Can anyone suggest how can I draw a tangent from a given point in line and get its intercept value on y axis?I

I have a langmuir probe data and I wish to calculate the Ion saturation current for which I have to plot a tangent to the curve that lies below 0 in the plot from any point on the curve and get its y intercept!

clc;

clear all;

close all;

a = csvread('scope_5.csv',300,1);%Reads the data from the given file and puts it in the defined variable%

Voltage = a(:,1);%Voltage variable is defined from a matrix by selecting all rows of first column%

Vprobe = a(:,2);%Vprobe is defined by selecting rows from second column%

Iprobe = Vprobe./5500;%Probe current is obtained by dividing the Probe voltage by the probe resistance R=5500ohm%

figure(1)

plot(Voltage,Iprobe,'.k','LineWidth',2,'MarkerSize',12);

hold on

grid on

set (gca,'fontsize',16, 'fontweight', 'b')

xlabel('Probe bias Voltage(V)');

ylabel('Probe current(A)');

title('I-V charecterstic curve');

index = find(a(:,2) < 0, 1, 'first');%Finds the cell number where the current changes the sign to give the floating potential value%

Vfloat = Voltage(index);%Finds the Voltage value corresponding to the index to give the floating potential%

display(Vfloat);

hold on

aa = ginput(2);%Takes input from user to define the Ion saturation current from the plot%

zoom on

p = polyfit(aa(:,1),aa(:,2),1);%Fits the four selected point linearly in form of a line%

Isaturation = polyval(p,aa(:,1));

display(Isaturation);%Shows the fitted current saturation data%

Isat=((Isaturation(1,1)+Isaturation(2,1)/2));%Calculates the average of 2 fitted data points and gives the ion saturation current%

plot(aa(:,1),Isaturation,'m','LineWidth',2,'MarkerSize',12);%Plots the linearly fitted line from four points%

hold on

Ie=Iprobe-Isat;

figure(2)

plot(Voltage,Ie,'r','LineWidth',2,'MarkerSize',12);%Plots the Electron current Vs probe bias voltage curve%

set (gca,'fontsize',16, 'fontweight', 'b');

xlabel('Probe bias Voltage(V)');

ylabel('Electron current(A)');

title('Electron current');

grid on

figure(3)

plot(Voltage,log(Ie),'g','LineWidth',2,'MarkerSize',12);%Plots the logarithmic curve of electron current and Bias voltage%

set (gca,'fontsize',16, 'fontweight', 'b');

xlabel('Voltage');

ylabel('Log(Electron current');

grid on

zoom on

bb = ginput(2);

x=polyfit(bb(:,1),bb(:,2),1);%Fits a line for the 2 given input points%

lnIe=polyval(x,bb(:,1));

T=((bb(4)- bb(3))/(bb(2)-(bb(1))));%Finds the Electron temperature from the slope of the fitted line between the 2 selected points%

Te=1/T;

display(Isat);

display(Te);

dI=diff(Ie);

dV=diff(Voltage);

L=dI./dV;

plot(Voltage(1:9701),L,'r','LineWidth',2,'MarkerSize',12);

hold on

grid on

set (gca,'fontsize',16, 'fontweight', 'b');

xlabel('Voltage','fontsize',16, 'fontweight', 'b');

ylabel('dI/dV','fontsize',16, 'fontweight', 'b');

%Calulating the number density%

d=0.001;%Probe diameter in meter%

A=0.785*d^2;%Probe diameter in m2%

k=1.38*10^-23;%Boltzmann constant%

e=1.6*10^-19;%electron charge%

vb=sqrt((1.38*10^-23*T*11604)/(40*1.67*10^-27));%Bohm Velocity in m/s%

nd=((-Isat)/(0.6*e*A*vb));%Number density per m3%

%Calcuating Debye length%

eps =8.85*10^-12;%Permittivity of free space%

DebyeLength=sqrt((eps*k*T)/(nd*e^2));%Debye length in meters%

%Displays all the calculated parameters%

display(vb);%Bohmvelocity%

display (nd);%Displays number density per m3%

display (DebyeLength);%Displays debye length in meters%

Answer 1

[Star Strider]

We do not have your data, however this illustrates the approach:

x = linspace(-5, 5);                                % Create Data
y = 1-(x./sqrt(1+x.^2));                            % Create Data

xi = 1;                                             % Choose x-Coordinate
yi = interp1(x, y, xi);                             % Use Interplation To Calculate y-Coordinate

h = x(2)-x(1);                                      % Use With ‘gradient’
dydx = gradient(y,h);                               % Numerical Derivative
dyi = interp1(x, dydx, xi);                         % Use Interplation To Calculate Slope At ‘xi’

yi_int = (yi - dyi*xi);                             % Y-Intercept Of Line

y_tngt = dyi*x + yi_int;                            % Calculate Tangent Line (Optional)

figure
plot(x, y)
hold on
plot(xi, yi, 'sg', 'MarkerSize',10)
plot(x, y_tngt, '-r')
hold off
grid
text(0, yi_int, sprintf('Y-intercept = %.3f \\rightarrow', yi_int), 'HorizontalAlignment','right')

Answer 2

[darova]

Just use simple equation of a line

Accept the answer please

[madhan ravi]

BHARAT SINGH RAWAT comments to darova:

If it was that simple i would not have asked it! And you are asking me to accept it.

[madhan ravi]

BHARAT SINGH RAWAT: Please don't use flags to make a comment. Flags are used to indicate rudeness.

Answer 3

[BHARAT SINGH RAWAT]

Attached is the data file

Answer 4

[inturi srivani]

Can you please tell me why you used 11604 in bohm velocity