Problem with fplot - doesnt plot anything but when plotting the data individually it works

Using this code, the aim is to calcuate the magnetic field produced by a solenoid within a 3D space. However, the fplot isnt working with my function, but when I substitute values into the fucntion, I get data points that make sense, and this is plotted using the for loop. Is anyone able to help fix this problem? As its an analytical solution, a function must be used to describe it. Thanks

The Code:

magnet.length = 0.25;

magnet.radius = 0.03;

magnet.position = [0, 0, 0];

magnet.I = 2.50E+01;

sphere.R = 0.25;

magnet.half_length = magnet.length / 2;

mu0 = 4*pi*1e-7;

syms x x1 % constants

f_magnet(x) = (mu0 * magnet.I * (magnet.radius^2)) / (2 * ((x-x1-magnet.position(1))^2 + magnet.radius^2)^(3/2)) * 1 / magnet.length; % Biot-Savart law

B_x_magnet(x) = int(f_magnet(x), x1, -magnet.half_length, magnet.half_length); % magnitude of B within the coil (uniform)

figure()

fplot(B_x_magnet(x), [-3*sphere.R 3*sphere.R])

hold off

figure()

for x = -3*sphere.R : 0.01 : 3*sphere.R

plot(x, B_x_magnet(x), 'o')

hold on

end

When plotting data using for loop:

When plotting using fplot:

[Dyuman Joshi]

There are many undefined parameters in the code, so we can not run your code to reproduce the result you got.

[Matt]

Hello, thanks for your reply, yes I have just realised that magnet.half_length and mu0 isnt included in the parameters.

magnet.half_length = magnet.length / 2;

mu0 = 4*pi*1e-7;

Thanks

Answer 1

[Dyuman Joshi]

You will have to define x as a real symbolic variable (or assume that the symbolic variable 'x' is a real number).

magnet.length = 0.25;
magnet.radius = 0.03;
magnet.position = [0, 0, 0];
magnet.I = 2.50E+01;
sphere.R = 0.25;
magnet.half_length = magnet.length / 2;

%% not defined, so value is assumed
mu0 = 1;

%% Define x as a real symbolic variable
syms x real
syms x1  % constants
f_magnet(x) = (mu0 * magnet.I * (magnet.radius^2)) / (2 * ((x-x1-magnet.position(1))^2 + magnet.radius^2)^(3/2)) * 1 / magnet.length  % Biot-Savart law 

f_magnet(x) = 

B_x_magnet(x) = int(f_magnet(x), x1, -magnet.half_length, magnet.half_length)  % magnitude of B within the coil (uniform) 

B_x_magnet(x) = 

figure()
fplot(B_x_magnet(x), [-3*sphere.R 3*sphere.R])

figure()
for x = -3*sphere.R : 0.01 : 3*sphere.R
    plot(x, B_x_magnet(x), 'o')
    hold on 
end 

[Dyuman Joshi]

The output of the integral without the assumption is a bit weird (for the lack of a better phrase), see below -

magnet.length = 0.25;
magnet.radius = 0.03;
magnet.position = [0, 0, 0];
magnet.I = 2.50E+01;
sphere.R = 0.25;
magnet.half_length = magnet.length / 2;

%not defined, so value is assumed
mu0 = 1;
syms x x1  % constants
f_magnet(x) = (mu0 * magnet.I * (magnet.radius^2)) / (2 * ((x-x1-magnet.position(1))^2 + magnet.radius^2)^(3/2)) * 1 / magnet.length;  % Biot-Savart law 
B_x_magnet(x) = int(f_magnet(x), x1, -magnet.half_length, magnet.half_length)  % magnitude of B within the coil (uniform) 

B_x_magnet(x) = 

[Matt]

Thanks so much for the help

[Dyuman Joshi]

You are welcome!