1.2
Energy E, the central quantity of this book, is given as the product of F times the
distance s,
Energy is usually measured in the unit of joule (J), named after the English physicist
James Prescott Joule (1818–1889). It is defined as the amount of energy required to apply
the force of 1 newton through the distance of 1 m, 1 J = 1 Nm.
Another important physical quantity is power P, which tells us the rate of doing work,
or, which is equivalent, the amount of energy consumed per time unit. It is related to
energy via
where t denotes the time. P is usually measured in the unit of watt (W), after the Scottish
engineer James Watt (1736–1819). 1 W is defined as one joule per second, 1 W = 1 J/s and
1 J = 1 Ws.
As we will see later on, 1 J is a very small amount of energy compared to human
energy consumption. Therefore, in the energy markets, such as the electricity market,
often the unit kilowatt hour (kWh) is used. It is given as
On the other hand, the amounts of energy in solid state physics, the branch of physics
that we will use to explain how solar cells work, are very small. Therefore, we will use the
unit of electron volt, which is the energy a body with a charge of one elementary charge (q
= 1.602 × 10
−19 C)
1
gains or loses when it is moved across an electric potential difference
of 1 volt (V),
Human energy consumption
After these somewhat abstract definitions we will look at the human energy consumption.
The human body is at a constant temperature of about 37 °C. It therefore contains thermal
energy. As the body is continuously cooled by its surroundings, thermal energy is lost to
the outside. Further, blood is pumped through the blood vessels. As it travels through the
vessels, its kinetic energy is reduced because of internal friction and friction at the walls of
the blood vessels, i.e. the kinetic energy is converted into heat. To keep the blood moving,
the heart consumes energy. Also, if we want our body to move, this consumes energy.
Energy E, the central quantity of this book, is given as the product of F times the
distance s,
Energy is usually measured in the unit of joule (J), named after the English physicist
James Prescott Joule (1818–1889). It is defined as the amount of energy required to apply
the force of 1 newton through the distance of 1 m, 1 J = 1 Nm.
Another important physical quantity is power P, which tells us the rate of doing work,
or, which is equivalent, the amount of energy consumed per time unit. It is related to
energy via
where t denotes the time. P is usually measured in the unit of watt (W), after the Scottish
engineer James Watt (1736–1819). 1 W is defined as one joule per second, 1 W = 1 J/s and
1 J = 1 Ws.
As we will see later on, 1 J is a very small amount of energy compared to human
energy consumption. Therefore, in the energy markets, such as the electricity market,
often the unit kilowatt hour (kWh) is used. It is given as
On the other hand, the amounts of energy in solid state physics, the branch of physics
that we will use to explain how solar cells work, are very small. Therefore, we will use the
unit of electron volt, which is the energy a body with a charge of one elementary charge (q
= 1.602 × 10
−19 C)
1
gains or loses when it is moved across an electric potential difference
of 1 volt (V),
Human energy consumption
After these somewhat abstract definitions we will look at the human energy consumption.
The human body is at a constant temperature of about 37 °C. It therefore contains thermal
energy. As the body is continuously cooled by its surroundings, thermal energy is lost to
the outside. Further, blood is pumped through the blood vessels. As it travels through the
vessels, its kinetic energy is reduced because of internal friction and friction at the walls of
the blood vessels, i.e. the kinetic energy is converted into heat. To keep the blood moving,
the heart consumes energy. Also, if we want our body to move, this consumes energy.