# Electricity – Electromagnetism

Electromagnetism

Electric current produces a magnetic field.

The magnetic field surrounding the wire persists as long as the current flows.

For a steady flow of charge through a surface, the current I (in amperes) can be calculated with the following equation:

I = Q/ t

where Q is the electric charge transferred through the surface over a time t.

More generally, electric current can be represented as the rate at which charge flows through a given surface as:

I = dQ / dt

Electromagnetic field

An electromagnetic field is a physical field produced by electrically charged objects.

It is one of the four fundamental forces of nature (the others are gravitation, the weak interaction, and the strong interaction).

The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents).

Electromagnetic energy transfer is better described as being carried in the form of packets called quanta (in this case, photons) with a fixed frequency.

Planck’s relation links the energy E of a photon to its frequency v through the equation:

E=h v

where h is Planck’s constant

(h = 6.626 × 10−34 J·s)

Electric field

An electric field surrounds electrically charged particles and time-varying magnetic fields.

The concept of an electric field was introduced by Michael Faraday.

Electromagnetic radiation is a form of energy emitted and absorbed by charged particles which exhibits wave-like behavior as it travels through space.

It has both electric and magnetic field components, which stand in a fixed ratio of intensity to each other, and which oscillate in phase perpendicular to each other and perpendicular to the direction of energy and wave propagation.

In vacuum, electromagnetic radiation propagates at a characteristic speed, the speed of light.

Planck–Einstein equation

E = h f = h c / λ

where

h is Planck’s constant

λ is the wavelength

c is the speed of light

F is the frequency

The momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:

P = E / c = h f / c = h / λ

Photoelectric effect

It states that light could exist in discrete particle-like quantities, which later came to be known as photons.