Force Between Two Parallel Wires: Ampere Definition



"A fact is a simple statement that everyone believes.  It is innocent, unless found guilty.  A hypothesis is a novel suggestion that no one wants to believe.  It is guilty, until found effective"
Edward Teller
magforce2wireseqn1
with direction given by "the" right-hand-rule.
magforce2wireseqn2
so that
magforce2wireseqn3

with a direction such that i2 is attracted towards i1 as shown at right.
Then the force per unit length on i2 is given by
magforce2wireseqn4
  • exclamation Note that if we reverse the roles of i1 and i2 we obtain the same expression for F1/l1 .  That is, i1 exerts a force on  i2 and  i2  exerts an equal, but opposite, force on i1  as expected from Newton's Third Law ("Action equals Reaction").
  • The above analysis applies for parallel currents - the two wires are attracted to each other.  If one of the currents is reversed, so that the currents are "anti-parallel", the force per unit length is unchanged, but the force acting on each wire is reversed.  In other words the wires are now repelled from each other.
exclamation "Like currents are attractive, unlike currents are repelled".  Note that this is opposite to the behavior of point electric charges.

  • Definition of the Ampere

The unit of electric current, the Ampere, is defined using the force between parallel wires carrying current.

tick"The Ampere is that current which when flowing in two infinite parallel wires one metre apart produces a force between them of 2 x 10-7 N/m"

Proof:

magforce2wireseqn5

What do you call a dinosaur with an extensive vocabulary ?  A thesaurus.

 
 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu