You are probably familiar with these types of energy from a physics
class. Energy is the capacity (ability, sort
of) to do work. You have a sense of what work is from regular life,
it's things that require effort. Energy and work have the same
units. Kinetic energy is the energy that comes
from motion. The equation for kinetic energy is
KE =
1
mv2
2
where KE is kinetic energy, m is mass, and v is velocity. This
definition should make sense: big things moving fast have the most
energy, the most ability to shove other things or knock them over,
etc. Potential energy is energy that comes
from position and a force. For instance, gravitational potential
energy is the energy that things have if they are high up. If they
fall, their potential energy will turn into kinetic energy because
they are accelerated by gravity. The equation for potential energy
from gravity is
PE = mgh
where PE is potential energy, m is mass, g is the acceleration of
gravity, and h is the height. This makes the units of energy very
clear: mass x distance x acceleration, or force x distance, which
comes to kgm2s-2. In chemistry, the force that
leads to potential energy is almost always the Coulomb force, not
gravity. In this case, the potential energy from 2 charges near each
other is
PE =
kQq
d
where q and Q are the 2 charges, d is the distance between them, and
k is a constant, 8.99 x 109 JmC-2. (Joules, J,
are the SI unit of energy, and coulombs, C, are the SI unit of
charge.) When the charges have the same sign, they repel and will
accelerate away from each other if allowed to move; the potential
energy has a positive sign. When the charges
have the opposite sign, they attract each other and have negative
potential energy. If they are allowed to get closer together, the
potential energy will get more negative. If they are separated, d
gets bigger and the potential energy approaches zero.
A pendulum. The red ball
is raised to position 1 and released; it swings through positions 2
3 and to position 4 on the other side.
Conservation of Energy
You probably learned about conservation of
energy already in a physics class. For instance, if you
have a pendulum as shown, at position 1 the weight has some
potential energy, but no kinetic energy. When you release the
weight, the weight falls, moving through position 2. At position 2,
some of the potential energy has been converted to kinetic
energy. Finally, at position 3, all the potential energy has been
converted to kinetic energy. As it passes 3, the process is
reversed, and kinetic energy is converted to potential energy. When
the weight reaches position 4, all the kinetic energy has been
converted back to the same amount of potential energy it started
with at 1. This is just one example of conservation of energy. It is
a general observation that the amount of energy in the universe
doesn't change, and the amount of energy in a particular system
doesn't change unless there is a flow of energy in or out.
