Introduction to physics 212 Slightly updated version (9/10/18), available via the course webpage and Blackboard Dr. Muhammad Ali Yousuf maliyousuf@towson.edu https://tinyurl.com/tuphys

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18.5 The Electric Field The electric field does not depend on the test charge. Specifically, the electric field E is defined to be the ratio of the Coulomb force to the test charge:

Electric field line simulator This slide is new https://academo.org/demos/electric-field-line-simulator/

Simulations This slide is new The Physics Classroom, https://www.physicsclassroom.com/Physics-Interactives/Static- Electricity/Electric-Field-Lines/Electric-Field-Lines-Interactive PhET Simulations, Electric Field Hockey, https://phet.colorado.edu/en/simulation/legacy/electric-hockey electrostatics and static current distributions in two dimensions, http://www.falstad.com/emstatic/

We use electric field lines to visualize and analyze electric fields (the lines are a pictorial tool, not a physical entity in themselves). The properties of electric field lines for any charge distribution can be summarized as follows: Field lines must begin on positive charges and terminate on negative charges, or at infinity in the hypothetical case of isolated charges. The number of field lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge. The strength of the field is proportional to the closeness of the field lines—more precisely, it is proportional to the number of lines per unit area perpendicular to the lines. The direction of the electric field is tangent to the field line at any point in space. Field lines can never cross. The last property means that the field is unique at any point. The field line represents the direction of the field; so if they crossed, the field would have two directions at that location (an impossibility if the field is unique).

Let’s solve some problems from the book …