Heat

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Heat
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Heat
Heat Energy Heat is thermal energy transferred
from one object to another. Do not confuse heat
and temperature!
There are three mechanisms of heat transfer 1)
Conduction highly energetic atoms collide with
less energetic ones giving them energy. 2)
Convection - highly energetic molecules move from
one place to another, carrying thermal energy
with them. 3) Radiation - atoms or molecules
emit electromagnetic waves the energy carried by
these waves is converted back to thermal energy
when the waves interact with some form of matter.
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• Conduction is the mechanism of heat transfer in
  which highly energetic atoms or molecules collide
  with less energetic atoms or molecules, giving
  them some energy.
• Ex a skillet on a stove

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Conduction FURTHER EXPLAINED If one end of a
metal rod is at a higher temperature, then energy
will be transferred down the rod toward the
colder end because the higher speed particles
will collide with the slower ones with a net
transfer of energy to the slower ones.
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• Gold, silver, copper, and aluminum
• are the best conductors.
• Most other solid substances are poorer conductors
  of heat.
• Liquids (except mercury)
• are mostly poor conductors. Materials which are
  very poor conductors of heat are called
• insulators or nonconductors.

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Convection is the mechanism of heat transfer in
which highly energetic molecules move from one
place to another.Ex water in a heated pot
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Convection heat transfer by mass motion of a
fluid such as air or water when the heated fluid
is caused to move away from the source of heat,
carrying energy with it.
Convection above a hot surface occurs because hot
air expands, becomes less dense, and rises. Hot
water is also less dense than cold water and
rises, causing convection currents which
transport energy.
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Convection
Heated water expands and becomes more buoyant.
Cooler, more dense water near the surface
descends and patterns of circulation can be
formed.
Heated air rises, cools, thenfalls.  Air near
heater isreplaced by cooler air, andthe cycle
repeats.
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 Natural Convection
                           Air above warmer groundrises.                             Inversion layer.  Air near ground is more dense thanair higher up no convectioncurrents to lift pollutants.                     Convection causes ocean water from the tropics to flow in currents towards higher latitudes
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• Radiation is the mechanism of heat transfer in
  which atoms or molecules emit electromagnetic
  waves.
• ex Sun

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3. Radiation thermal energy transfer in which
hot atoms emit heat in the form of infrared
electromagnetic waves. The energy carried by
these waves is converted back to thermal energy
only when the waves strike some form of matter.
Radiation is the method by which heat from the
sun travels through the vacuum of space to reach
earth.
Dark colors absorb heat better than lighter
colors. Shiny surfaces reflect heat better that
dull surfaces.
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Heat transfer among objects or substances on
earth usually involves convection and/or
conduction too.
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Specific Heat Capacity
Substance s.h.capacity (J/g C)
Fresh water 4.18
Ocean water 3.89
Air 1.00

• The amount of heat needed to change the
  temperature by 1C for 1g of substance.
• Each substance has a unique specific heat
  capacity.

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Qmc?T

• Q energy absorbed
• m mass of substance
• c specific heat capacity of substance
• ?T change in temperature

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Sample problem

• What is the heat absorbed (Q) if 2.0g water is
  heated from 10C to 25C ?
• Qmc?T
• 2.0g x 4.18J/g C x (25 10 C)
• 125.4 J
• J joules, the unit for energy

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Practice

1. What is the Q, if 5.2kg sample of limestone has a
   change in temp. of 16.3 C? (c 0.92)
2. What is the change in temp, if a 9.2g sample of
   mud absorbs 950J of energy? (c 2.51)
3. An unknown sample absorbs 6000J and has a temp
   change of 62 C. It has a mass of 2.6kg, what is
   the specific heat capacity?

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• 1) What is the Q, if 5.2kg sample of limestone
  has a change in temp. of 16.3 C?
• Solution Q5.2 x 0.92 x 16.3
• Q 77.98 KJ
• 2) What is the change in temp, if a 9.2g sample
  of mud absorbs 950J of energy?
• Solution 950 9.2 x 2.51 x T
• 950/23.092
• T 41.13
• 3) An unknown sample absorbs 6000J and has a temp
  change of 62 C. It has a mass of 2.6kg, what is
  the specific heat capacity?
• Solution 6000J 2.6 x c x 62
• 6000 / 161.2 c
• 37.2 J/goc c

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Heat of Fusion QmHfus

• Amount of heat required to melt 1.0g of a solid
  at the freezing point into a liquid.
• Amount of heat needed to freeze 1.0 g of a liquid
  at the freezing point into a solid
• Water has a large heat of fusion.

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Heat of vaporization QmHvap

• Amount of energy required to change 1.0g of a
  liquid at the boiling point into a gas at the
  boiling point.
• If the gas returns back into a liquid, then the
  same amount of heat must be lost. (condensation)

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Latent Heats for a Substance
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Latent Heat Sample Question

• Ted is making ice cubes that each contain 50 g of
  pure water. When the water reaches a temperature
  of 0 degrees, it begins to freeze. How much heat
  is lost in the process of the water freezing for
  each ice cube. (LHF of water is 333 J/g)
• Q mHfus 50(333) 16,650 J per ice cube