The Nature of: Energy

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The Nature of Energy
What is Energy?

• The ability to do work or produce heat

Potential Energy?
Energy due to position or composition
Kinetic Energy?
Energy due to motion. (Depends on velocity and
mass)
2
The Nature of Energy
Can energy be converted from one form to another?

• Yes! Kinetic ? ? Potential

Can you make Energy?
No!
Can you destroy Energy?
No!
3
The Nature of Energy
The law of conservation of energy energy can be
converted from one form to another but can not be
created or destroyed.
4
The Nature of Energy
State function some property of a system (in
this case energy) that changes independently of
its pathway.
Example Traveling from Seattle to Stevens Pass.
Your altitude changes from sea level to 1236M no
matter which road you take there.
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The Nature of Energy
State function some property of a system (in
this case energy) that changes independently of
its pathway.
Example A ball rolling down a hill.
It ends up with the same amount of energy
independently of how many bumps there are or how
fast it gets there.
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Temperature Heat
What is temperature?

• The measure of the random motions of the
  components of a substance

What is heat?
The flow of energy due to a temperature
difference. (Hot ? Cold)
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Temperature Heat
For the example I used last week?

• Tfinal Thot initial Tcold initial 50?C
  10?C 30?C
• 2 2

?Thot 50?C - 30?C 20? C
?Tcold 30?C - 10?C 20?C
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Exothermic Endothermic
Exothermic?

• Energy flows out of the system

Endothermic?
Energy flows into the system
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Exothermic Endothermic
System?

• The part of the universe we are focusing on.

Surroundings?
Everything else in the universe.
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Surroundings
Surroundings
energy
energy
System
System
Exothermic ? heat out ? (-)
Endothermic ? heat in ? ()
?E lt 0
?E gt 0
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Energy Equivalents
calorie (cal)?

• The amount of energy (heat) required to raise the
  temperature of one mL of water by one degree
  Celsius.

Kilocalorie (Calorie or C)?
1000 calories.
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Energy Equivalents
joule (J) ?

• The joule is an SI unit.
• 1 calorie 4.184 joules

Abbreviations?
1 cal 4.184 J 1 C 1000 cal 4184 J
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Problem
Express 34.8 cal of energy in units of joules.
Express 47.3 J of energy in units of calories.
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Problem
Calculate the amount of energy (heat) in joules
of energy to raise the temperature of 45.0g of
water from 15.0?C to 90.5?C.
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Demonstration
Predict the result. A 200g weight made of steel
is placed in boiling water and brought to a
temperature of 100?C. The weight is then placed
in 200 grams of room temperature water (about
22?C). When the temperature equalizes in a few
minutes, what will the temperature of the water
be?
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Specific Heat

• So far weve seen that the energy (heat) required
  to change the temperature of a substance depends
  on
• The amount of substance being heated (grams)
• The temperature change (?T in number of degrees)

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Problem
Calculate the amount of energy (heat) in joules
of energy to raise the temperature of 45.0g of
water from 15.0?C to 90.5?C.
Mass (m) in grams of substance 4.184 J/g?C ? 1g
H2O ? 1?C Change in temperature ?T Tinitial
-Tfinal
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Problem
Calculate the amount of energy (heat) in joules
of energy to raise the temperature of 45.0g of
water from 15.0?C to 90.5?C.
45.0g x 4.184 J/g?C x (90.5?C-15.0?C) 14200 J
or 1.42 x 104J
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Specific Heat
There is another important factor
The identity of the substance.
4.184 J/g?C raises 1g of H2O ? 1?Celsius The
same amount of energy (heat) raises one gram (g)
of gold (Au) ? 32?C
20
Specific Heat
The two substances, water and gold, have
different heat capacities.
The amount of energy required to change the
temperature of one gram of a substance by one
Celsius degree is called its SPECIFIC HEAT
CAPACITY or just specific heat
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Some Specific Heats
Substance Specific Heat Capacity (J/g ?C )
Water (l) 4.184 Water (s) 2.03 Aluminum
(s) 0.89 Iron (s) 0.45 Silver (s) 0.24 Gold
(s) 0.13
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Problem
What quantity of energy (in joules) is required
to heat a piece of iron weighing 2.0g from 25?C
to 30?C?
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Problem
In the problem, to calculate the energy(heat)
required we took the product of the specific heat
capacity, the sample size in grams, and the
change in temperature in Celsius degrees.
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We can represent this using the following
equation Q s x m x ?T Q energy
(heat) required s specific heat capacity
m mass in of sample in grams?T change in
temperature in ?C