function mat13(l,z,mu_r,n,choice)

%function em(l,z,mu_r,n,choice)
%Aim	: To calculate the force per unit area between an E-M and soft iron.
%Sketch	: Scanned HTML GIF. (h3p19.gif)
%Vars	: - knowns	:l,mu_r,n,I=z
%	  - unknowns	:B
%ouput	: 1 - B; 2 - force per unit area, f;  

mu=4*pi*1e-7;

g='B=(mu*mu_r*n*z)/l';
h='f=(B^2)/mu';

op=solve(g,h,'B,f');

if choice==1
mat20(eval(op.B),'T')
end

if choice==2
mat20(eval(op.f),'N/m2')
end
