# Where does the magnetic field energy of a charged particle moving with a uniform velocity come from?

Consider a charged particle initially at rest with respect to an inertial frame.

Let a force act on it so that it gains a velocity 'v'.

It now produces a magnetic field that has some energy associated with it.

My question is where does this energy come from? If it comes from the work done by the force acting on the particle, does it mean that $W = ΔKE$ is not valid in this case?

• This is a hard question, because that energy is likely to diverge as you integrate closer to the particle, like the electrostatic self energy does, which then suggests that the two are tightly linked. – Emilio Pisanty Sep 12 '17 at 17:36
• So basically you mean that the magnetic field energy was already existing in some form just like the electric field energy? – User Sep 13 '17 at 16:44
• No. What I'm saying is that this is a hard question. – Emilio Pisanty Sep 13 '17 at 16:45
• So is there a very complicated answer or no answer? – User Sep 13 '17 at 16:46
• Reading your discussion maybe it's helpful to read physics.stackexchange.com/a/357141/46708 – HolgerFiedler Sep 14 '17 at 10:49

You're right that the work done on the charge is not equal to the change in the charge's kinetic energy in this case. An accelerated charged body (let's assume it's of finite size, to avoid the infinite energy problem of point charges) will have a change in its "near field", and it will send off electromagnetic radiation. The work done by this external agency on the charged body will be equal to the total energy imparted to all three of these things: $$W = \Delta KE + \Delta E_\text{near field} + E_\text{radiated}.$$
It is possible to account for this "deficit" in the resulting kinetic energy of the charge by defining a so-called radiation reaction force, which can in some sense be thought of as the force that the charged body exerts on itself as it accelerates. In this case, you still have $W = \Delta KE$, so long as you define $W$ to be the work done both by the external agency and by the radiation reaction force. However, this force has some weird properties (it's proportional to the time derivative of the acceleration, for one thing), which is why you usually don't hear about it in intro E&M classes.