Questions and Answers
What are physical quantities?
Physical quantities are those quantities that can be measured directly or indirectly.
What are fundamental quantities?
Fundamental quantities are physical quantities that can be treated as independent of other physical quantities.
What are derived quantities?
Derived quantities are physical quantities whose defining operations are based on other physical quantities.
What is measurement?
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What is a unit?
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What is a system of units?
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Which of the following are base units in the SI system? (Select all that apply)
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What is the unit for plane angle?
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What is the solid angle unit?
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Provide an example of a derived unit.
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What are the benefits of the SI system? (Select all that apply)
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What is 1 light year in meters?
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What is the definition of an astronomical unit?
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What is a fermi?
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Why is reporting measurements in significant figures important?
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Study Notes
Physical Quantities
- Physical quantities can be measured directly or indirectly.
- Fundamental quantities are independent of other quantities.
- Derived quantities are defined through other physical quantities.
Measurement
- Measurement involves comparing an unknown physical quantity with a known one.
- A unit is a standard quantity used for comparison, e.g., meter (m), centimeter (cm), gram (g).
Systems of Units
- A system of units is a complete set used for measuring fundamental and derived quantities.
- Common systems include:
- cgs (centimeter, gram, second)
- fps (foot, pound, second)
- mks (meter, kilogram, second)
- SI (International System of Units)
SI System
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SI comprises seven base units:
- Length: metre (m)
- Mass: kilogram (kg)
- Time: second (s)
- Temperature: kelvin (K)
- Amount of substance: mole (mol)
- Electric current: ampere (A)
- Luminous intensity: candela (cd)
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Two additional defined units:
- Plane angle: radian (rad)
- Solid angle: steradian (sr)
Derived Units
- Derived units are formed from fundamental units.
- Examples include:
- Speed: meters per second (m/s)
- Volume: cubic meters (m³)
Advantages of the SI System
- Coherent: all derived units can be expressed through multiplication or division of base units.
- Rational: one unit per physical quantity, e.g., energy in joules.
- Metric: multiples/submultiples are easily expressed as powers of 10.
- Absolute: does not use gravitational or local units (no 'g').
- Internationally accepted, facilitating global communication in science and engineering.
Common Practical Units
- Fermi (femtometre): 1 fm = 10⁻¹⁵ m, used for nuclear sizes.
- Angstrom: 1 Å = 10⁻¹⁰ m, used for light wavelengths.
- Nanometre: 1 nm = 10⁻⁹ m, also for light wavelengths.
- Micron (micrometre): 1 µm = 10⁻⁶ m, a unit of distance.
Practical Units for Large Distances
- Light Year: distance light travels in one year, calculated as 1 ly = 9.467 x 10¹⁵ m.
- Astronomical Unit: mean distance from Earth to the Sun, defined as 1 AU = 1.496 x 10¹¹ m.
- Parsec: distance where an arc of 1 AU subtends an angle of 1 second of arc.
Significant Figures
- Every measurement carries inherent errors, thus results should reflect precision.
- Reported measurements include all significant digits to indicate reliability.
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Description
This quiz covers the basics of physical quantities, including their classification into fundamental and derived categories. It also delves into the systems of units used for measurement, such as SI and its seven base units. Test your understanding of measurement principles and unit systems with this comprehensive quiz.
