Unit 3 Physics Energy

Energy• the ability to do work

7 Types of Energy1. Light - energy traveling in waves

2. Mechanical - energy of lifting, bending, etc… (work)

3. Thermal - molecular motion

4. Nuclear – energy in atomic nuclei (holds p+ & neutrons together)

5. Chemical - energy stored in chemical bonds

6. Electrical – moving electrons

7. Sound - vibrations

Law of Conservation of Energy• energy cannot be created or destroyed

only transferred from one form to another

Electric Light Chemical Mechanical

Work-transfer of energy through motion *Object must move in direction force applied!

Work• Equation: W = Fd

W = workF = forced = distance

• Work is measured in Joules (J)J = N m

*Joule is always (N)(m), never (N)(cm)!!

A teacher pushed a 10kg desk across a floor for a distance of 5m. She exerted a horizontal force of 20N. How much work was done?F = 20Nd = 5m

W = Fd

W = (20N)(5m)

W = 100J

A force of 10,000N is applied to a stationary wall. How much work performed on the wall?

F = 10,000Nd = 0m

W = Fd

W = (10,000N)(0m)

W = 0J

Work – Energy Theorem

• All work is energy,but not all energy is work

2 major forms of energy

1. Kinetic energy (KE)-energy due to motionex: anything movingKE = ½ mv2

(J) (kg)(m/s)2

(N) (m)

Calculate KE

KE = ½ mv2

m = 1216kgv = 10m/sec

KE = ½ (1216kg)(10m/sec)2

KE = 60,800J

KE = ½ (1216kg)(100(m/s)2 )

A 1216kg car traveling at 10m/sec.

2. Potential energy (PE)-energy due to position or conditionex: book high on a shelf

stretched rubber bandwound up clock

-the only PE we calculate is gravitational potentialPE = m g h(J) (kg)(m/s2)(m)

(N) (m)

Calculate PE

PE = m g h

m = g =

500N

h = 100m

PE = (500N) (100m)

PE = 50,000J

Check up:

• Which of the 3 vehicles has the greatest KE?• Which car has the greatest KE?

Check up:

• Where does the pendulum have greatest PE?• Where does the pendulum have greatest KE?

Greatest PE

Greatest PE

Greatest KE

Physics Unit 3

Heat, Thermal Energy, Temperature and Specific Heat

Thermal Energy (Q)•

the total

amount of kinetic energy of all the

molecules that make up a substance•

Example: If you have a couple of molecules with low KE (low Q)) vs. a couple of molecules with high KE (more Q).

•

Example: If you have a few molecules with high KE (low Q) vs. a bunch of molecules with the same KE (more Q).

•

measured in joules (J)•

Q = mΔTCp

Temperature (T)•

A measure of the average

kinetic energy of

the molecules of a substance-deals with how fast molecules moving

•

measured in degrees Celsius (°C)

Heat•

Transfer of thermal energy

•

Examples:-bath tub has lots of thermal energy; when you climb in you feel heat.-radiator has lots of thermal energy; when you lean on it you feel heat.

Specific Heat (Cp

)•

Amount of energy required to raise 1kg of a substance 1°C

•

Cp

water = 4190J/kg°C•

Cp

sand = 664J/kg°C •

Example: Why when you go to the beach is the sand so hot and the water so cold?

-sand has lower Cp

so needs less energy to raise its temperature

•

Equation: Cp

= Q / mΔT

A 0.05kg sample of a material requires 660J of thermal energy to have its temperature raised

from 20°C to 80°C. What is the specific heat?

m = 0.05kgQ = 660JΔT = 60°C

Cp

= Q / mΔT

Cp

= 660J / (0.05kg)(60°C)

Cp

= 220 J/kg°C

What is the amount of thermal energy required to raise the temperature of 0.5kg

of water from 0°C to 100°C m = 0.5kgCp

= 4190J/kg°CΔT = 100°C Q = mΔTCp

Q = (0.5kg)(100°C)(4190J/kg°C)

Q = 209,500J

Check upCould a lighted candle and a pitcher of ice water

have the same amount of thermal energy?

Answer: Yes!! Candle has few molecules moving quicklyIce water has lots of molecules moving slowly

Check upWhich contains more energy?A) mug of boiling waterB) bath tub of boiling waterC) both contain the same amount of energyAnswer: B -

even though they both have the

same average KE (Temperature), the tub has many more molecules to give it a greater total KE (thermal energy)

Unit 3 Physics

Heat Transfer

3 Ways of Heat Transfer1. Conduction

-”contact”-heat transfers from direct contact between 2 objects at different temperatures (from high temp. to low temp.)-example:

-pan on a stove

2. Convection-”carry with”-heat travels with moving fluids

(a gas or a liquid)*heat Δ

KE of molecules, molecules move

faster, have more collisions, and move farther apart; matter becomes less dense and rises.

-example: -cooking in an oven

3. Radiation-”to give off”-heat given off in waves(does not require matter)Example:

-toasting marshmallows-sun

Check up

•

How are many homes heated?A. ConductionB. ConvectionC. Radiation

•

B. convection -

Many homes have furnaces in the basement & use convection currents.