# Magnetic field

1. Jun 18, 2010

### adiputra

I just want to clarify whether electric field is the one which can transfer its energy to some object while magnetic field cannot. If yes, how come magnetic field cannot transfer its energy?

2. Jun 18, 2010

### gabbagabbahey

Magnetic fields cannot transfer energy to objects because (classically at least) they can't do any work. In classical electrodynamics, the only sources/sinks are are static or dynamic (currents) charge distributions. The fundamental force law governing how these sources/sinks interact with magnetic fields is the Lorentz force law, which says that the force on a charge element is always perpendicular to the charge's instantaneous velocity, $\textbf{v}=\frac{d\textbf{r}}{dt}$, and hence the magnetic field never does any work.

However, a current distribution placed in a magnetic field will have some associated energy, $W=\frac{1}{2}\int \textbf{H}\cdot\textbf{B} d^3x$ (See section 5.16 of Jackson's Classical Electrodynamics 3rd ed. for a good brief discussion of energy in a magnetic field), since there are forces at play inside the distribution (usually electric) which do an amount of work which depends on the external magnetic field (See example 5.13 of Griffiths' Introduction to Electrodynamics 3rd ed.).

3. Jun 18, 2010

### Naty1

Just search "magnetic field" here and you'll turn up many discussions about your question.

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