Thermal properties and temperature

• Solids, liquids and gasses expand when they are heated as atoms vibrate more and this causes them to become further apart, taking up a greater volume
• When heated, due to differences in molecular structure of the different states of matter, expansion is greatest in gases, less so in liquids and lowest in solids
• Applications and consequences of thermal expansion:

  • Overhead cables have to be slack so that on cold days, when they contract, they don’t snap or detach.
  • Gaps have to be left in bridge to allow for expansion
• Temperature can be measured by observing a physical property that changes with temperature. Examples include alcohol and mercury - used in thermometers.
• Fixed points are definite temperatures at which something happens and are used to calibrate a thermometer. For example, melting and boiling point of water
• Sensitivity: Change in length or volume per degree
• Range: The values which can be measured using the thermometer
• Linearity: Uniform changes in the physical property with a change in temperature over the measured temperature values.
• Responsiveness: How long it takes for the thermometer to react to a change in temperature
• Calibrating a thermometer:

  • Place thermometer in pure water.
  • Place the thermometer above the steam of the pure boiling water, this is 100 °C.
• Liquid-in-glass thermometer:

  • As temperature rises or falls, the liquid (mercury or alcohol) expands or contracts.
  • Amount of expansion can be matched to temperature on a scale.

  
• To increase sensitivity:

  • Thinner capillary
  • Less dense liquid
  • Bigger bulb
• The range of a thermometer is defined depending on the melting and boiling point of the liquid being used.
• The linearity of a thermometer depends on the liquid being used
• Thermocouple thermometer:

  • The probe contains 2 different metals joined to form 2 junctions.
  • The temperature difference causes a tiny voltage which makes a current flow.
  • A greater temp. difference gives a greater current.
  • Thermocouple thermometers are used for high temperatures which change rapidly and have a large range (-200C° to 1100°C)