Download What is Energy? Energy

What is Energy?
Course Webpage
www.yorku.ca/mearl/energy/energy.htm
Course email [email protected]
• Energy is the ability to do work
• Energy can neither be created nor
destroyed in only changes form
• Can be measured in a number of units,
calories, BTU’s, ergs.. but we will use
Joules (metric system) for the most part in
this course
Energy
• There are two main types of energy
• stored energy which we call potential
energy
• Kinetic energy - the energy of something
in motion
1
Energy types
• Chemical Energy – this includes energy either
used or derived by chemical reactions. In any
chemical reaction the bonding patterns between
molecules are changed. When bonds are
broken and reformed energy is either released
(exothermic or exergonic reactions) or energy is
absorbed/stored (endothermic or endergonic
reactions). Combustion or burning of any kind is
a release of stored chemical energy. Most
biological processes involve the release and
storage of chemical energy.
• .
• Nuclear Energy – this comes in two forms.
• Fission is the break down of larger atoms
that releases energy.
• Fusion is the amalgamation of smaller
atoms to make bigger ones that also
releases energy.
• This is the form of energy for nuclear
bombs, nuclear power plants, and fusion is
the energy source for the sun.
• Mechanical Energy - this is the energy of
some sort of movement (type of kinetic
energy) like the work done by our muscles,
by wind turning a turbine etc.
2
• Thermal energy – this is the energy
derived from heat (how the energy is
actually extracted in electrical generating
plants that either burn fossil fuels or use
nuclear reactions). Here we use what is
called a heat engine.
• Radiant or light energy – this is the
energy that comes from light (what powers
solar cells and photosynthesis)
• Electrical energy – this is the energy of
flowing electrons. It is a major way in
which we transmit energy
• Gravitational Energy - this is the energy
that results from objects moving in a
gravitational field (generally stored as
potential energy). This is where the
energy for hydroelectric dams comes from
3
• With any of these forms of energy, one of
the major things we try to accomplish is
the transformation from one form to
another. (Table 2.2). The problem with
any energy transformation is that by the
laws of thermodynamics they can never be
100% efficient.
Some Basic physics
In dealing with energy we deal often with
motion.. Water moving over a turbine,
wind turning a wind turbine, so we need
some basic defintions
Newton’s 3 Laws of motions (see special
topics chapter 2) describe the rules by
which objects move
Motion
• We can quantify motion with a few basic
equations
– Velocity and speed.. These are not the same
thing in a strict physics definition.
– Speed is the rate at which something moves
– Velocity is a vector, which means it is the
speed and the direction of an object
4
• In a simple sense we can apply this
direction generally by saying something
like up is positive and down is negative.
(though this is a simplification, usually
vectors involve angles for direction)
• Though this concept won’t be used in the
context of this course, it does often appear
in really motion and physics calculations
• Energy is ability to do Work
• Work is the force exerted over a distance
W = F•d
• Force comes from Newton’s Second Law
of Motion
• Force is equal to mass times acceleration
F = ma
5
• Mass is how much matter is present.
• Weight is how much force gravity is
exerting on you.
• Acceleration is how fast you are changing
velocity.
v2 − v1
t
where v 2 = final velocity
v1 = initial velocity
t = time
a=
• Velocity is rate at which your distance
changes
d 2 − d1
t
d 2 is final distance
d 1 is initial distance
v=
6
Units
• Units – we will always work in the metric
system.
• Distance is always measured in metres.
(m)
• d= 50m
• Time is always measured in seconds. t =
200 s
• Velocity is always be measured in m/s
Units
• Acceleration is always measured in m/s2
• Force is always kg* m/s2 is also called
Newtons (N)
• Mass is always measured in kilograms kg
• Work and energy is always measured in
Joules = N*m (J)
• Some quantities will sometimes have a negative
sign attached, these would include: distance,
velocity, acceleration, force, and work this is
because they are vector quantities. A vector has
both an amount and a direction as part of the
quantity.
• Forward = + (positive)
• Backward = - (negative)
• Other quantities are scalars – direction is
irrelevant, this includes energy, time and mass.
7
• When work is done – energy is either
expended or stored by an object. (it is the
total change of the energy in a system we will discuss this in the next lecture)
Kinetic Energy
• The kinetic energy of an object depends on it’s
velocity and mass
• KE (Ek) = ½ mv2
• Heat energy or Thermal Energy (TE) is a form of
kinetic energy since it is a measure of the motion
of the molecules in a substance. When we deal
with heat and temperature however because we
do averages, we tend to calculate as a separate
quantity.
Potential Energy
• The other type of energy is potential energy. (PE or Ep)
• This is energy that has been stored in some form or
another.
• One major type is gravitational potential energy
•
• E = m g h
• PE Ep
• m= mass
• g= gravitational constant 9.8 m/s2
• h= height
8
Power
• Power – this is the rate at which energy is
used, or the rate at which work is done
• P = E /t = W/t
• Units are Watts (W) = J/s
Sample Problems
•
•
•
•
•
•
•
Problem 1 page 59 text
F = ma
m = 50 kg
F= 12 N
a = F/m = 12 / 50
= 0.24 m/s2
Sample Problems
•
•
•
•
•
•
•
•
•
•
Problem 9 page 73
Here m = 80 kg
h= 6m
t = 12 s
P = E/t
E = mgh since we are dealing with someone moving to
a certain height
E = 80(9.8) 6
= 4704 J
P= 4704 /12
= 392 W
9
•
•
•
•
•
•
Problem 6 page 73
a = 4 m/s2
t=6s
a = v/t (here initial v is zero)
v=at= 4 x 6
V = 24 m/s
10