電荷是否一定存在於一個有質量的物質之中?

I would the charges are one of the proporties of the matter, it is not indepent proporty .for example quarks are "carrying" different charge from -2/3 to 2/3, while leptons (electron is one of them) carry -1 to 1.

For a massless charged particles (this is an assumption), it will only affected by electro-magnetic force. In you question, your want to know what happen to a charge with mass.

In our daily life, for example, there is a electron gun in the old type of TV (not LCD), the electrons would acelerated by a electric field and finally strike at the screen to give a bright spot. For the particle, the source of the force is not important to calculating its acceleration. E-field and the acceleration is different, the force is come from the E-field, to accelerate a mass particle only. On the other hand, I can accelerate the charged particle by striking lots of other particles on it, the equ. F=ma is still hold.

You should analysis your question into several parts, the EM part and mechanic part. Rest of this is only maths. You need to figure out the physics question, and the relation between them.

Summary:

The source of force --> net force on the particle --> the balance of the equ. --> maths

2007-06-15 10:10:38 補充：
the first sentence is "I would say that the ......", I miss the word "say"