Direct Energy, 2018a

10 MISCELLANEOUS ENERGY CONVERSION DEVICES 239
ees. These detectors must be sent in to a lab to be analyzed, and both the
amount and the type of radiation can be determined [153].
10.4 Biological Energy Conversion
The human body can be considered an energy conversion device. Humans
take in chemical energy in the form of food and convert it to kinetic energy,
heat, and other forms of energy. Some components of the human body are
also energy conversion devices. Muscles convert chemical energy to kinetic
energy. Photoreceptors in the retina of the eye convert optical energy of
photons to electrical energy of neurons. The ear converts sound waves to
energy stored in the pressure of the uid of the inner ear, kinetic energy of
moving hairs that line the cochlea of the inner ear, and electrical energy of
neurons. The human body also stores energy. Muscles can store energy as
they stretch and contract. Human fat cells store energy in chemical form.
When you walk, your center of mass moves up and down storing energy in
pendulum-like motion [155]. Additionally, bone, skin, and collagen exhibit
piezoelectricity [156].
Neurons are nerve cells that convert chemical energy to electrical energy.
The human brain has around 10 11 neurons [157, p. 135]. They are
composed of a cell body, an axon, dendrites, and synapses [158]. The axon
is the brous part that transmits information to other neurons. The dendrites
are the brous part that receives information from other neurons. A
synapse is a gap between neurons. Ions, such as Na + , K + , or Cl − , build
up on the membrane or in the gap between two neurons, and the charge
separation of the ions causes an electrical potential [157]. Ions sometimes
cross the gap between neurons. Neurons may be classied as sensory afferents,
interneurons, or motoneurons [157]. Sensory aerents transmit a
signal from sensory receptors to the nervous system. Interneurons transmit
the signal throughout the nervous system, and motoneurons transmit the
signal from the nervous system to muscles [157]. Electrical signals transmitted
along the nervous system involve pulses with a duration of a few
milliseconds [157]. The information is encoded in the frequency rate of the
pulses [157].
10.5 Resistive Sensors
Sensors may be made from capacitive, inductive, or resistive materials.
These sensors may involve direct energy conversion or may involve multiple
energy conversion processes. In Chapters 2 and 3 capacitive energy
conversion devices were discussed. The capacitance C of a parallel plate