Forces and Fields Lesson 6

Fields Michael Faraday - idea of fields Definition: a field is a region of interaction of influence Evidence of a field - existence of a force

Two Kinds of Fields Scalar fields –magnitude only eg) heat, sound, pressure, temperature 2.Vector fields –magnitude and direction eg) gravitational, magnetic, electric

To examine fields, field strength must be determined and direction must be assigned Gravitational fields Definition: a gravitational field exists about any mass m1 Earth m2 r Fg A force of gravity Fg acts onto m2 Fg is produced or generated by the earth

Fg can be calculated with the formula from Physics 20 If m2 was released, the direction of Fg is toward the center of the earth

Determining the Gravitational Field Strength Eg) If a 1.00 kg mass were placed on a spring scale, the weight, Fg, would be 9.81 N. The gravitational field strength is represented by g.

Fg can also be determined by: This means there are 2 equations to determine g. g is the acceleration due to gravity or the gravitational field strength

Definitions of the 2 Equations Smaller mass placed within the gravitational field Larger mass that generates the gravitational field

Electric Fields Electric charges (+ or –) have a region of electrostatic influence surrounding them in which an electric force exists Electric field E or |E| - a region of influence surrounding a charge q1 q2 Fe q2 experiences an electrostatic force which will either attract or repel it from the larger charge

To check electric field strength, a test charge (an infinitely small, always positive charge) is used to measure the source charge’s electric field Since test charge is always positive, if it moves away from the source charge, then the source charge must be positive If test charge moves toward it, then we know the source charge must be negative

Electric Field Strength Equation Where: E is the electric field strength (N/C) Fe is the electric force (N) q is the test charge placed within the electric field in (C)

Example: A 5.00 C charge experiences a force of 40.0 N when placed within an electric field. Determine the electric field strength.

2nd Electric Field Strength Equation Used to determine the electric field strength at some distance from the center of a source charge, q

Example: Determine the electric field strength 6.00 cm from a 5.00 x 10-2 C charge. 6.00 cm q1 5.00 x 10-2 C

Two Equations to Determine Electric Field Strength Smaller charge placed in the electric field Larger charge creating the electric field

Warning: Note: To indicate an electric field the E must have the vector arrow on it If the arrow is missing, the symbol stands for energy which is a scalar and shows incorrect usage for field strength

Example: Two positively charged spheres, A and B, with charges of 2.50uC and 3.00uC, are 4.2 cm apart. Determine the net electric field at a point P midway between them. 1. Determine distance to point P from each charge. 2. Determine the electric field created by A: 3. Determine the electric field created by B:

Example: Continued Determine the net force using vector addition:

Assignment Read p. 544 – 550 in the textbook Do handout #7 on Fields Do Workbook p. 98 #1, 3, 4, 7, 8, 11, 12, 15