Before we look into
the physical principles of photovoltaics, let’s define some basic connections between
the three very important physical quantities: energy, force and power. These connections are taken from classical
mechanics, but are generally valid. The learning objectives for this video are to understand
how force, energy and power are related. We will learn to define and relate
different types of energy. Finally, we will look at the first
and second law of thermodynamics and discuss what their implications are for solar cells. We will start with the force. The force can be defined as any influence
on an object that changes its motion. A force is exerted as a result of an interaction
between two or more objects. The objects can be in physical contact, or at a distance, like gravitational forces or magnetic forces. In this figure a very strong man
exerts a force on a granite block. The force causes the block to accelerate. According to Newton’s second law,
the acceleration ‘a’ depends on the magnitude of the exerted force ‘F’ and the mass ‘m’
of the object that undergoes the acceleration. As shown in this equation. We can gather from the figure that the force
and acceleration are vectors, or directional quantities. The units of force are kilogram times meter
per second squared, which is known as a newton, denoted by ‘N’. The amount of energy used in this interaction
is given by the product of the force and the distance ‘s’. Energy is a very useful concept to evaluate
an amount of effort to accomplish a system change, or to put it simply, the ability of a system to do work. Let’s put this definition of energy in practice. Here we have a tomato. We lift the tomato up to a height of 1 meter, for which we have to overcome
the gravitational force ’G’. Since the required amount of energy is equal
to the force times the distance, it is equal to ‘G’ times the height ‘h’. The gravitational force near the surface of
the earth, is equal to the mass of the tomato times the gravitational acceleration,
which is denoted by the lowercase ‘g’. The amount of energy required to lift the
tomato is therefore equal to 1 kilogram times meter squared per second squared. This is known as a joule, after the
English physicist James Prescott Joule. Since energy is equal to
a force applied over a certain distance, 1 joule can also be defined as the amount
of energy required to apply a force of 1 Newton over a distance of 1 meter. We just defined energy as the ability to exert
a force over a certain distance. But that is not the only definition of energy. Energy can be converted from one form to another
and all these manifestations of energy that we can observe are related through the great
physical constants that define our universe. We have related energy to the mass of an object
and the height of that object, with respect to the surface of the earth,
through the gravitational acceleration, which is derived from the gravitational constant. According to Einstein’s famous equation,
energy and mass are also interchangeable, through the speed of light in vacuo,
denoted by c-naught. The energy contained in a voltage,
which is a difference in electric potential charge, is related through the elementary charge, denoted by ‘q’. The elementary charge is the charge of a single
proton or electron, which have similar charge but of opposite sign. The elementary charge is expressed in Coulomb,
after the French physicist Charles-Augustin de Coulomb, which is equal to an ampere times a second. The amount of thermal energy present in a
temperature is defined using Boltzmann’s constant, denoted by ‘k-B’. Finally, the discrete particles of energy
contained in an electromagnetic radiation, like light, is a function of only its frequency, denoted by ‘nu’. Energy and frequency are related though
Planck’s constant, denoted by ‘h’. All these constants, and implicitly
all these definitions of energy, play an important role in the field of photovoltaics. Now, on to power. The power ‘P’ is defined as the amount
of energy used per unit time, denoted by ‘t’. Power is therefore expressed in joules per second. 1 Joule per second is also known as a Watt,
after Scottish engineer James Watt. As an example, with 70 joules of energy we could power
this 70 Watt incandescent light for exactly one second. We could also power this 7 Watt LED light
for 10 seconds, or 10 of these LED lights for 1 second. Joules are not the only unit used to express
a certain amount of energy. 1 joule is a very small amount compared
to the human energy consumption. Therefore a different unit of energy is used
for the production and consumption of electrical energy, namely the kiloWatt-hour. 1 kiloWatt-hour, as the name implies,
is the amount of energy consumed if a power of 1 kiloWatt is applied for one hour. Since 1 kiloWatt is equal to 1000 joules per
second, and there are 3600 seconds in an hour, 1 kiloWatt-hour is equal to 3.6 megajoules. The amounts of energy used in the
atomic physics relevant for solar cells, on the other hand, are very small. We therefore use the unit electronvolts. An electronvolt is the amount of energy a
body with a charge of one elementary charge gains or loses when it is moved across the
electric potential difference of 1 volt. 1 electronvolt is therefore equal to the
elementary charge times 1 volt, which equals 1.6 times 10 to the power minus 19 joules. For the large scale production and consumption of
energy, a unit is used that is much larger than the joule, namely the ton of oil equivalent, or toe. A toe is defined as the amount of energy released
by the burning of 1 ton of crude oil. 1 toe is equal to 4.2 times 10 to the power 10 joules. Finally, in the food industry the unit of calorie is used. 1 calorie is defined as the amount of energy
required to raise the temperature of 1 gram of water by 1 degree Celsius, at a pressure of 1 atmosphere. One calorie equals to 4.2 joules. We discussed how energy can be converted
from one form into another form. Practical examples are wind turbines
that convert the kinetic energy, contained in the particle flow that we know
as the wind, into mechanical energy, which is then converted into electrical energy. Chemical energy stored in fossil fuels are
converted into thermal energy, which is in turn converted into mechanical energy
and finally electrical energy. Solar cells convert the energy contained in
electromagnetic radiation directly in electrical energy. The first law of thermodynamics tells us that
throughout all of these conversions, in a closed system, the total amount
of energy is conserved. This means that, in a closed system like the
universe, the total amount of energy does not change. The second law of thermodynamics, however,
states that the entropy of a system can only ever increase or stay the same. So, what exactly is the entropy of a system? The entropy is a measure of amount of dispersion
of matter and energy in a closed system. The fact that entropy only increases, implies
that the amount of dispersion will only ever increase. This is illustrated by the figure on the right. We can see a closed system, with a large blue
area containing a liquid at a low temperature, and a small red area containing a liquid
at a high temperature. The two areas are thermally separated by a membrane. We know from experience that when we remove
the membrane, the hot liquid will diffuse into the cold liquid, producing a mix
with a temperature somewhere in between. Where first the hot molecules were contained
in a small area, they now have dispersed throughout the system and the entropy has increased. Since the hot liquid could more easily
be converted into other forms of energy, the quality of the energy contained
in the closed system has decreased. The entropy can therefore also be interpreted
as a measure of the usefulness of a type of energy. The lower the entropy of a type of energy,
the easier it is to convert that energy into another type, so the higher its usefulness. Mechanical and electrical energy are very useful,
since they can be converted into one-another with an efficiency of over 90%. Both can be converted into thermal without any loss. Thermal energy however can not be converted
into electrical energy directly, and its conversion into mechanical energy
occurs at an efficiency of under 60%. Thermal energy is the most distributed,
and least useful form of energy. Furthermore, the usefulness of thermal energy,
and its conversion efficiency into other forms of energy, decreases strongly with its temperature. For any natural energy conversion
or even every natural process, the quantity of energy will remain the same. The quality of the energy will decrease. This has some implications for photovoltaics as well. When an electric current, which is flow
of charge carriers, moves through an object its usefulness will decrease. During this flow some power
is dissipated as thermal energy. The amount of dissipated heat is equal to the current
squared times the resistance of the material. 1 Joule of thermal energy is therefore dissipated
when an electric current of 1 ampere passes through a resistance of 1 ohm for 1 second. In the last week of this course we will see
how the thermodynamic laws fundamentally limit the performance of a solar cell. In summary, we defined the force as any influence
on an object that changes its motion and the energy as ability to apply a force
over a certain distance. The power we defined as the energy
exerted per unit time. We introduced a number of alternative definitions
of which the kiloWatt hour and electronvolt are of special interest to us as solar cell engineers. We discussed how one form of energy
can be converted into another, without changing the total amount
of energy in a closed system. Then we found that even though the quantity
of the energy in a system does not change, the quality of the energy will only ever
decrease or stay the same. Thermal energy is the most dispersed,
and least useful form of energy and we discussed the thermal energy dissipation in a material. In the next video we will discuss the output
of our greatest source of energy, the sun.

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1 thought on “Energy”

  1. Edwin Rojas says:


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