Energy
Quantities of Energy
The key quantities of energy calculations are presented in table 1.
The SI unit of energy is the joule, but many times in people's everyday life other energy
units are used. The unit kilocalorie, or kcal, is used a lot in nutrition
matters, and for example, the unit kWh, or kilowatt hour, can be found in the
electricity bill charged by the electricity company. When discussing calories,
people often mean kilocalories, even though they are talking about calories.
For example a person's daily energy requirement is something about 2000 kcal,
i.e. 2000 kilocalories, not 2000 calories.
The conversion factor between joules and calories is 4.1868 J = 1 cal. 2000 kcal is therefore
about 8400 kJ. The conversion factor between kilojoules and kilowatt hours is
3600 kJ = 1 kWh. This comes from the idea that one hour is 3600 seconds. One
joule is one watt times second.
With energy matters, the prefixes kilo, mega, giga, tera and even peta are used a lot. A
kilo is a thousand, a mega is a million, a giga is a billion, etc. Changing
these prefixes to another always means just moving the decimal separator. Table
2 shows the usage of prefixes with energy quantities.
Picture Pixabay.
The SI unit of energy is the joule, but many times in people's everyday life other energy
units are used. The unit kilocalorie, or kcal, is used a lot in nutrition
matters, and for example, the unit kWh, or kilowatt hour, can be found in the
electricity bill charged by the electricity company. When discussing calories,
people often mean kilocalories, even though they are talking about calories.
For example a person's daily energy requirement is something about 2000 kcal,
i.e. 2000 kilocalories, not 2000 calories.
The conversion factor between joules and calories is 4.1868 J = 1 cal. 2000 kcal is therefore
about 8400 kJ. The conversion factor between kilojoules and kilowatt hours is
3600 kJ = 1 kWh. This comes from the idea that one hour is 3600 seconds. One
joule is one watt times second.
With energy matters, the prefixes kilo, mega, giga, tera and even peta are used a lot. A
kilo is a thousand, a mega is a million, a giga is a billion, etc. Changing
these prefixes to another always means just moving the decimal separator. Table
2 shows the usage of prefixes with energy quantities.
Picture Pixabay.
Formulas
With matters of energy, we take up here three formulas, the first of which says that energy
is equal to power multiplied by time. From this formula, it is easy to see
where the unit kWh comes from. A kilowatt is a unit of power and an hour is a
unit of time. The next two formulas are related to mechanics. Formula (2) is
the kinetic energy formula and formula (3) is the lifting work formula. Both
formulas give a unit joule, because joule consists of units J = Nm = kg × m × m / s2. The s2 in
the denominator is a second to the power of two, which can also be written s2
= s × s. Power or potency tells how many times the thing has been multiplied
by itself. Yes, the word ”power” can here mean the quantity of power with units
of watts, or the mathematical operation.
E = Pt (1)
Ekin= ½ mv2 (2)
W = mgh (3)
In formula (2), the subscript "kin" refers to kinetic energy. This formula can
therefore be used to calculate, for example, the kinetic energy of a moving
car, if the car's mass m and velocity v are known. It can be seen from the
formula that if the speed doubles, the kinetic energy quadruples. This also
means a quadrupling of the braking distance to stop the car. In the formula (3)
there is a "W", which refers to the idea of work. Work is a change
of energy, so here we are dealing at the same thing. In the formula, h is the
change in height and g is the gravitational acceleration, which on Earth
averages 9,81 m/s2 approximately.
Picture Pixabay.
Picture Pixabay.
Example 1.
Here are examples of calculations with energy matters.
