Can someone please help me answer this question, I have no idea what to do.

The question I have been given is:

The relationship between voltage, v, and current, i, in an inductor, L, is given by

v = L* (di/dt)

Given that the current, i, is (e^(-0.5*(t-5)^2)) * (sin(10t) + 0.5sin(30t))

Use MATLAB’s diff function to approximate the voltage and hence plot the current and voltage of the inductor for 0<t<10 on the same axes.

You may assume that L = 1.

I have written:

% Since L approx = 1
% v = 1*(di/dt)
t = 0:10;
i = exp(-0.5*(t-5).^2).*(sin(10*t)+(0.5*sin(30*t)));
v = diff(i);

plot(v,t)
hold on
plot(i,t)
hold on

But i get the error

"Error using plot. Vectors must be the same length.

Error in assesment2question2 (line 7) plot(v,t)"

Im not sure why, but the value for v is is not a 1x11 double like t and i, and im not sure why? Sorry if I am being stupid. Any help would be much appreciated.

[Star Strider]

Your classmate seems to be having problems with this assignment as well.

Answer 1

[Stephen23]

Consider what the difference between values actually is, and try some examples:

>> diff([2,5])
ans =
    3
>> diff([2,5,9])
ans =
    3    4
>> diff([2,5,9,23])
ans =
    3    4   14

Notice how the diff output always one element shorter than the input vector.

Some would recommend to simply shorten the time vector by one element.... however this gives a misleading alignment of the two data vectors. A more representative approach is to average adjacent time values:

Ti = 0:0.001:10;
Tv = (Ti(1:end-1)+Ti(2:end))/2;
I = exp(-0.5*(Ti-5).^2).*(sin(10*Ti)+(0.5*sin(30*Ti)));
V = diff(I)./diff(Ti);
plotyy(Ti,I, Tv,V)
legend('I','V')

[Jonathan Whiting]

Thanks alot

Answer 2

[TADA]

diff Calculates The Difference Between Every Two Adjacent Points In A Vector, Therfore, It's Output Is A Vector With A Length Shorter Than It's Input By One.

To Plot It, use A Shorter t:

plot(t(2:end), v);