Electric Fields Physics Chapter 22 A

Electric Field Definition: Michael Faraday (1791-1867) Electric field exists around any charged object Area where a test charge would be given a force by the charged object Test charge is a positive charge of small magnitude Magnitude of field is a measure of the force exerted by the charged object Area of field indicated by field lines Show the path that would be taken by the test charge Note: electric field intensity is a VECTOR!

Electric Field Electric field is said to exist in the region of space around a charged object: the source charge. Concept of test charge: Small and positive Does not affect charge distribution Electric field intensity: Existence of an electric field is a property of its source; Presence of test charge is not necessary for the field to exist; + + + + + + + + + September 18, 2007

The electric field will be directly proportional to the charge setting up the field and inversely proportional to the square of the distance between that charge and where you are measuring the strength of the electric field. In other words, Electric field = (Coulomb’s constant) (Charge on object setting up the field) (distance away from the charge)2 E is in N/C K=9x109 Nm2 C 2 Q is in Coulombs Distance is in meters E = kq d2

Example of Field Lines Electric Field Lines

Problem p 370 A positive test charge of 4.0 x 10-5 C is placed in an electric field. The force acting on the test charge is 0.60 N at 10o. What is the electric field intensity at the location of the test charge? USE THE STEPS! What we know: Q = 4.0 x 10-5C F = 0.60 N at 10o Equation: E = F/q

Problem continued Substitute in values: Solve the math: Check answer! E = 0.60 N 4.0 x 10-5C Solve the math: E = 1.5 x 104 N/C at 10o Check answer! Reasonable answer? Units???

Problem : What is the electric field 0 Problem : What is the electric field 0.2 meters away from a +4 Coulomb charge? Equation: E = kq/d2 E = (9x109 Nm2/C2)(4C) (0.2 m)2 The direction of the electric field would be away from the +4C charge since the direction is always the direction a small positive test charge would move. E=9x1011 N/C