixed numerical value of elementary charge $e$ to be $1.602176634 \times 10^{-19}$ coulombs.$