diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index f83576c..078f17d 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -21,6 +21,7 @@
  * */
 
 #include "PenningTrap.hpp"
+#include "constants.hpp"
 #include <algorithm>
 #include <stdexcept>
 
@@ -85,7 +86,21 @@ arma::vec PenningTrap::total_force_external(int i)
 
 arma::vec PenningTrap::total_force_particles(int i)
 {
+    Particle p = this->particles.at(i);
 
+    arma::vec::fixed<3> res;
+
+    for (int j=0; j < this->particles.size(); j++) {
+        if (i == j) {
+            continue;
+        }
+
+        res += this->force_on_particle(i, j);
+    }
+
+    res *= K_E*p.q/p.m;
+
+    return res;
 }
 
 arma::vec PenningTrap::total_force(int i)