chapter 10

• Temperature

  : physical quantity that measures how hot an object is

  • kelvin (K)

  _ lowest temperature 0K -> absolute zero
• boiling water poured into glass cup

  • inside of glass expands more than the exterior
  • unequal expansion breaks the glass
• bimetallic strip (brass on top, steel below)

  _ brass expands, contracts more than steel

  • when cooled: bends upward (brass is shorter)
  • when heated: bends downward (brass is longer)
  • used as thermostats -> regulate temperature of electrical appliances eg. iron, air con, fridge, oven
• Provision for expansion

  • gaps in railway tracks, roads, bridges, building structures
  • prevent warping and buckling of railway tracks, or cracks to building structures on hot days
  • expansion rollers under bridge

  _ in hot weather: surface of bridge gets longer

  _ in cold weather: surface of bridge gets shorter

  so, one end of bridge is fixed —— other end is allowed to move over rollers

  • pipes carrying hot water or steam

  _ expansion loops

  _ without them, the stress produced by the expanding pipes can cause them to bend and break
• Liquid-in-Glass thermometer

  • temperature around bulb (increases/ decreases)
  • liquid in bulb (heats up/ cools down), (expands/ contracts) and (rises up/ falls back down) the narrow glass tube
• Hot air balloon

  • air in hot air balloon is heated -> volume of air increases
  • mass of air remains the same -> density decrease
  • hotter, less dense air rises -> balloon floats
• Hot Riveting -> join steel plates and beams

  • when red-hot, rivet is put into position
  • hammered into a head, allowed to cool
  • as rivet cools, it contracts and clamps steel plates together
• Thermostats

  • Fire alarms

  _ during a fire, bimetallic strip gets hot -> bends toward the contact, completes electrical circuit

  _ bell connected to the circuit starts ringing

  
• Thermostat in electric iron

  1. iron is switched on -> heating coil gets very hot
  2. bimetallic strip is heated -> brass expands more than iron -> bends upward (away from contact) -> heater turns off

  • when iron cools down, bimetallic strip bends toward contact, straightens
  • circuit is closed, heater switches on
  • process is repeated to keep iron at constant temperature
• Bimetallic thermometer

  • bimetallic strip made into a coil
  • temperature change results in unequal expansion or contraction of the two metal strips, causing the coil to expand or tighten
  • pointer moves over a scale to show the temperature
• Conduction: transfer of heat energy through a material without the obvious movement of the material itself

  • heat moves by conduction from hot end to cool end of the rod

  1. metal atoms closest to heat source gain heat energy and vibrate faster
  2. these atoms hit neighbouring atoms, transfer some energy to them -> cause them to vibrate faster too
  3. process is repeated from atom to atom

  • iron, pot, woollen blanket (traps air), double-glazed windows
• Convection (only fluids)

  : transfer of heat energy from one place to another by the movement of the material itself

  • involves movement of matter from hot regions to cooler regions due to differences in densities

  1. when water is heated, it expands
  2. volume increases -> density decreases
  3. warm water less dense than cold water -> rises
  4. as warm water rises, it is replaced by cooler, denser water from above
  5. a convection current is set-up
• applications of convection

  • kettle, hot water system, air con
  • sea breeze

  _ on a hot day, land heats up faster than sea

  _ hot air over land rises — cold air over water moves to replace hot air rising up

  • land breeze (at night!) -> opposite of sea breeze
• conduction

  • solids
  • heat flows in any direction
  • solid itself does not move

  vs

  convection

  • liquid and gases
  • heat flows upward
  • hot fluid moves freely
• thermal conductivity

  : how well a material conducts energy when it is heated

  • in metals, free electrons gain energy and move during heating

  _ can move fast over short distances -> transfer heat quickly when they collide with other particles
• Radiation

  : transfer of heat energy without the need for a medium

  • can take place in vacuum, travels in the form of waves
  • eg. energy from sun

  — when an object is warmer than its surroundings, it radiates more heat energy than it absorbs

  — when radiation is absorbed, it changes into heat
• Greenhouse effects (radiation)

  • greenhouse gases prevent heat from leaving Earth’s atmosphere by radiation
  • increased greenhouse effect -> increased temperature -> global warming
  • leads to melting ice caps, expansion of water bodies -> rising sea levels -> floods
• variables affecting rate of radiation

  1. surface area (S.A. increase, radiation/ absorption increase)
  2. temperature of body (greater the temperature difference between body and surroundings, higher the radiation)
  3. colour and texture of surface

  _ black, dull, rough: good a/r

  _ white, shiny, smooth: poor a/r
• applications of radiation

  • petrol tankers (white surface)

  _ prevent flammable contents from igniting

  • kettle (shiny)
  • spacesuits (white)

  _ temperature in space is sometimes very high or very low

  _ reduce amount of heat radiation absorbed and rate of heat loss from body
• Thermal vacuum flask

  • silvered walls -> poor radiator
  • vacuum between walls -> prevent heat movement by conduction and convection
  • stopper (cork/ plastic) -> poor conductor
  • glass walls -> poor conductor, reduce heat gain/ loss