Heat

Heat:

• Heat is a form of energy which makes us feel the sensation of coldness and hotness
• Heat always flows from hotter body to colder body
• one cannot rely on sense of touch to decide whether an object is hot or cold

Temperature:

• The quantity by which we can compare the hotness or coldness of bodies

Thermometer:

• thermo - heat; meter - to measure (greek)
• instrument to measure the temperature
• unit:
  • celsius -
    • designed by Swedish Astronomer - ANDERS CELSIUS (1701 - 1744)
    • degree celsius C
    • lower fixed point is 0 and upper fixed point is 100
  • fahrenheit -
    • designed by German scientist - GABRIEL DAVID FAHRENHEIT (1686 - 1736)
    • degree fahrenheit F
    • lower fixed point is 32F and upper fixed point is 212F {WHY?}
  • kelvin - used for scientific purposes

Relation between C AND F:

• C / 100 = (F-32) / 180
• C/ 5 = (F - 32) / 9
• F = (9C / 5) + 32
• C = 5 / 9 (F - 32)

Measuring Temperature:

• measured by a principle of expansion
• Mercury - Thermometric liquid
• thermometers that use mercury are called mercury thermometers

Reasons for using mercury:

1. remains in liquid state for a wide range of temperature
2. melting point: - 38.87 degree celsius
3. Uniform Expansion Rate: expands and contracts uniformly with a rise and fall in temperature
4. good conductor of heat
5. low specific heat capacity
6. does not take much heat from the body when measured
7. silver grey colour - easily visible
8. does not stick to the glass tube

Clinical thermometers:

• Doctor's thermometer
• long, narrow and uniform glass tube
• bulb on one end - contains mercury
• constriction (kink) - to prevent the back flow of mercury into the bulb
• shiny mercury thread through the bulb
• temperature read from 35 to 42 degree celsius
  • this is because normally the temperature of a person is always between 35 to 42 degree celsius
• earlier India used 94 to 108 degree fahrenheit, but later adopted celsius scale

Precaution:

• Wash with antiseptic before using it on another person

• ensure the mercury is below 35 before use, if not shake it well

• do not hold the bulb while checking the temperature

• There is a kink in the stem just above the bulb

• when the thermometer is taken out of the patient's mouth, the liquid contracts and begins to fall

• then mercury column breaks at the kink

• therefore the mercury's levels remains the same

Maximum & Minimum thermometer:

• used to measure the weather temperature
• marked in both C and F
• from - 28 to +121F or -34 to +50C
• mercury is used to indicate the measurements

Laboratory thermometer:

• to measure the temperature of other objects
• thick walled fine bore glasss capillary tube with mercury at one end in a small bulb
• range: -10 to +110C

Precaution:

• kept upright not titled
• bulb should be surrounded from all sides by the substance
• alcohol is used because it can be used at very low temperatures and its expansion is 6 times greater than mercury
• but it cannot be used in hot conditions as it boils at low temperature than mercury
• bulb should not touch the container's sides

Transfer of heat

• heat flows from higher temp to lower temp
• 3 modes of transfer of heat
  • conduction
  • convection
  • radiation

Conduction

• transfer of heat from hotter to colder objects through vibration of particles without actual movement of the object
• ex: hold - copper - aluminium - iron rods in the hand - heat it at one end - you will feel the heat at different times with different objects

Conditions:

• both the objects should be contact
• their temp should be different
• if the temp are same, then there is no heat transfer and this state isi known as Thermal Equilibrium

Conductors:

• allows heat to pass through
• silver, copper, aluminium, iron, etc

Insulators:

• bad conductors of heat
• plastic, wood, rubber, thermacoal, leather, air, water, etc

Applications of conductors:

• mercury in thermometer
• cooking vessels
• soldering rod
• cooling coil of ac and fridge are made of copper to conduct the heat away
• copper tubing in automobile radiation to take out the heat from the hot water coming from side of the engine

Applications of insulators:

• trapped air in double glass window acts as insulator to stop heat flow out from the room and keep it warm
• newly made quilt or bed has more air trapped in the cotton, therefore it feels warm
• ice covered ini gunny bags, as it has air trapped in it
• igloo made out of ice traps large amount of air inside, which acts as insulator
• handles of utensils made of bakelite or wooden or plastics

Convection:

• in liquids and gases only
• heat transferred by the movement of molecules, due to weak molecular force
• ex: take water in a beaker and boil it - the upper layer is chill in the beginning but after some time it becomes hot
• ex: candle and place the hand near it - you feel the heatness

Applications of convection:

• room heater, chimneys, fridge - freezer at top, exhaust fan
• land and sea breeze in coastal areas

Land and sea breeze

• convection plays a imp role in moderating the temp of coastal areas
• Sea breeze:
  • in day, land heats faster and hot air moves above
  • to fill this place, the cold air from the sea comes
• Land breeze:
  • after sunset, the land cools faster
  • the air over sea is hot and moves above
  • to fill this the cold air from land moves in

Specific heat capacity:

• The amount of heat energy required to raise the temperature of a unit mass of the substance by one degree Celsius (or one Kelvin).
• c = m / ΔTQ​ Where:
  • Q is the amount of heat energy supplied (in joules),
  • m is the mass of the substance (in kilograms),
  • ΔT is the change in temperature (in degrees Celsius or Kelvin).
• The unit of specific heat capacity is joules per kilogram per degree Celsius (J/kg°C) or joules per kilogram per Kelvin (J/kg K).
• Substances with low specific heat capacity heat up and cool down quickly because they require less heat energy to change their temperature.
• Substances with high specific heat capacity can absorb or release large amounts of heat with only a small change in temperature.
• Materials with low specific heat capacity are often used in heat exchangers and cooking utensils because they transfer heat efficiently.

Radiation:

• transfer of heat directly without any heating space in between
• fastest mode
• air is a poor conductor of heat, so conduction cannot take place
• air when heated rises above not sidewards, hence convection also doesn't take place
• heat transferred from sun to earth through vacuum - no molecules in between

Absorption / Radiation of radiant heat:

• when any object absorps heat, its temp rises
• some amount of received heat is reflected as well

Absorption and Emission Based on Color

• Black:
  • Absorbs the most radiation across the entire spectrum of visible light, leading to faster heating.
  • Black surfaces emit heat well but also cool down quickly when the heat source is removed.
• White:
  • Reflects most of the visible light and absorbs very little, leading to slower heating.
  • White surfaces do not emit heat as efficiently as black surfaces.
• Dark Colors (e.g., dark blue, dark green, dark red):
  • Absorb more radiation than light colors but less than black.
  • They heat up faster than light colors but slower than black.
• Light Colors (e.g., light blue, light green, light yellow):
  • Reflect more light and absorb less radiation compared to dark colors.
  • They heat up more slowly than dark colors but faster than white.