Second

Definition

The second is the SI unit of time, defined as the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom. It is a fundamental unit in physics and essential for precise measurements.

5 Must Know Facts For Your Next Test

1. The second is one of the seven base units in the International System of Units (SI).
2. It was historically based on Earth's rotation but redefined using atomic clocks for greater precision.
3. Cesium-133 atoms are used to define the second due to their consistent vibration frequency.
4. Atomic clocks can measure time to within billionths of a second per day.
5. The definition of a second impacts various scientific fields, including mechanics, oscillations, and wave calculations.

Review Questions

• What physical phenomenon is used to define a second?
• Why was the cesium-133 atom chosen for defining the second?
• How does an atomic clock improve the precision of time measurement compared to astronomical methods?

Related terms

Meter: The SI unit of length, defined as the distance light travels in a vacuum in $1/299,792,458$ seconds.

Kilogram: The SI unit of mass, defined by setting Planck's constant $h$ exactly equal to $6.62607015 \times10^{-34}$ joule-seconds ($J\cdot s$).

Hertz: The SI unit of frequency, equivalent to one cycle per second.