## Unit of Time- Class 11 | Chapter – 2 | Physics Short Notes Series PDF for NEET & JEE

Unit of Time: The measurement of time is based on the regular recurrence of events, such as the rotation of the Earth on its axis, the orbit of the Earth around the sun, or the oscillations of an atomic clock. The most accurate clocks are based on the vibrations of atoms or molecules, which can be counted and used to define the duration of a second. The following are the most regularly used time units:

• Second
• Minute
• Hour

The study of time and its measurement is essential in many areas of science and engineering, including physics, astronomy, chemistry, and engineering. The accurate measurement of time is critical in many fields, such as navigation, communications, and the synchronization of computer systems.

## SI Unit of Time

The SI (International System of Units) unit of time is the second (symbol: s). It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom.

The second is the base unit of time in the SI system and is used to express various time intervals, such as seconds, minutes, hours, days, weeks, months, and years. Other units of time, such as milliseconds, microseconds, and nanoseconds, are also commonly used in scientific and technological applications, particularly in the fields of computing, electronics, and telecommunications.

The use of the second as the standard unit of time has enabled a high degree of accuracy in time measurement and has allowed for precise synchronization of clocks and other time-keeping devices around the world.

 Units of Time Minute 60 seconds Hour 60 minutes, or 3,600 seconds Day 24 hours, or 86,400 seconds Week 7 days, or 604,800 seconds Month 28-31 days, or 2,419,200-2,678.400 seconds Year 365.25 days, or about 31,557,600 seconds

## Measurement of Time

Time can be measured using various devices and methods, depending on the required level of precision and the context of the measurement. Some of the most common methods for measuring time include:

• Sundials: Sundials are the oldest known instruments for measuring time and work by using the shadow cast by the sun to indicate the time of day.
• Hourglasses: Hourglasses, also known as sandglasses, measure time by allowing sand or another fine material to flow from one chamber to another through a narrow opening. The time elapsed is determined by the amount of sand that has passed through the opening.
• Mechanical clocks: Mechanical clocks, which include pendulum clocks and spring-driven clocks, measure time using a mechanical mechanism that regulates the movement of hands on a dial.
• Quartz clocks: Quartz clocks use the regular vibration of a quartz crystal to measure time. The crystal is electrically charged, causing it to vibrate at a specific frequency that can be used to accurately measure time.
• Atomic clocks: Atomic clocks are the most accurate time-keeping devices available and are based on the vibrations of atoms or molecules. The most common type of atomic clock uses the vibrations of caesium atoms to measure time.
• Electronic devices: Modern electronic devices, such as digital watches and smartphones, use a quartz crystal or other electronic components to measure time.

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