Energy has the same unit, the joule with symbol J, but energy can appear or be transferred in different ways. Energy may show up as motion, position, a thermal state related to temperature, chemical bonds, light, or the state of electric charges.
In the International System of Units (SI), every form of energy can still be expressed in joules. What changes is how we recognize or calculate the energy.
Identify Energy from Its Visible Clue
The compact diagram follows the U.S. Energy Information Administration (EIA) reading: forms of energy can be grouped as potential energy and kinetic energy. In the formula discussion, we still treat heat separately as energy transferred because of a temperature difference. EIA's Forms of Energy page can be opened through this source link.
The following forms appear often in renewable-energy discussions because electricity generation usually involves motion, height, heat, or moving charges.
Energy form
When it appears
Common mathematical model
Renewable-energy example
Kinetic energy
an object moves
Ek=21mv2
wind turns a turbine
Gravitational potential energy
an object has height relative to a reference level
Ep=mgh
stored water is higher than a turbine
Heat
energy transfers because of a temperature difference
Q=mcΔT
geothermal fluid heats a power system
Electrical energy
charge moves because of electric potential difference
E=VIt
a generator sends energy into the grid
Beyond these four examples, biomass stores chemical energy, solar panels receive radiant energy from the Sun, and batteries store energy in chemical-electrochemical states. The four formulas in the table are the ones most useful for many basic calculations in grade 10 physics.
Kinetic energy is the energy an object has because of its motion.
Ek=21mv2
Symbol guide:
Symbol
Meaning
SI unit
Ek
kinetic energy
J
m
object mass
kg
v
object speed
m/s
If the air mass passing a turbine is 2 kg and its speed is 3 m/s, its kinetic energy is:
Ek=21mv2=21(2 kg)(3 m/s)2=9 J
OpenStax College Physics 2e explains kinetic energy through work and energy on the Kinetic Energy and the Work-Energy Theorem page, which can be opened through this source link.
Gravitational potential energy is energy related to an object's position in a gravitational field. For objects near Earth's surface, the common model is:
Ep=mgh
Symbol guide:
Symbol
Meaning
SI unit
Ep
gravitational potential energy
J
m
object mass
kg
g
gravitational acceleration
m/s2
h
height relative to a reference level
m
The value of g near Earth's surface is about 9.8 m/s2. In many school problems, g is rounded to 10 m/s2 so the arithmetic is shorter.
Heat is energy transferred because of a temperature difference. So an object does not "store heat" like it stores an item in a drawer. An object has internal energy, and heat flows when there is a temperature difference.
Q=mcΔT
Symbol guide:
Symbol
Meaning
SI unit
Q
heat or transferred energy
J
m
object mass
kg
c
specific heat capacity
J/(kg K)
ΔT
temperature change
K
OpenStax College Physics 2e discusses Q=mcΔT on the Temperature Change and Heat Capacity page, which can be opened through this source link.
Electrical energy can be read from electric power acting over a time interval. The model below is used when V and I are treated as constant during the time interval.
PEE=VI=Pt=VIt
Symbol guide:
Symbol
Meaning
SI unit
E
electrical energy
J
P
electric power
W
V
electric potential difference
V
I
electric current
A
t
time
s
Some books use W for electrical energy as electrical work. Nakafa uses E so it is not confused with W as the unit watt.
OpenStax College Physics 2e discusses the relationship between electric power and energy on the Electric Power and Energy page, which can be opened through this source link.
A power plant does not use only one form of energy. Hydropower, for example, is easier to read as a chain of energy forms.
gravitational potential energy of water→kinetic energy of flowing water→kinetic energy of turbine→electrical energy
A solar panel is also not simply "making electricity". A solar panel receives radiant energy from sunlight, then changes part of that energy into electrical energy.
solar radiation→electrical energy
This is why forms of energy matter for renewable energy. We need to know the starting form, the desired output form, and the transformation path.
Kinetic energy is treated as depending only on mass
Kinetic energy also depends on the square of speed, written as v2
Heat is treated as a substance stored inside an object
Heat is energy transferred because of a temperature difference
kWh is treated as a unit of power
kWh is a unit of energy because it comes from P×t
All energy from a renewable source is treated as directly becoming electricity
There is always an energy transformation process, and not all input energy becomes useful electrical output
When reading renewable-energy technology, ask about the energy form before asking about the device. Wind carries kinetic energy, elevated water stores gravitational potential energy, geothermal heat carries thermal energy, biomass stores chemical energy, and sunlight carries radiant energy.
