Vectors must be the same length

Hello,

Although my code is at a beginner's level (see below), I can't get rid of the error you see in the attached photo.

I haven't really understood the problem, so I can't find a way to correct the length of these vectors.

Maybe there's an easier way to write my code.

Thanks for helping me out ;)

%   --------------
%   | EXERCISE 3 | - RLC Circuit
%   --------------
%   DE useful to describe the behaviour of the 
%   RLC Circuit's components over time
%********************************************************
%   Non-homogeneous equation
%             | ay''(t) + by'(t)+cy(t) = u(t) |
% Kirchhoff's voltage law in RLC Circuits
%           | Lq''(t) + Rq'(t) + 1/Cq(t) = u(t) |
%   with L Inductance (Henry H)
%        R resistor's resistance (ohms)
%        C capacitor's capacitance (Farads F)
%        q(t) capacitor's charge at a time t (Coulombs C)
%        i(t) current through the circuit at time t (Amperes A)
%        u(t) applied voltage (Volts V)
% Remove all variables and functions from the memory
clear;
close all;
disp('start');

start

% Given parameters
R = 1;
L = 0.2;
C = 0.5;
U = 1;
T = 3;  % Time span
% Initial conditions
q0 = 0;       % Initial capacitor's charge
q_dot_0 = 0;  % Initial charge's derivative, = current
% Time span for plotting
tspan = linspace(0, T, 100);
% Solve the DE with ODE45
[t, q] = ode45(@(t, q) (U - (R * q) / L - q / (L * C)), tspan, [q0, q_dot_0]);
% Calculate current and voltages
i = diff(q(:, 1)) ./ diff(t);  % Current derivative
U_R = R * i;                   % Resistor's voltage
U_L = L * diff(i);             % Coil's voltage
U_C = q(:, 1) / C;         % Capacitor's voltage
% Plot Graph
figure;
plot(t, U_R, 'LineWidth', 1.5);

Error using plot
Vectors must be the same length.

hold on;
plot(t, U_L, 'LineWidth', 1.5);
plot(t, U_C, 'LineWidth', 1.5);  % Remove the initial value to match lengths
hold off;
titleText = sprintf('Evolution of the voltage in the components of a RLC-circuit (R = %.2f, L = %.2f, C = %.2f, U = %.2f)', R, L, C, U);
title(titleText);
xlabel('Time t');
ylabel('Voltage (V)');
legend({'U_R', 'U_L', 'U_C'}, 'FontSize', 12);
grid on;
disp('done');

Answer 1

[Matt J]

i = gradient(q(:, 1),t);  % Current derivative
U_R = R * i;                   % Resistor's voltage
U_L = L * gradient(i,t);             % Coil's voltage

disp('done');

done