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Let us consider a long straight wire carrying a current in
a magnetic field . Each charge q in the wire will experience
a force, and it is possible to find the total force on the wire by the
following arguments. Suppose there are n charges per unit volume in
the wire of crosssectional area A and length l , as in
Fig. 1.4.
Figure 1.4:
Current carrying wire in a magnetic field

If each charge has a charge q and is moving with a drift velocity
,
then there will be a total force on the wire given by
where we recall that
= nqA and also that
by convention is in the direction of the flow of positive
charges.
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10/9/1997