Energy

1
Energy you cant have change without it!

• Unit 2

2
What is energy no, it is not coffee!

• Energy is the ability to do work, it comes in
  many different forms
• Chemical the potential energy that is stored in
  chemical substances
• Heat measure of energy of molecules atoms
• Potential energy of position (think top of the
  mountain)
• Kinetic energy of motion
• Light
• Electrical the flow of electrons
• Nuclear the energy released in fusion or
  fission

3
The big rule!

• Energy can not be created or destroyed, it just
  changes forms
• Example
• Mechanical ? Heat what happens when you rub a
  block of wood with sand paper
• Heat ? Mechanical When steam is used to turn
  turbines in a steam locomotive
• Chemical ? Heat what happens when you burn oil
  to heat your home

4
Heat vs. Temperature

• Heat is the amount of kinetic energy that
  molecules have, the more motion the more heat,
  heat is measured in Joules
• Temperature is just a relative scale of comparing
  how hot or cold something is to something else
  measured in Fahrenheit, Celsius, Kelvin
• Heat and Temperature are NOT synonymous!

5
Temperature Scales

• Fahrenheit First Temperature Scale, based on
  freezing and boiling points of water (32º - 212º)
• Celsius Also based on freezing and boiling
  points of water, on a 100 point scale (0º - 100º)
  used to be called centigrade
• Kelvin not based on a physical object, but on
  the complete lack of molecular motion (absolute
  zero)

6
Conversions Celsius to Fahrenheit

• F 9/5 (temp in C) 32
• Example
• What is 20ºC in Fahrenheit?
• F 9/5(20) 32 68º
• Now you! What is 15ºC in Fahrenheit?

7
Fahrenheit to Celsius

• Answer to prior slide 5ºF
• C 5/9 (temp in F 32)
• Example
• What is 75ºF in Celsius
• C 5/9(75 32) 24ºC
• Now you! What is 20ºF in Celsius?

8
Celsius? Kelvin

• Answer to prior slide -7ºC
• To change Celsius to Kelvin
• Celsius 273 Kelvin
• Example 31ºC in Kelvin is
• 31 273 304 K,
• Note! There is NO degree symbol for Kelvin
• Now you
• -23ºC to Kelvin

9
Kelvin ? Celsius

• Answer to prior slide 250K
• To change Kelvin to Celsius
• Kelvin - 273 Celsius
• Example 403K in Celsius
• 403K- 273 , 130 ºC
• Now you
• 159K is what in Celsius?

10
Heat - it is needed for change

• Answer to prior slide -114ºC
• Heat needs to be added or removed to change
  temperature or change the state of a substance.
• Heat Mass º Specific Heat º Change in
  temperature
• Q m Cp ?T
• or
• Q m Cp (Tf Ti)
• Specific Heat (Cp)is a constant for the
• substance with the unit of J/kgºC

11
Example 1

• What is the heat needed to raise the temperature
  of a bottle of water (mass of 0.750kg) from 45.0
  to 90.0 degrees Celsius if the specific heat of
  water is 4.19 J/kgºC?
• Q m Cp (Tf Ti)
• Therefore
• 0.75 kg (4.19 J/kgºC)(90.0 45.0)
• 141J

12
Example 2

• What is the heat released when the temperature of
  a block of steel (mass of 2.50kg) is cooled
  from 145.0 to 30.0 degrees Celsius if the
  specific heat of steel is 450J/kgºC?
• Q m Cp (Tf Ti)
• Therefore
• 2.50 kg (450 J/kgºC)(30.0 -145.0 )
• -1.3x105J (the negative sign means released)