Medical Biophysics: Quantities and Units

Introduction to Medical Biophysics

• Biophysics is a field that applies principles of physics to biological systems.
• A quantity is something that can be quantified, meaning it can be assigned a numerical value.
• A physical quantity is a physical property that can be expressed in numbers.
• Physical quantities are measured using units.
• A unit is a known fixed physical quantity.
• A unit is used as a standard for measurement.
• Physical quantities can be divided into base quantities and derived quantities.
• Base quantities are expressed in base units.
• Derived quantities are expressed in derived units.
• There are seven base quantities and seven base units in the SI system.

SI Base Quantities and Units

• Length is measured in meters (m).
• Mass is measured in kilograms (kg).
• Time is measured in seconds (s).
• Electric current is measured in amperes (A).
• Thermodynamic temperature is measured in kelvin (K).
• Amount of substance is measured in moles (mol).
• Luminous intensity is measured in candelas (cd).

SI Derived Quantities and Units

• Derived units are defined as products of powers of the base units.
• Some derived units are expressed directly in terms of base units.
  • Square meters (m²) and meters per second (m/s) are examples of derived units expressed directly in base units.
• Some derived units have special names, often after scientists.
  • Newtons (N) and Pascals (Pa) are examples of derived units with special names.
  • Area: Symbol A, Unit: m²
  • Volume: Symbol V, Unit m³
  • Speed/Velocity: Symbol v, Unit: m/s
  • Acceleration: Symbol a, Unit: m/s²
  • Density: Symbol ρ, Unit: kg/m³
  • Force: Symbol F, Unit: kg m/s² or N
  • Pressure: Symbol P, Unit: N/m² or Pa
  • Energy/Work: Symbol E/W, Unit: N m or J
  • Power: Symbol P, Unit: J/s or W
  • Electrical Charge: Symbol Q, Unit: A s or C
  • Electrical Potential Difference: Symbol V, Unit: W/A or V
  • Electrical Resistance: Symbol R, Unit: V/A or Ω
  • Moment of Force (Torque): Symbol τ (or M), Unit: N m

Scalar and Vector Quantities

• A scalar quantity has magnitude only and is completely described by a number with appropriate units.
  • Distance is a scalar quantity like 7 m.
  • Mass, time, and temperature are scalar quantities.
• A vector quantity has both magnitude and direction and can be represented by a straight line in a particular direction.
  • Displacement is a vector quantity.
  • Velocity, force, and momentum are vector quantities.
• The formula for a vector quantity accounts for positive and negative values.
• Acceleration is a vector quantity; negative acceleration is deceleration.
• Negative values for vector quantities indicate direction.
• Temperature is a scalar quantity even with negative values, as these do not represent direction.

Units

• Three common systems of units exist:
  • SI units (Système International d'Unités) use meter, kilogram, and second.
  • The British engineering system (imperial system) uses foot, pound, and second.
  • The cgs system uses centimeter, gram, and second.

Common SI Prefixes for Units

Multiples and Submultiples of SI Units - The Metre