physics - physical quantities

• A physical quantity

  • can be measured
  • has its own unit
  • expressed as numerical value with a unit
• Physical quantity = numerical value x unit
• All physical quantities can be classified into base (or fundamental) quantities, or derived quantities.
• SI unit of mass:

  kilogram, kg
• SI unit of length:

  metre, m
• SI unit of time:

  second, s
• SI unit of electrical current:

  ampere, A
• SI unit of temperature:

  kelvin, K
• Amount of substance:

  mole, mol
• SI unit of luminous intensity:

  candela, cd
• Derived quantities are quantities that are obtained using a formula that comprises equations such as the addition, subtraction, multiplication, and/or division of base quantities.

  The corresponding derived units can be expressed as products or quotients of the base units.
• Derived quantity: force (newton, N)

  Force = mass x accelertion

  kg m s^-2
• Derived quantity: pressure (pascal, Pa)

  Pressure = force / area

  kg m^-1 s^-2
• Derived quantity: work done (joule, J)

  Work done = force x distance in direction of force

  kg m^2 s^-2
• Derived quantity: power (watt, W)

  Power = work done / time

  kg m^2 s^-3
• Derived quantity: electric charge (coulomb, C)

  Electric charge = current x time

  A s
• Derived quantity: electromotive force (volt, V)

  Electromotive force = work done / electric charge

  kg m^2 s^-3 A^-1
• Derived quantity: frequency (hertz, Hz)

  Frequency = 1 / period

  s^-1
• Prefix for 10^12

  Tera-, T
• Prefix for 10^9

  Giga-,G
• Prefix for 10^6,

  Mega-, M
• Prefix for 10^3

  Kilo-,k
• Prefix for 10^-1

  Deci-, d
• Prefix for 10^-2

  Centi-, c
• Prefix for 10^-3

  Milli-, m
• Prefix for 10^-6

  Micro-, the funny u thing
• Prefix for 10^-9

  Nano-, n
• Prefix for 10^-12

  Pico-, p
• The typical thickness of a piece of paper is 0.1mm
• The thickness of a strand of hair is 100 micro-metre
• The diameter of an atom is 0.1nm
• For order of magnitude estimations, the estimated value should not be expressed in more than 1 significant figure due to large uncertainty of a human estimate.

  Note: uncertainty could be thought of as a quantified value of the amount of doubt towards the value.
• A number in standard form is a number that is expressed as a decimal number, between 1.0 and 10.0 (1.0 < x < 10.0), multiplied by a power of 10.
• Some common instruments used to measure lengths are tape measure, metre rule, calipers, digital calipers and the digitaal micrometer screw gauge.
• The suitability of an instrument depends on the extent of precision necessary.

  The precision of an instrument is the smallest unit that the instrument can measure.
• Metre rule

  Range: several centimeters to one metre

  Precision: 0.1cm
• Tape measure

  Range: several metres

  Precision: 0.1 cm
• Digital calipers

  Range: between 1 cm to 15 cm

  Precision: 0.01mm
• Digital micrometer screw gauge

  Range: between 1 mm to 25 mm

  Precision: 0.001 mm
• The key parts of the digital calipers are:

  • Inside jaws - used to measure the internal diameter of an object
  • Outside jaws - used to measure the external diameter or width of an object
  • Digital display - shows the numerical value of measurement
  • Zero button - used to reset the value to zero
  • Depth rod or tail - used to measure the depth of an object