Energy – you can’t have change without it! Unit 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

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

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!

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)

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?

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?

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

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

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 C p ΔT or Q = m C p (T f – T i ) Specific Heat (C p )is a constant for the substance with the unit of J/kgºC

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 C p (T f – T i ) Therefore… = 0.75 kg (4.19 J/kgºC)(90.0 – 45.0) = 141J

Example # 2 What is the heat released when the temperature of a block of steel (mass of 2.50kg) is cooled from to 30.0 degrees Celsius if the specific heat of steel is 450J/kgºC? Q = m C p (T f – T i ) Therefore… = 2.50 kg (450 J/kgºC)( ) -1.3x10 5 J (the negative sign means released)