# Summing over indices using symsum

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RH il 17 Mar 2023
Modificato: Torsten il 17 Mar 2023
I am trying to calculate electric field at position due to charges
, at positions .
The equation is .
If I can figure out how to for example just get this sum , I would be able to figure out the equation for the electric field. I tried
u=symunit;
syms k
e0=8.854E-12*(u.s)^4*(u.A)^2/((u.kg)*(u.m)^3);
Q1=10E-9*(u.C);
Q2=15-9*(u.C);
Q3=7E-9*(u.C);
r1=[1 4 2]*1E-3*(u.m);
r2=[-2 0 5]*1E-3*(u.m);
r3=[-1 -2 3]*1E-3*(u.m);
r=[5 5 5]*1E-3*(u.m);
Q=symsum(Qk,k,1,3)
Unrecognized function or variable 'Qk'.
That obviously didn't work. Is there a way to make it work with 'symsum'?
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Walter Roberson il 17 Mar 2023
If you have somehow created a list of numbered variables that is too long to make it practical to put them into a list by name, then what you should do is...
Not create a list of numbered variables that is too long to make it practical to put them into a list by name.
Dyuman Joshi il 17 Mar 2023
Modificato: Dyuman Joshi il 17 Mar 2023
Note that the sum isn't just over Q, but rk as well
(Without syms and symunit but values according SI units)
e0=8.854E-12;
Q=[10E-9 15E-9 7E-9];
R=[1 4 2;-2 0 5;-1 -2 3]*1E-3;
r=[5 5 5]*1E-3;
r0=vecnorm(r-R,2,2).^3;
out=sum(Q.*(r-R)./(4*pi*e0*r0),1)
out = 1×3
1.0e+06 * 4.3424 3.1998 1.5735

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### Risposta accettata

Torsten il 17 Mar 2023
u=symunit;
Q(1)=10E-9*(u.C);
Q(2)=15E-9*(u.C);
Q(3)=7E-9*(u.C);
Qsum = sum(Q)
Qsum =
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Torsten il 17 Mar 2023
Modificato: Torsten il 17 Mar 2023
u=symunit;
e0=8.854E-12*(u.s)^4*(u.A)^2/((u.kg)*(u.m)^3);
Q(1)=10E-9*(u.C);
Q(2)=15E-9*(u.C);
Q(3)=7E-9*(u.C);
rk(1,:)=[1 4 2]*1E-3*(u.m);
rk(2,:)=[-2 0 5]*1E-3*(u.m);
rk(3,:)=[-1 -2 3]*1E-3*(u.m);
r=[5 5 5]*1E-3*(u.m);
Qsum = zeros(1,3);
for k = 1:3
Qsum = Qsum + Q(k)*(r-rk(k,:))/(norm(r-rk(k,:)))^3;
end
Qsum = Qsum/(4*pi*e0);
simplify(Qsum.')
ans =
Data = separateUnits(Qsum.');
double(Data)
ans = 3×1
1.0e+06 * 4.6438 2.2614 2.1837

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### Più risposte (1)

Walter Roberson il 17 Mar 2023
You cannot use a symbolic variable as an index.
I think it is unlikely that it will ever be supported to use symbolic variables as indices.
Create the definite values and sum() the definite values.
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