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. Thermal
These 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 = mgh
Remember: 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 MASS
7KG
2KG
2KG
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 CONDUCTORS
1. Heat moves faster by conduction
in SOLIDS and LIQUIDS than in
gases
Gases are more
WHY??
spread
out/collisions are
less frequent
2. The BEST conductors = METALS
WHY??
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 Energy
Review: 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
WORK
RECALL: Force = push or pull
- In order for WORK to be done, a FORCE
must make something MOVE.
-So: If you push your hand straight
down on the table right now (apply a
force) and the table doesn’t move,
have you really done any work??
Doing Work
Two Things are Required for Work to be
done:
- The force must make object move
- Movement must be in the same
direction as the force
Force is Up,
Movement is Up
Work and Energy
Another 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 on
something, you INCREASE its
ENERGY
-Energy is always transferred
from the object doing work to
the object on which work is
done.
CALCULATING WORK
Amount of Work done depends on:
1. Amount of Force used
2. Distance over which force was applied (distance
the 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=5m
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=W
t
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)
Q = 20(25-15)(700) =
Mass = m=20kg
Ti = 15 oC
Tf = 25 oC
Specific heat = C= 700 J/(kgoC)
20(10)(700) =
140,000 J
THERMODYNAMICS
There 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
2 warms your
hands by holding them closeSituation
to the fire.
hands up more!
Situation 2: Sitting by a fire, you warm your
hands by holding them close to the fire and rubbing them
together
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
THERMODYNAMICS
First 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??
THERMODYNAMICS
Second 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??
Download

Energy Work and Power