Work by Friction

1
Work by Friction

• A box slides 10 m across a surface. A frictional
  force of 20 N is acting on the box.
• What is the work done by friction?
• What happened to this energy?

2
Work by Friction

• A box slides 10 m across a surface. A frictional
  force of 20 N is acting on the box.
• What is the work done by friction? 200 J
• What happened to this energy?
• It turned into sound and heat.

3
Heat Energy

• 3U Physics

4
Remember the 2nd Law

• You cant break even.
• Youre always going to get less useful energy out
  than was put in because some energy will be lost
  to entropy.

5
The 2nd Law

• Entropy refers to the disorder of a system.
• Entropy always increases.

6
The 2nd Law

• For our purposes, the 2nd Law means that heat
  will always move from materials at higher
  temperatures to materials at lower temperatures.

7
Some definitions

• Temperature is a measure of the average kinetic
  energy of the particles (measured in oC or K).

8
Some definitions

• Temperature is a measure of the average kinetic
  energy of the particles (measured in oC or K).
• Thermal energy is the total kinetic energy and
  potential energy of the particles of a material
  (measured in J).

9
Some definitions

• Temperature is a measure of the average kinetic
  energy of the particles (measured in oC or K).
• Thermal energy is the total kinetic energy and
  potential energy of the particles of a material
  (measured in J).
• Heat is a measure of the thermal energy
  transferred from a warm body to a cooler one.

10
Heat Transfer

• Heat may be transferred by
• conduction
• convection
• radiation

11
Conduction

• In a solid, every atom is physically bonded to
  its neighbours in some way. If heat energy is
  supplied to one part of a solid, the atoms
  vibrate faster and these vibrations are passed on
  to the next atom, and so on

12
Conduction

• In metals, not only do the atoms vibrate more
  when heated, but the free electrons move around
  more as well. These transfer the energy much
  faster than just vibrations in bonds.

13
Convection

• Actually, heat doesnt rise.
• Heated gas or liquid rises because it is less
  dense than the cooler material surrounding it
  (which will sink to replace the rising material).

14
Convection

• Note that radiators actually heat primarily by
  convection, not radiation.

15
Radiation

• Radiation is the transfer of heat in the form of
  infrared (long wavelength) light.
• It travels in straight lines in every direction
  and can even travel through a vacuum.

16
Radiation

• Warm-blooded creatures typically maintain a body
  temperature warmer than their surroundings and
  will lose heat energy by radiating infrared light.

17
Radiation

• Cold-blooded creatures will not.

18
Specific Heat Capacity

• The specific heat capacity (c) of a material is
  the amount of energy that must be added to raise
  the temperature of 1.0 kg of material by 1oC or
  1K.
• c is different for different materials.

19
Specific Heat Capacity

• c is different for different materials, e.g.
• csteel 452 J/kgoC
• cglass 840 J/kgoC
• cwater 4186 J/kgoC

20
Heat

• The amount of heat (Q) required to raise the
  temperature of a quantity m of a material by an
  amount DT is therefore
• Q mcDT

21
Heat Example 1

• Calculate the energy required to raise the
  temperature of 5.0 kg of water by 75oC.

22
Heat Example 1

• Calculate the energy required to raise the
  temperature of 5.0 kg of water by 75oC.
• m 5.0 kg
• c 4186 J/kgoC
• DT 75oC
• Q ?

23
Heat Example 1

• Calculate the energy required to raise the
  temperature of 5.0 kg of water by 75oC.
• m 5.0 kg Q mcDT
• c 4186 J/kgoC Q (5.0 kg)(4186
• DT 75oC J/kgoC)(75oC)
• Q ? Q 1.6 x 106 J

24
Conservation of Energy

• Heat will be transferred from a hot object to a
  cold object such that the heat lost by the hot
  object is equal to the heat gained by the cold
  object.
• Qlost Qgained

25
Heat Example 2

• A 0.500 kg pot of hot water for tea has cooled to
  40.0oC. How much boiling water must be added to
  raise the temperature of the tea water to 65.0oC?

26
Heat Example 2

• A 0.500 kg pot of hot water for tea has cooled to
  40.0oC. How much boiling water must be added to
  raise the temperature of the tea water to 65.0oC?
• So the cold water needs its temperature raised
  from 40.0oC to 65oC DTcold 25oC

27
Heat Example 2

• A 0.500 kg pot of hot water for tea has cooled to
  40.0oC. How much boiling water must be added to
  raise the temperature of the tea water to 65.0oC?
• So the cold water needs its temperature raised
  from 40.0oC to 65oC DTcold 25oC
• And the hot water needs its temperature lowered
  from 100.0oC to 65.0oC DThot 35.0oC

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Heat Example 2

• Heat lost by hot water Heat gained by cold
  water
• mhotcDThot mcoldcDTcold

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Heat Example 2

• Heat lost by hot water Heat gained by cold
  water
• mhotcDThot mcoldcDTcold
• mhotDThot mcoldDTcold

30
Heat Example 2

• Heat lost by hot water Heat gained by cold
  water
• mhotcDThot mcoldcDTcold
• mhotDThot mcoldDTcold
• mhot mcoldDTcold/DThot

31
Heat Example 2

• Heat lost by hot water Heat gained by cold
  water
• mhotcDThot mcoldcDTcold
• mhotDThot mcoldDTcold
• mhot mcoldDTcold/DThot
• mhot (0.500 kg)(25.0oC)/(35.0oC)

32
Heat Example 2

• Heat lost by hot water Heat gained by cold
  water
• mhotcDThot mcoldcDTcold
• mhotDThot mcoldDTcold
• mhot mcoldDTcold/DThot
• mhot (0.500 kg)(25.0oC)/(35.0oC)
• mhot 0.357 kg

33
Heat More Practice

• Homework Set 9 Heat Energy