generating 3D vector in MATLAB

I want to write a script to generate a 3D vector field of the electric flux density of 8 different point charges in a [-2,2]x[-2,2]x[-2,2] box in 3D space.

I have a function definition in a separate .m file as follows:

    function[Dx,Dy,Dz]= question3function(Q,Loc,XX,YY,ZZ)

    Q=1e-6;
    Loc=[];

    XX=(2,-2);
    YY=[2,-2];
    ZZ=[2,-2];

    % Position vector from the point charge
    Rx=(XX)-Loc([]);
    Ry=(YY)-Loc([]);
    Rz=(ZZ)-Loc([]);

    % Distance between position in interest and the point charge
    R=sqrt(Rx.*Rx+Ry.*Ry+Rz.*Rz);

    % Unit Position vector
    Ax=Rx./R;
    Ay=Ry./R;
    Az=Rz./R;

    % Electric flux density XYZ components
    K=Q./(4*pi*R.^2);
    Dx=K.*Ax;
    Dy=K.*Ay;
    Dz=K.*Az;

And then in my main script I have the function calls:

    %function calls
    [Dx1,Dy1,Dz1]=question3function(Q,[1 1 1],XX,YY,ZZ);
    [Dx2,Dy2,Dz2]=question3function(Q,[1 1 -1],XX,YY,ZZ);
    [Dx3,Dy3,Dz3]=question3function(Q,[1 -1 1],XX,YY,ZZ);
    [Dx4,Dy4,Dz4]=question3function(-Q,[1 -1 -1],XX,YY,ZZ);
    [Dx5,Dy5,Dz5]=question3function(2*Q,[-1 1 1],XX,YY,ZZ);
    [Dx6,Dy6,Dz6]=question3function(-2*Q,[-1 1 -1],XX,YY,ZZ);
    [Dx7,Dy7,Dz7]=question3function(-Q,[-1 -1 1],XX,YY,ZZ);
    [Dx8,Dy8,Dz8]=question3function(-Q,[-1 -1 1],XX,YY,ZZ);

    Dx=Dx1+Dx2+Dx3+Dx4+Dx5+Dx6+Dx7+Dx8;
    Dy=Dy1+Dy2+Dy3+Dy4+Dy5+Dy6+Dy7+Dy8;
    Dz=Dz1+Dz2+Dz3+Dz4+Dz5+Dz6+Dz7+Dz8;

    quiver3(XX,YY,ZZ,Dx,Dy,Dz);
    axis square equal;
    xlabel('X'); ylabel('Y'); zlabel('Z');
    title('Electric Flux Density of the sum of 8 Point Charges');

I receive the following errors when I try to run my function file:

    ??? Error using ==> minus
    Matrix dimensions must agree.

    Error in ==> question3function at 11
        Rx=(XX)-Loc([]);

Could somebody please help me and explain how I can fix this? I will add I am not very experienced with using MATLAB.

Answer 1

[Stephen23]

The error message tells us the error occurs on this line:

Rx=(XX)-Loc([]);

Those variables are hardcoded as:

Loc=[];
XX=(2,-2);

Presumably you have not copied your code correctly, because that XX definition is not valid MATLAB code, and will immediately cause an error:

   >> XX=(2,-2);
  ??? XX=(2,-2);
           |
  Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

Once we fix that by replacing the parentheses by square brackets we can try that code out. The code Loc([]) takes no elements of an empty vector, giving an empty vector. So your code is equivalent to this:

>> [2,2] - []
??? Error using ==> minus
Matrix dimensions must agree.

What output do you expect to get when you subtract an empty vector from a 1x2 vector?

When doing matrix arithmetic either:

• one array can be scalar, or
• both arrays must be the same size.

This is clearly stated in the minus/subtraction documentation. You should should read it.