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is the amount of power
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
(time rate of energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
transfer) per unit volume.
In energy transformers like batteries, fuel cell
A fuel cell is a device that converts the chemical energy from a fuel into electricity through a chemical reaction with oxygen or another oxidizing agent. Hydrogen is the most common fuel, but hydrocarbons such as natural gas and alcohols like methanol are sometimes used...
s, motors, etc. but also power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
units or similar, power density refers to a volume. It is then also called volume power density
which is expressed as W/m3
Volume power density is sometimes an important consideration where space is constrained.
In reciprocated internal combustion engines, power density- power per swept volume is an important metric.
- Energy density
Energy density is a term used for the amount of energy stored in a given system or region of space per unit volume. Often only the useful or extractable energy is quantified, which is to say that chemically inaccessible energy such as rest mass energy is ignored...
energy per unit volume
- Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...
/Specific energy - energy per unit mass
- Specific absorption rate
Specific absorption rate is a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency electromagnetic field; although, it can also refer to absorption of other forms of energy by tissue, including ultrasound . It is defined as the power absorbed per mass of...