# Force on current carrying conductor

I found this sentence in my textbook very confusing:

"...where k is constant of
proportionality. If we follow SI units, the value of k is 1."
Thus in
SI units, F=kILBsin<theta>=ILBsin<theta>

Why is the value of k taken as 1 only because we're following the SI units???

## Answers and Replies

Set k = 1 allows you to define the Tesla in SI units. If a wire is carrying a current of 1 Amp and is 1 metre long and is orientated so the direction of current flow is 90 degrees to a magnetic field of 1 Telsa then the force acting on the wire will be 1N. The sin theta bit is for when the current or moving charge is at any other angle than 90 degree to the magnetic field direction.

Doc Al
Mentor
Why is the value of k taken as 1 only because we're following the SI units???
Systems of units are set up to make the fundamental equations simpler. (SI Units are not the only system of units.)

You would agree, I trust, that if you measured the length in units of one-half meter (instead of the usual meter), you'd need to modify that force equation by choosing a different constant of proportionality? (k = 1/2)