# Light, Current, & Magnetism Relationship?

1. Feb 18, 2008

### hobbesglobin

Light is the result of an electron moving to a lower energy state, giving off a photon a particular wavelength in the electromagnetic spectrum. Current is the movement of electrons which induces a magnetic field. A magnetic field can create current by moving charges.

Is there a relation between the energy contained in light, and the energy of magnetic or electric field (as suggested by the name "electromagnetic spectrum")?

More questions:
Does a magnetic field exist anywhere there is a potential difference between positive and negative charges? If so, how is this different from an electrical field other than magnetic poles cannot be separated (unlike charged particles)?

Why does the magnetic field around a conductor form concentric circles when in the inverse situation (a charged particle entering a magnetic field) current spirals around the magnetic field needing an initial velocity in order to interact with the field?

Correct me if I'm wrong, I'm new to this, but very curious.

2. Feb 18, 2008

### pam

"Is there a relation between the energy contained in light, and the energy of magnetic or electric field (as suggested by the name "electromagnetic spectrum")?"

The intensity of light equals $$\frac{c}{8\pi}\sqrt{\frac{\epsilon}{\mu}}|E_0|^2$$
(in Gaussian units), where E_0 is the amplitude of the oscillating electric field.

Last edited: Feb 18, 2008
3. Feb 19, 2008

### per.sundqvist

"Why does the magnetic field around a conductor form concentric circles when in the inverse situation (a charged particle entering a magnetic field) current spirals around the magnetic field needing an initial velocity in order to interact with the field?"

The first comes from Biot-Savarts law, which is a version of one of Maxwells equations:
rot(B)=j/mu. The latter case from the Lorentz Force: F=q*vxB. Losely speaking Maxwells equations tells how EM-fields are created from charges, at rest -or in motion. They does not tell where the charges comes from (unfortunately). Lorentz force is connected to conservation of energy: Force*velocity=change in energy /time unit (This is obtained from the contraction of the covariant em-field tensor and the four-velocity), which results in the equation for the Lorentz force.

Note also the classical relativity effect: E=vXB/c, when we are in the electrons rest-frame and the source of magnetic field is in motion with velocity v (towards the electron) -A electric field E is created when the magnetic field B moves!