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I Few questions about Lorentz force

  1. Sep 26, 2016 #1
    I have some confusion about the Lorentz force. First of all, I found that there are two equations for the Lorentz force: one of them is F = qE + qv × B , and the other one is just F = qv × B . What's the difference between them and how do you know which one to use?

    My other question is: Is the Lorentz force always in the same direction as the Poynting Flux? since the Poynting Flux is also E cross B. If they are always in the same direction then what's the reason behind that?
  2. jcsd
  3. Sep 26, 2016 #2

    Simon Bridge

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    ... you should be able to figure that out for yourself. What do the symbols mean and what does the presence of the qE in the first and it's absense in the second imply about the situation?

    Again - you should be able to figure this out for yourself by considering what the symbols mean: compare the meanings of the symbols and be specific about which equations you are comparing. Is ##\vec E\times\vec B## always in the same direction as ##\vec v\times\vec B## ? What about when ##\vec E=0##, yet ##\vec v \neq 0##?
  4. Sep 26, 2016 #3
    From my understanding v is the velocity of the particles in the field, but doesn't this velocity exist because there is an electric field? how can you have v but no E?
  5. Sep 26, 2016 #4

    Simon Bridge

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    The charge could be a bb pellet that was fired from a gun ... maybe the charge is a particle emitted from nuclear decay.... maybe it comes from solar radiation ... maybe it's glass bead someone rubbed on their sweater and then they threw it between magnets ... maybe the accelerating E field was 10000000ly away and it was very local? Maybe someone switched the electric field off but kept the magnets?
  6. Sep 26, 2016 #5


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    You always use the first one. You'll only see the second one when we're working with just magnetic fields, meaning that ##E## is 0 - and if ##E## is zero the two are equivalent.

    But the Lorentz force is ##qv\times{B}## not ##E\times{B}##... These aren't the same thing and there's no reason to expect that they're pointing in the same direction.
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