You're not seeing anything because you're plotting 1001 lines, each of which has only one point. You can fix this by plotting them as markers, or by saving all the points to a vector and using one call to plot (vastly preferred approach). Additionally, the condition in your else statement doesn't do anything, and you have an assignment to the variable r which doesn't do anything, I've removed both below:
1001 line objects containing a marker (heavyweight solution):
a = 1;
b = 2;
pv = 5*10^(-3);
Q = pv*((4*pi*b^3)/3) - pv*((4*pi*a^3)/3);
epsilon = 8.8541878176*10^(-12);
hold on
for r = 0:0.01:10
if r<a
E = -(Q/(4*pi*epsilon*r^2));
elseif r>b
E = (Q/(4*pi*epsilon*r^2));
else %a<r<b
E = (pv*r)/(3*epsilon) - (pv*a^3)/(3*r^2*epsilon);
end
plot(E,r,'r.');
end
hold off
1 line object (lightweight solution):
clf
a = 1;
b = 2;
pv = 5*10^(-3);
Q = pv*((4*pi*b^3)/3) - pv*((4*pi*a^3)/3);
epsilon = 8.8541878176*10^(-12);
rr = 0:0.01:10;
E = nan(length(rr),1);
for i = 1:numel(rr)
r=rr(i);
if r<a
E(i) = -(Q/(4*pi*epsilon*r^2));
elseif r>b
E(i) = (Q/(4*pi*epsilon*r^2));
else %a<r<b
E(i) = (pv*r)/(3*epsilon) - (pv*a^3)/(3*r^2*epsilon);
end
end
plot(E,rr,'r');
