diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index 83b7be7..f1c62df 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -36,7 +36,14 @@ void PenningTrap::add_particle(Particle particle)
 
 arma::vec PenningTrap::external_E_field(arma::vec r)
 {
+    arma::vec::fixed<3> res;
+    double x = r(0), y = r(1), z = r(2);
+    double f = this->V_0/2*this->d*this->d;
+    res(0) = f*2*x;
+    res(1) = f*2*y;
+    res(2) = -f*4*z;
 
+    return res;
 }
 
 arma::vec PenningTrap::external_B_field(arma::vec r)