Energy flux is the rate of energy transfer per unit area, often used to compare the power carried by particle beams.Energy flux measures how much energy passes through a unit area per unit time. In physics, it is a way to quantify the intensity of energy transfer—whether carried by light, sound, particles, or electromagnetic fields. The concept is essential for comparing the power of different sources, such as particle beams, radiation from celestial objects, or ultrasound waves, because it normalizes energy flow to a standard area.
The concept has roots in the 19th-century development of the Poynting vector for electromagnetic energy flow, but the general term 'flux' was formalized in continuum mechanics and field theory in the early 20th century.
Energy flux is calculated as the energy passing through a surface divided by the area of that surface and the time interval. For a beam of particles, it depends on the number of particles per second, their individual energy, and the cross-sectional area of the beam. For waves, it relates to the amplitude and frequency. In practice, measuring flux involves placing a detector of known area and counting the energy it absorbs over time.
It is used to compare the power of particle accelerators, to calibrate radiation therapy doses (as in the article about electron beams against brain tumors), and to analyze energy transport in plasma physics and astrophysics.
The energy flux of sunlight at Earth's orbit is about 1,361 watts per square meter, a value known as the solar constant.
| \dot{M} | variable | Mass loss rate of the planet — mass loss rate kilogram per second |
| R_p | variable | Planet radius — radius metre |
| F_{XUV} | variable | Incident XUV flux at the planet — energy flux watt per metre squared |
| M_p | variable | Planet mass — mass kilogram |
| K | variable | Efficiency factor (dimensionless, often ~0.1-1) |
| G | constant | Newtonian gravitational constant — gravitational constant = 6.67430e-11 metre cubed per kilogram per second squared |
