Summing over indices using symsum
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I am trying to calculate electric field 
at position 
due to charges
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
, I would be able to figure out the equation for the electric field. I triedu=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)
That obviously didn't work. Is there a way to make it work with 'symsum'?
5 个评论
Torsten
2023-3-17
Why do you use symbolic variables if all variables involved have numeric values ?
RH
2023-3-17
u=symunit;
syms k
e0 = sym(8854)*sym(10)^(-15)*(u.s)^4*(u.A)^2/((u.kg)*(u.m)^3);
Q1 = sym(10)*sym(10)^(-9)*(u.C);
Q2 = sym(15)*sym(10)^(-9)*(u.C);
Q3 = sym(7)*sym(10)^(-9)*(u.C);
r1=[1 4 2]*sym(10)^(-3)*(u.m);
r2=[-2 0 5]*sym(10)^(-3)*(u.m);
r3=[-1 -2 3]*sym(10)^(-3)*(u.m);
r=[5 5 5]*sym(10)^(-3)*(u.m);
Qs = [Q1, Q2, Q3];
Q = sum(Qs)
Walter Roberson
2023-3-17
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.
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)
采纳的回答
u=symunit;
Q(1)=10E-9*(u.C);
Q(2)=15E-9*(u.C);
Q(3)=7E-9*(u.C);
Qsum = sum(Q)
1 个评论
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.')
Data = separateUnits(Qsum.');
double(Data)
更多回答（1 个）
Walter Roberson
2023-3-17
1 个投票
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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