Energy is the ability to do work or cause change, and work is the transfer of energy from one form to another. There are many forms of energy such as heat, light, kinetic, mechanical, electrical, chemical, and nuclear energy, or other forms; however they all fall into two categories – potential or kinetic energy.
Potential energy is stored energy and the energy of position (gravitational). It exists in various forms such as chemical, nuclear and stored mechanical energy as well as gravitational energy.
Chemical energy is the energy stored in the bonds of atoms and molecules. Biomass, petroleum, natural gas, propane and coal are examples of stored chemical energy.
Nuclear energy is the energy stored in the nucleus of an atom - the energy that holds the nucleus together. The nucleus of a uranium atom is an example of nuclear energy.
Stored mechanical energy is energy stored in objects by the application of a force. Compressed springs and stretched rubber bands are examples of stored mechanical energy.
Gravitational energy is the energy of place or position. Water in a reservoir behind a hydropower dam is an example of gravitational energy. When the water is released to spin the turbines, it becomes motion energy.
Kinetic energy is energy in motion- the motion of waves, electrons, atoms, molecules and substances. Radiant, thermal and electrical energy, motion and sound are the most known kinetic energy forms.
Radiant energy is electromagnetic energy that travels in transverse waves. Radiant energy includes visible light, x-rays, gamma rays and radio waves. Solar energy is an example of radiant energy.
Thermal energy (or heat) is the internal energy in substances- the vibration and movement of atoms and molecules within substances. Geothermal energy is an example of thermal energy.
Electrical energy is the movement of electrons. Lightning and electricity are examples of electrical energy.
The movement of objects or substances from one place to another is motion. Wind and hydropower are examples of motion.
Sound is the movement of energy through substances in longitudinal (compression/rarefaction) waves.
Electrical and chemical energy are high-grade energy, because the energy is concentrated in a small space. Even a small amount of electrical and chemical energy can do a great amount of work. The molecules or particles that store these forms of energy are highly ordered and compact and thus considered as high grade energy. High-grade energy like electricity is better used for high grade applications like melting of metals rather than simply heating of water.
Heat is low-grade energy. Heat can still be used to do work (example of a heater boiling water), but it rapidly dissipates. The molecules, in which this kind of energy is stored (air and water molecules), are more randomly distributed than the molecules of carbon in a coal. This disordered state of the molecules and the dissipated energy are classified as low-grade energy.
According to the law of conservation of energy, the total energy of a system remains constant, though energy may transform into another form. Two billiard balls colliding, for example, may come to rest, with the resulting energy becoming sound and perhaps a bit of heat at the point of collision. The SI unit of energy is the joule (J) or Newton-meter (Nm).
Electricity is the flow of electrical charge or power and a basic part of nature. It is one of most widely used forms of energy.
Electricity is actually a secondary energy source, also referred to as an energy carrier. That means that we get electricity from the conversion of primary energy sources, such as thermal, nuclear, or solar energy. The energy sources we use to make electricity can be renewable or non-renewable, but electricity itself is neither renewable nor nonrenewable.
Electricity is measured in terms of current (amperes, amps, A) and force (voltage, volts, V). The energy used is electrical power (watts, W) equal to the product of current and force.
Electric current is the rate of flow of charge. The ampere (A) is the basic unit of electric current. It is that current which produces a specified force between two parallel wires, which are 1 meter apart in a vacuum. The flow of electrical charge can be continuous in one direction (direct current electricity, DC) or it can be reversing on a fixed frequency (alternating current electricity, AC). The number of times per second that AC current reverses polarity from positive to negative is the frequency given in hertz (Hz).
The voltage between two points is the name for the electrical force that would drive an electric current between those points. Specifically, voltage is equal to energy per unit charge.
A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance. The unit of resistance is ohm (Ω).
Ohm's law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature and other external conditions remain constant.
Thermal Energy, or heat, is the internal energy in substances; it is the vibration and movement of the atoms and molecules within substances. The more thermal energy in a substance, the faster the atoms and molecules vibrate and move.
Thermal energy is originated by increased movement of molecules in a substance, which in turn causes temperature to rise accordingly. There are many natural sources of a thermal energy on Earth, making it an important component of alternative energy like geothermal energy and solar energy.
Temperature and pressure are measures of the physical state of a substance. They are closely related to the energy contained in the substance, whereas pressure specifies the stored energy. As a result, measurements of temperature and pressure provide a means of determining energy content.
Pressure is the force per unit area applied to surface of a body. When we heat a gas in a confined space, we create more force so that it follows a increasing of the pressure. Therefore steam at high pressures contains much more energy than at low pressures.
The laws of thermodynamics explain the energy in the form of heat can be exchanged from one physical object to another. Temperature characterizes the amount of thermal energy available, whereas heat flow represents the transfer of thermal energy from place to place as a result of the temperature difference.
Heat is a form of energy, a distinct and measurable property of all matter. The quantity of heat depends on the quantity and type of substance involved. Heat will always be transferred from higher temperature to lower temperature independent of the mode. The energy transferred is measured in Joules (or kcal). The rate of energy transfer, more commonly called heat transfer, is measured in Joules/second (or kcal/hr).
Heat is transferred by the primary modes conduction, convection and radiation.
The conduction of heat takes place, when two bodies are in contact with one another. If one body is at a higher temperature than the other, the motion of the molecules in the hotter body will vibrate the molecules at the point of contact in the cooler body and consequently result in increase in temperature. The amount of heat transferred by conduction depends upon the temperature difference, the properties of the material involved, the thickness of the material, the surface contact area, and the duration of the transfer. Good conductors of heat are typically substances that are dense as they have molecules close together. This allows the molecular agitation process to permeate the substance easily. So, metals are good conductors of heat, while gaseous substance, having low densities or widely spaced molecules, are poor conductors of heat.
The transfer of heat by convection involves the movement of a fluid such as a gas or liquid from the hot to the cold section. There are two types of convection, natural and forced.
In case of natural convection, the fluid in contact with or adjacent to a high temperature body is heated by conduction. As it is heated, it expands, becomes less dense and consequently rises. This begins a fluid motion process in which a circulating current of fluid moves past the heated body, continuously transferring heat away from it.
In the case of forced convection, the movement of the fluid is forced by a fan, pump or other external means. A centralized hot air heating system is a good example of forced convection.
Convection depends on the thermal properties of the fluid as well as surface conditions at the body and other factors that affect the ability of the fluid to flow. With a low conductivity fluid such as air, a rough surface can trap air against the surface reducing the conductive heat transfer and consequently reducing the convective currents.
Thermal radiation is a process in which energy is transferred by electromagnetic waves similar to light waves. These waves may be both visible (light) and invisible. A very common example of thermal radiation is a heating element on a heater. When the heater element is first switched on, the radiation is invisible, but you can feel the warmth it radiates. As the element heats, it will glow orange and some of the radiation is now visible. The hotter the element, the brighter it glows and the more radiant energy it emits.
The key processes in the interaction of a substance with thermal radiation are:
Absorption: the process by which radiation enters a body and becomes heat
Transmission: the process by which radiation passes through a body
Reflection: the process by which radiation is neither absorbed nor transmitted through the body; rather it bounces off
Objects receive thermal radiation when they are struck by electromagnetic waves, thereby agitating the molecules and atoms. More agitation means more energy and a higher temperature. Energy is transferred to one body from another without contact or transporting medium such as air or water. In fact, thermal radiation heat transfer is the only form of heat transfer possible in a vacuum.
All bodies emit a certain amount of radiation. The amount depends upon the body's temperature and nature of its surface. Some bodies only emit a small amount of radiant energy for their temperature, commonly called low emissivity materials (abbreviated low-E). Low-E windows are used to control the heat radiation in and out of buildings. Windows can be designed to reflect, absorb and transmit different parts of the sun's radiant energy.
The condition of a body's surface will determine the amount of thermal radiation that is absorbed, reflected or re-emitted. Surfaces that are black and rough, such as black iron, will absorb and re-emit almost all the energy that strikes them. Polished and smooth surfaces will not absorb, but reflect, a large part of the incoming radiant energy.
The laws of thermodynamics describe the transport of heat and work in thermodynamic processes.
The first law of thermodynamics, an expression of the principle of conservation of energy, states that energy can be transformed (changed from one form to another), but cannot be created or destroyed.
The second law of thermodynamics states that the entropy of an isolated macroscopic system never decreases, or (equivalently) that perpetual motion machines are impossible, i.e. some amount of energy will be dissipated as heat in any conversion of energy from one form to another and no energy conversion is 100 % efficient