Now open the simulation. Click on "Grid" and "Values" to select those options. Place a 1 nC positive (red color) charge on the grid. This is sometimes called a "source charge" since it's the source of the electric field we are going to measure. To make a measurement of electric field, grab an E-field sensor and place it where you want to measure the electric field. The arrow of the sensor indicates the direction of the E-field at that point and the length of the arrow is proportional to the strength of the electric field