Direction of magnetic field in lightning strike

Posted on 2006-03-19
Last Modified: 2006-11-18
In one of the previous Physics exam papers that I've got, there's a question which has a diagram of a negatively charged cloud, above a positively charged ground. There's a line from the cloud down to the ground, which signifies a lighting strike.

There is a point P slightly to the right of the lightning strike. I'm asked to state the direction that the magnetic field due only to the lightning strike is..

I'm guessing that I'd simply use flemings left-hand rule...

I know the direction of the current (cloud to ground); but I also need to know the direction of the force.

But, it doesn't seem to give this information...

The correct answer for the direction of the magnetic field at point P is 'in to the page'. But where have they deduced the direction of the force/motion ?

Question by:InteractiveMind
    LVL 22

    Assisted Solution

    You only need to know the direction of the current to deduce the magnetic field.

    LVL 25

    Author Comment

    How?  :|
    LVL 25

    Author Comment

    Oh yeah !!!
    LVL 25

    Author Comment

    I partially remember a corkscrew being turned such that it moves in the direction of the current... and the clockwise pattern of the rotation of the corkscrew represents the magnetic field...

    But.. if that's the case.. then shouldn't the field be coming OUT of the page at a point to the right of the lightning strike (on the page) ? Rather than going IN.. ?
    LVL 26

    Accepted Solution

    But where have they deduced the direction of the force/motion ?


    Force and motion are two different things. One cannot answer that question until it is asked.
    The direction of motion of what , where? The force on what?

    I personally do not like left hand rules although they are often (correctly) used. But you sometimes forget wheather one is to use the right hand rule or the left hand rule. I answer that question by saying

    electric current (by convention considered to flow in external circuits from positive to negative, in this case is flowing up to the cloud. The right hand rule says the thumb points in the direction of current flow (as defined above) and the fingirs naturally curl in the direction of the magnetic field. (Hence into the paper in this case.)

    In your case electrons do go from the cloud to the ground hence a left hand rule can be used. But the air is ionized so there will be a flow of positive ions from the ground to the cloud. The usual rignt hand rule will give the magnetic field as above. (Note that the field produced by the electrons goes in the same direction.)
    LVL 25

    Author Comment

    Yup, doh. I was 'turning the corkscrew' in the direction of electron flow, now conventional.  :(

    Thanks very much !

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