Heat and Temperature

1
Heat and Temperature

• Chapter 14

2
Temperature

• Temperature is a measure of the average kinetic
  energy of all the particles within an object.
• We measure temperature using a thermometer.

3
Temperature

• Thermometers work on a principle called thermal
  expansion.
• As the temperature rises, the particles of a
  substance gain kinetic energy and move faster.
  With this increased motion, the particles of that
  substance move further apart and expand the
  volume of a substance.

4
Temperature Scales

• Around the world, three temperature scales are
  used Fahrenheit (oF), Celsius (oC), Kelvin (K)

5
Temperature Scales
212

• What is the freezing point and boiling point of
  water on the Fahrenheit scale?

32
6
Temperature Scales
100

• What is the freezing point and boiling point of
  water on the Celsius scale?

0
7
Temperature Scales

• The Kelvin scale is based on the principle of
  absolute zero.
• Absolute zero is the coldest possible
  temperature, and it is also where an objects
  energy is minimal.
• Absolute zero is -273.13 oC. The Kelvin scale
  was created so that absolute zero was in fact
  recorded as 0 kelvin or 0 K. (Kelvin has no
  negatives!)

8
Temperature Scales
373

• What is the freezing point and boiling point of
  water on the Kelvin scale?

273
9
Temperature Conversions

• How to convert from Celsius to Fahrenheit
• Fahrenheit temp. (1.8 x Celsius temp.) 32.0
• TF 1.8 TC 32.0

10
Temperature Conversions

• How to convert from Fahrenheit to Celsius
• Celsius temp. (Fahrenheit temp. - 32.0)
• 1.8
• TC (TF - 32.0)
• 1.8

11
Temperature Conversions

• How to convert from Celsius to Kelvin and vice
  versa
• Celsius to Kelvin
• TK TC 273
• Kelvin to Celsius
• TC TK - 273

12
Temperature Conversions

• If the boiling point of liquid hydrogen is
  -252.87 oC, what is its boiling point in oF?
• TF 1.8 TC 32.0
• TF 1.8 (-252.87) 32.0
• TF -423.17 oF

13
Temperature Conversions

• If the temperature of a winter day at the North
  Pole is -40.0 oF, what is the temperature in oC?
• TC (TF - 32.0) / 1.8
• TC (-40.0- 32.0) / 1.8
• TC -40.0 oC

14
Temperature Conversions

• The melting point of gold is 1064 oC, what is its
  melting point in K?
• TK TC 273
• TK 1064 273
• TK 1337 K

15
Temperature Conversions

• The air temperature in a typical living room is
  294 K, what is its temperature in oC?
• TC TK 273
• TC 294 K 273
• TC 21 oC

16
Temperature Conversions

• The metal in a running car engine will typically
  get as hot as 388 K, how hot is this in oF?
• TC TK 273
• TC 388 K 273
• TC 115 oC

• TF TC1.8 32.0
• TF (115 oC) 1.8 32.0
• TF 239 oF

17
Temperature Conversions

• The air temperature on a summer day in the desert
  is typically 110 oF, what is the temperature in
  K?
• TC (TF - 32.0) / 1.8
• TC (110 oF - 32.0) / 1.8
• TC 43 oC

TK TC 273 TK 43 oC 273 TK 316 K
18
Heat

• Hot and cold has nothing to do with
  temperature.
• Cold is felt when we lose energy to an object
  with less kinetic energy.
• Hot is felt when we gain energy from an object
  with more kinetic energy.

19
Heat

• Heat is the transfer of thermal energy from the
  particles of one object to those of another
  object due to a temperature difference between
  two objects.

20
Heat Transfer

• The warmer object always transfers heat to the
  cooler object.
• The greater the temperature difference, the
  faster the energy transfer.

21
Heat Transfer

• Three methods of energy transfer are conduction,
  convection, and radiation.

22
Heat Transfer

• Thermal conduction is the transfer of energy as
  heat between particles as they collide within a
  substance or between two objects in contact.

23
Heat Transfer

• Convection is the transfer of energy by the
  movement of fluids with different temperatures.
• A convection current is the flow of a fluid due
  to heated expansion followed by cooling and
  contraction.

24
Heat Transfer

• Radiation is the transfer of energy by
  electromagnetic waves.
• Radiation does not require contact.
• It is the only method of energy transfer that can
  occur in a vacuum (outer space).

25
Conductors and Insulators

• Conductors are a material through which energy
  can be easily transferred as heat.
• Solids, especially metals like copper and silver,
  are the best conductors.

26
Conductors and Insulators

• Insulators are a material that is a poor energy
  conductor.
• Wood, rubber, fiberglass, Styrofoam, wool, etc.

27
Laws of Thermodynamics

• First Law of Thermodynamics the total energy
  used in any process is conserved, whether the
  energy is transferred as a result of work, heat,
  or both.

28
Laws of Thermodynamics

• Second Law of Thermodynamics the energy
  transferred as heat always moves from an object
  at a higher temperatures to an object at a lower
  temperature.

29
Specific Heat Capacity

• Specific heat capacity is the amount of energy
  transferred as heat that will raise the
  temperature of 1 kg of a substance by 1 K.
• Specific heat capacity is a physical property and
  will be the same any pure substance. Therefore,
  we can use it to identify substances.
• energy specific heat capacity x mass x change
  of temp.
• q c x m x ?T

(J)
(kg)
(K)
(J/kg K)
30
Specific Heat Capacity

• Specific heat capacity is the amount of energy
  transferred as heat that will raise the
  temperature of 1 kg of a substance by 1 K.

(J)
(kg)
(K)
(J/kg K)
31
Ch. 14 Section 2 Energy Transfer

• A 10 g piece of iron absorbs 1000 joules of heat
  energy, and its temperature changes from 25C to
  100C. Calculate the specific heat capacity of
  iron.

q ?T c m
1000 J
c q / (m ?T)
c 1000J/(10g75oC)
100-25 75 oC
q
c 1000J/(750goC)
?
c 1.3 J/goC
(c m ?T)
10 g
32
Ch. 14 Section 2 Energy Transfer

• To what temperature will a 20 g piece of glass
  raise if it absorbs 1000 joules of heat and its
  specific heat capacity is 0.50 J/gC? The
  initial temperature of the glass is 10.0C.

?T q / (c m)
?T 1000J/(0.50 J/goC 20g)
q ?T c m
1000 J
?T 1000/(10 oC)
Tf-10 ?
q
?T 100 oC
0.50 J/goC
?T Tf To
(c m ?T)
100oC Tf 10 oC
20 g
10oC100oCTf 10oC10oC
Tf 110oC
33
Ch. 14 Section 2 Energy Transfer

• 100 g of 5.0C water is heated until its
  temperature is 20C. If the specific heat of
  water is 4.18 J/gC, calculate the amount of heat
  energy needed to cause this rise in temperature.

q ?T c m
?
q c m ?T
q 4.18 J/goC 100 g 15oC
15 oC
q
q 6,270 J
4.18 J/goC
(c m ?T)
100 g