Part II – Energy
Work is said to be done when matter moves as a result of the application of a force upon
(There has to be some force involved in order for work to be done; if there is no motion
no work is done)
The ability of a system to move matter through some distance.
Ie. The ability a system to perform mechanical work.
Forms of Energy:
Energy comes in several different forms. Some are closely associated with tangible
physical objects, such as atoms. Other forms, such as the energy carried by light, are less
- We will be mainly concerned with
o Kinetic energy
o Potential and internal energy
o Thermal energy
o Electromagnetic energy
Kinetic Energy: energy is associated with motion. A moving object has the ability to do physical
work. For example in a collision.
Intutively, a faster object can do more work than a slow moving object and a more massive
object can do more work than a light object.
Kinetic energy increases with speed and mass
Potential Energy: represents the ability to do work that is stored in a physical system that isn’t
in motion. There are many ways to store energy:
- Height above ground (gravity):
- Bending of a solid object (elastically):
- Difference of electric charge between objects (electricity) (Battery, electric generator)
- Binding of two or more atoms (chemical energy)
Stored energy is the result of mechanical work that was done against a force at some prior time.
Thermal Energy: Energy associated with heat. Associated with the random motion of the
molecules making up a substance is called thermal energy. It is also called heat.
Temperature is a measure of the average amount of thermal every per molecule in an object.
More thermal (kinetic) energy
Faster moving molecules
Hotter object How does a thermometer measure thermal energy?
- Suppose a fast moving molecule in a hot object collides with a slow moving molecule in
a cold one. The collision transfers thermal energy to the slow molecule, and thus to the
(in a microscopic object there are always collisions occurring)
- Thermal energy flows form hot to cold until both objects share the available thermal
energy equally. After this there is no longer any net every transfer between the objects. A
steady state is reached and both objects have the same temperature.
A thermometer is initially either hotter or colder than the object whose temperature it is to
The HOT thermometer reading stops COLD
changing when the average thermal energy per
molecule in the thermometer is the same as
that in the object being measured
In other words, the steady state thermal energy of the thermometer reflects the thermal energy
of the object under study.
The kinetic energy of a piano hammer is transferred to the struck string, which starts to vibrate
and sets the surrounding air into motion. A pressure wave is created, which our ears interpret as
Similarly, vibrating electric charges cause vibrations in the surrounding electromagnetic (EM)
field, creating a EM wave which moves away from the vibrating charge.
Vibrations of the EM field carry energy just as sound waves do. EM waves travel at the speed of
light (3c 10^3 m/s in vacuum). Any given EM wave has a specific frequency and wavelength.
Wavelength: the distance in space between successive cycles.
Frequency: the number of full cycles of the wave passing a fixed point in space each second.
Frequency and wavelength are related in such a way that
Higher frequency Shorter wavelength Greater Energy
The energy carried by an EM wave depends on the frequency (wavelength) of the oscillations,
with higher frequencies (shorter wavelengths) corresponding to greater energy.
The electromagnetic Spectrum
The motion/vibration of a charged particle creates an E.M wave (ie. The charge does work on
the ambient EM field)
An EM wave can do work on a charged particle, setting it into motion.
Heat and Electromagnetic Radiation:
Heat and EM radiation are intimately connected in a way that is crucial to understand climate
and climate charge.
Consider a rock exposed to sunlight. It absorbs EM energy continuously.
- The rocks molecules speed up
- The rocks thermal energy (temperature) increases.