Energy, Work, Power

ENERGY-The ability to cause CHANGE

-Anything that causes change must have ENERGY!!

-You use energy to function

Different Forms of Energy

1. Electrical

2. Chemical

3. Radiant

4. ThermalThese are like different forms of

money

KINETIC ENERGY (KE)-The energy of MOTION

-All moving objects have Kinetic Energy!

-Depends on: MASS and SPEED of the object

-Equation:

Kinetic Energy (KE) Joules = 1/2mass(kg) X speed2

KE = 1/2mv2

-Joule (J) = SI unit for Energy

Example Problem with Kinetic Energy

-A jogger whose mass is 60kg is moving at a speed of 3 m/s. What is the jogger’s Kinetic Energy?

Mass = 60kg

Velocity = 3 m/s

KE = ??J

KE = 1/2mv2

KE = 1/2(60kg)(3m/s)2

KE = 270 J

POTENTIAL ENERGY (PE)-The energy of REST

-Objects at REST have POTENTIAL ENERGY

-Potential Energy is CHANGED into KINETIC ENERGY when MOTION occurs

TYPES OF POTENTIAL ENERGY

1. Elastic Potential Energy = Energy stored by something that can be STRETCHED or COMPRESSED

Ex: Rubber band

2. Chemical Potential Energy = Energy stored in CHEMICAL BONDS

Ex: Food, Natural gas

3. Electrical Potential Energy = Energy stored due to ELECTRICAL CHARGES

4. Nuclear Potential Energy = Energy stored in the NUCLEI OF ATOMS

TYPES OF POTENTIAL ENERGY

5. Gravitational Potential Energy: Energy stored by objects due to their POSTION ABOVE EARTH

-Ex: Anything with the potential to FALL

-Depends on: MASS and HEIGHT above ground

-Equation:

GPE (J) = Mass (kg) X gravity (m/s2) X height (m)

GPE = mghRemember: Gravity on Earth = 9.8 m/s2

GPE Example Problem-What is the GPE of a ceiling fan that has a mass of 7kg

and is 4m above the ground?

GPE = mgh

Gravity = 9.8 m/s2

Mass = 7 kg

Height = 4 m

GPE = (7kg)(9.8m/s2)(4m)

= 274 kgm2/s2 = 274 J

To INCREASE GPE

1: INCREASE object’s HEIGHT

2. INCREASE object’s MASS2KG

2KG7KG

2KG

The Change of GPE to KE

-As objects fall, GPE is changed into KE

-KE is LARGEST right before the object hits the ground, thus GPE is the

SMALLEST right before hitting the ground

-Objects with MORE GPE move FASTER because they have more KE

Converting between KE and PE

-MECHANICAL ENERGY = Total amount of POTENTIAL and KINETIC energy in a system

Mechanical energy = PE + KE

What happens to the mechanical energy as PE and KE are converted into each other?

ME stays the same!! As PE and KE are converted, the FORM of energy

changes, but the TOTAL AMOUNT STAYS THE SAME

The Law of Conservation of Energy

-States that: ENERGY CAN’T BE CREATED OR DESTROYED!!

-So does this mean the total amount of energy in the Universe is the same at all times???

Friction and Air Resistance

These forces can cause some mechanical energy to change into THERMAL ENERGY!!

TEMPERATURE-TEMPERATURE = Measure of the AVERAGE KINETIC

ENERGY of the particles in an object

-As temp. INCREASES = Average SPEED of particles INCREASE = KE INCREASES

-SI UNIT: Kelvin (K) or Celsius (0C)

THERMAL ENERGY

Sum of the KINETIC and POTENTIAL energy of all the particles in an object

THERMAL ENERGY & TEMPERATURE

-As KE INCREASES = TEMPERATURE INCREASES

THUS

-As TEMP INCREASES = THERMAL ENERGY INCREASES

THERMAL ENERGY & MASS-As MASS INCREASES = total KE INCREASES

THUS

-AS MASS INCREASES = THERMAL ENERGY INCREASES

HEAT-Thermal energy that FLOWS from something at a HIGHER

TEMP to something at a LOWER TEMP

-Form of ENERGY

-Measured in JOULES (J)

***Always flows from warmer to cooler***

SPECIFIC HEAT

-Amount of HEAT needed to RAISE THE TEMP of 1kg of a material by 10C

-Measured in: J/(kg 0C)

-As a substance absorbs HEAT, its TEMP change depends on the composition of the substance. This is called the

Specific Heat of the substance!

Ocean water v/s

Beach Sand

(see next slide)

SPECIFIC HEAT EXAMPLE PROBLEM

1 kg of sand takes 6x less heat to raise it 10C than 1 kg of water

Which has the higher specific heat??

WATER

Which can absorb more heat with changing its temp?

WATER

TRANSFERRING THERMAL ENERGY

-3 Ways to transfer thermal energy from place to place

1. CONDUCTION

2. CONVECTION

3. RADIATION

CONDUCTION-The transfer of energy by COLLISIONS or TOUCH

-This happens because particles are in CONSTANT MOTION

-Thermal Energy is transferred by collisions between molecules with more KE to molecules with less KE

**HEAT is transferred by COLLISIONS, not by movement of matter**

HEAT CONDUCTORS1. Heat moves faster by conduction in SOLIDS and LIQUIDS than in gases

WHY??

2. The BEST conductors = METALS

WHY??

Gases are more spread

out/collisions are less frequent

All the e – aren’t bonded to atoms, so

they move more freely

CONVECTION-The transfer of thermal energy in a FLUID (gas or liquid) by movement of WARMER and COOLER fluid from place to place

-Uses CONVECTION CURRENTS to transfer the energy from WARM TO COOL

-Still uses collisions to transfer as well

-As a fluid increases its temp = it EXPANDS and its DENSITY DECREASES

WHY??

-Particles have more KE = move faster = spread out

RADIATION-The transfer of energy by ELECTROMAGNETIC WAVES

-Waves can travel through space even though there is no matter THUS Radiation is the only form of energy

transfer in space

-Energy = RADIENT ENERGY

RADIATION-Radiation can be ABSORBED, REFLECTED, or

TRANSMITTED thru an object

-This depends on: MATERIAL OF OBJECT

-LIGH-COLORED = Reflect more

-DARK-COLORED = Absorb more

-Radiation can be TRANSMITTED thru:

1. SOLIDS

2. LIQUIDS

3. GASES

INSULATORS-A material in which HEAT FLOWS SLOWLY

-Good Conductors = BAD INSULATORS

-Good Insulators = Wood, plastic, fiberglass, air

-Gases are usually the BEST INSULATORS

CONTROLLING HEAT FLOW

-You can use various materials to control heat flow

Ex: Jacket/sweater

-Living organisms have special features to help control the flow of heat

Ex: Fur, Blubber, Scales (reflect), Color

Transferring EnergyReview: 1st: Discussed transferring energy as

HEAT

-Heat flows from an object that is warmer (more ke) to an object that is cooler (less ke)

New Stuff: 2nd way to transfer energy

-Work = transfer of energy that occurs

when a force makes an object move

WORKRECALL: Force = push or pull

- In order for WORK to be done, a FORCEmust make something MOVE.

-So: If you push your hand straightdown on the table right now (apply a

force) and the table doesn’t move, have you really done any work??

Doing WorkTwo Things are Required for Work to be

done:

- The force must make object move

- Movement must be in the samedirection as the force

Force is Up, Movement is Up

Work and EnergyAnother way to think about energy is:

-ENERGY is the ability to do WORK

-If something has energy, it can transfer it to another object by

doing work on that object.

-When you do WORK onsomething, you INCREASE itsENERGY

-Energy is always transferredfrom the object doing work tothe object on which work isdone.

CALCULATING WORKAmount of Work done depends on:

1. Amount of Force used

2. Distance over which force was applied (distancethe object moves)

Work (in joules) = applied force (newtons) X distance (meters)

W = Fd

Example Problem

You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how

much work do you do?

F= 100 N

d = 5 m

W = ??

W= Fd

W = (100N)(5m)

W = 500Nm

W = 500 J

POWER

= The amount of work done in one second/the RATE at which WORK is done

For example: If you and a friend push two boxes (identical in mass) for the same distance = YOUR

WORK DONE IS EQUAL

BUT: If your friend pushes the box faster than you, your friend is MORE POWERFUL!

CALCULATING POWER

Power (in watts) = Work (in Joules) time (in seconds)

P = Wt

SI Unit for Power = Watt (W)

1 Watt = 1 J of work done in 1 sec (small unit)

Use kilowatt (kW) to often express power

1 kW = 1000 W

Example Problem

You do 900 J of work in pushing a sofa. If it took 5 sec to move the sofa, what was your power?

W = 900 J

t = 5 sec

P = ??Watt

P = W = 900 = 180 J/s = 180 Watts

t 5

Power and Energy

Power is also the RATE at which ENERGY is TRANSFERRED

Power (in Watts) = energy transferred (in joules)

time (in seconds)

P = E

t

Ex: A light-bulb transfers electrical energy into light and heat.

Calculating Changes in Thermal Energy

Changes in thermal energy (J) =

Mass (kg) X change in temp. (oC) X specific heat (J/kg oC)

Q = m (Tf-Ti) C

Example: Find the change in thermal energy of a 20-kg wooden chair that warms from 15oC to 25oC if the specific heat is 700 J/(kg oC)

Mass = m=20kg

Ti = 15 oC

Tf = 25 oC

Specific heat = C= 700 J/(kgoC)

Q = 20(25-15)(700) =

20(10)(700) =

140,000 J

THERMODYNAMICSThere is another way to increase thermal energy besides

adding heat

Thermodynamics = The study of the relationship between thermal energy, heat, and work.

System = Anything you can draw a boundary around. Heat transferred to a system is the amount of heat flowing into the system across that boundary.

First Relationship: Heat and Work both increase Thermal Energy

Ex. Situation 1: Sitting by a fire, you warm your hands by holding them close to the fire.

Situation 2: Sitting by a fire, you warm your hands by holding them close to the fire and rubbing them together

Situation 2 warms your hands up more!

Open System = Thermal energy of this system can change

Closed System = processes can happen within the system/but no energy is entering or leaving = Thermal Energy of this system doesn’t change

THERMODYNAMICSFirst Law of Thermodynamics

States: The INCREASE in THERMAL ENERGY of a system = the WORK DONE plus the HEAT TRANSFERRED to the system or

The increase in energy of a system = the energy added to the system

90 J of energy added to the system

Increase of 90 J within the system

1st law explains the following:

-When Heat flows from a warm object to a cool object, the Thermal Energy of the Warm object decreases = the Increase in Thermal Energy of the Cool Object

So: Can heat flow spontaneously from a cool object to a warm object??

THERMODYNAMICSSecond Law of Thermodynamics: It is IMPOSSIBLE for HEAT to FLOW from a COLD OBJECT to a WARM OBJECT unless WORK IS DONE

Also explains that it is impossible to build a device that is 100% efficient

Friction between objects can completely convert work into heat.

-Is it possible to completely convert heat into work?

2nd Law of Thermodynamics FORBIDS this from happening.

2nd Law makes it IMPOSSIBLE to build a device that converts heat completely into

work!

HEAT ENGINE: A device that converts heat into work.

Ex: Car’s Engine - converts chemical energy (gas) into heat, then transfers some of the thermal enegy into work by rotating the wheels.

Only 25% of the heat is converted to work. Where does the rest go??