Conservation of
Energy
PHYSICAL SCIENCE
The Law of Conservation of Energy
Energy cannot be created or destroyed; it may be transformed from one
form into another, but the total amount of energy never changes.
What is energy?
“the ability to do work”
The combination of energy and matter make up the universe:
◦ Matter is substance, and energy is the mover of substance.
Energy is
nature’s way of keeping score.
We sense energy only
when the score changes,
either a transformation
from one form of energy
to another,
or a transfer of energy
from one point to
another.
Work
is equal to the force that is exerted times the distance over which it is
exerted.
W=Fxd
The unit of work combines the unit of force (N) with the unit of
distance (m)
Newton-meter (N-m) aka Joule.
You carry a 20 kg suitcase upstairs, a distance
of 4m. How much work did you do?
W=Fxd
F = ma
= (20 kg) (10m/s2) = 200 N
W=Fxd
= (200 N) (4m)
= 800 J
Power
omeasures the rate of work done.
oor the rate at which energy is expended.
oPower is the amount of work done, divided by the time it takes to do
it.
oPower (watts) = work (joules) / time (sec)
oP = W/t
Power
Since work performed equals energy expended,
Power (watts) = energy (joules) / time (sec)
The watt is defined as the expenditure of
1 joule of energy in 1 second.
(75 watt light bulb consumes 75 J/sec)
Important formulas and units
Quantity
Definition
Units
Force
mass x accel.
newtons
Work
force x distance
joules
Energy
power x time
joules
Power
work / time
watts
Heat and
Temperature
THIS LESSON BROUGHT TO YOU BY:
THE LET TER 3 AND THE NUMBER M
Temperature
and Heat
Temperature and heat are
NOT THE SAME.
Temperature
How hot or
cold it is.
Measured in degrees Celsius.
Heat
The amount of thermal energy,
measured in joules or J.
A cup of hot tea has heat energy in
the form of kinetic energy from its
particles.
A swimming pool at 30°C is at a lower
temperature than a cup of tea at 80°C.
BUT the swimming pool
contains more water, so it stores
more thermal energy or heat.
The small beaker of water boils
first
The large beaker
contains more water
and needs more
thermal energy or
heat to reach 100°C.
Transfer
of Heat
Heat
A form of energy associated with the motion of
atoms or molecules.
Transferred from higher temperature objects to
objects at a lower temperature.
How Heat Can Be Transferred
Conduction
Convection
Radiation
Conduction
Transfer of heat through direct contact.
Occurs anytime objects at different temperatures are touching each
other.
As long as the objects are in contact, transfer of heat will continue until
the temperature of the objects is the same.
Conduction (continued)
Example: If you leave a metal
spoon in a pan of soup that you
are heating on the stove, it may
burn your fingers. The spoon is in
direct contact with the hot soup
and heat is transferred to the
spoon.
Conductors and Insulators
Some materials conduct heat better than others.
Materials that transfer heat well are called
conductors.
Metals are usually good conductors.
Wood, paper and plastic are not.
Materials that stop the transfer of heat are called
insulators (styrofoam, wool, fiberglass).
Convection
The transfer of energy in a liquid or gas.
When part of a gas or liquid is heated, the particles it is made up of
move faster and spread out more.
The moving particles bump into other particles, causing them to move
faster and spread out more.
Convection Currents
When particles in the air
spread out, they become less
dense and generally rise above
the unheated, more dense
particles around them.
The denser masses of the gas
or liquid move in to fill the
space left by the heated
particles.
The particles that move away
from the source of heat
become cooler and more
dense.
Radiation
Energy transferred in the form of rays or waves or particles.
We will concentrate on the type of radiation that travels as
electromagnetic waves.
Heat From the Sun
You can feel the sun warm your
skin on a sunny day.
This is because the energy causes
the particles in your skin to move
faster = more heat energy.
Electromagnetic Waves
Include visible light, microwaves
and infrared light
Can travel through space.
The sun is our major source.