Electric Fields

Outcomes You will define vector fields You will compare forces and fields You will explain, quantitatively, electric fields in terms of intensity (strength) and direction, relative to the source of the field and to the effect on an electric charge

Forces Contact forces vs. forces that act at a distance Contact forces occur when objects touch each other directly in order to exert a force on each other Contact forces cannot explain a falling object or two like charges repelling one another

Faraday, first name Michael Introduced the idea of a “field” to explain the phenomena of “action at a distance” An object within a field will be affected Mmm, yes, quite.

Types of Fields 1. Scalar Fields: Magnitude, but no direction (ex. Heat from a fire) 2. Vector Fields: Magnitude and direction (ex. gravitational, electrical etc.)

Electric Fields Electric fields are vector fields that exist around any charge (+ or -) If charges are close enough, their fields will exert a force on one another ELECTRIC FORCE ≠ ELECTRIC FIELD (a field can exert a force, but it is not the same thing)

Fields and Point Charges The direction of an electric field is defined as the direction a positive test charge will move when placed in the field ***The test charge is an infinitely small, positive charge. It is a mathematical creation…they don’t really exist***

Depicting Electric Fields The strength of the electric field is represented by the density of the lines So, more lines = a stronger field

Sample Question You have a steel ball that has an unknown charge on it (this is your source charge). When you place a test charge to the right of the source charge, you see the test charge move away, to the right. Determine if the steel ball is positive or negative.

Mathematics of Electric Fields There are two equations for calculating electric fields: 1. Using a test charge and the force acting on the test charge: 2. Using the attributes of the source charge (ie. The charge producing the field) No vector hat = ENERGY! VECTOR!

Sample Problem You place a +3.7 C test charge 2.7m to the right of a -7.94C source charge. If there is an attractive force of 2.45N acting on the test charge, determine the field strength of the source charge at that location (0.66N/C left)

Sample Problem (8.4x10¯³ N [E]) You now place a -4.81x10¯² C charge at that spot in the electric field. Determine the force acting on this charge. (8.4x10¯³ N [E])

Sample Problem (3.05x10⁵ N/C towards the ball) A tiny metal ball has a charge of –3.0x10¯ C. Determine the magnitude and direction of the electric field it produces at a point, P, 30cm away. (3.05x10⁵ N/C towards the ball) 6

Multiple Source Charges You will run in to problems where several source charges are interfering with each other to make one electric field Simply calculate the individual electric fields, and then add them as vectors, taking into account directions and angles as necessary

Sample Problem – Multiple Source Charges Two negatively charged spheres are arranged as shown in the diagram below. Determine the electric field strength at a point exactly half way in between (2.9x1011 N/C [right])

Homework p. 553 #1-6, 8, 9a. & 10 p. 41 #1-13 (odd)