1. Forces and Fields Lesson 6

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

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

4. 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

5. 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

6. 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.

7. 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

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

9. 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

10. 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

11. 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)

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

13. 2nd Electric Field Strength Equation
Used to determine the electric field strength at some distance from the center of a source charge, q

14. 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

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

16. 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

17. 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:

18. Example: Continued
Determine the net force using vector addition:

19. 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