Medical Physics: Temperature and Kinetic Energy

Temperature and Heat

• Temperature is a measure of the thermal state distinguishing hot from cold bodies.
• Molecules in matter possess kinetic energy, which correlates with temperature.
• Increasing molecular temperature can be achieved via heat transfer, often using a flame.
• Heat removal reduces temperature, entering a cryogenic state.

Temperature Scales

• Celsius Scale (°C):

  • Fixed points are the ice point (0 °C) and steam point (100 °C).
  • Scale divided into 100 equal parts, with each equivalent to 1°C.

• Fahrenheit Scale (°F):

  • Freezing point is 32°F and boiling point is 212°F.
  • Normal body temperature is approximately 98.6°F.

• Kelvin Scale (°K):

  • Absolute scale with 0°K at absolute zero, equivalent to -273.15°C.
  • Shares divisions with the Celsius scale.

• Rankine Scale (°R):

  • Developed by William John Macquorn Rankine; related to the Fahrenheit scale.

• Body temperature can be converted:

  • 98.6°F = 37.44°C = 310.15°K.

Thermometry

• Temperature is typically measured indirectly through properties that change with temperature.
• Mercury thermometers operate on the principle that temperature increase causes mercury expansion.

Pressure

• Defined as the effect of normal force acting on a given area, measured in N/m² (Pascal).
• 1 bar equals 100,000 Pa (105 N/m²).
• Pressure equates to force per unit area; differs in meaning when applied to solids (termed stress).

Pressure Units

• Commonly used pressure units include:
  • Kilopascal (kPa) where 1 kPa = 10³ Pa.
  • Megapascal (MPa) where 1 MPa = 10⁶ Pa.
  • 1 atm = 101,325 Pa = 14.696 psi.

Pressure in the Body

• High blood pressure is a common health issue; blood pressure is a routine measurement in medical exams.
• Other pressure measurements taken include:
  • Respiratory pressures
  • Bladder pressure
  • Foot pressure
  • Ocular pressure
  • Middle-ear pressure

Measurement of Pressure

• SI units for pressure measurement include:
  • Dynes per square centimeter
  • Newton per square meter (Pa)
• Clinically, pressure is often measured in mmHg (height of mercury column).
• Example: A peak systolic pressure of 120 mmHg indicates a liquid mercury column of this height exerts the same pressure.