1. 21-1 Creating and Measuring Electric Fields

2. Electric Field
Vector quantity that relates the force exerted on a test charge to the size the test charge
Electric charge ,q, produces an electric field that is measureable
Field Strength: Stronger the force, stronger the field
Field Directions: Away from (+), towards (-)

3. Equation F = force measured in Newtons (N) E = F on q’
q = charge in Coulombs (C)
E = Field Strength in Newton/Coulombs (N/C).

4. Example
If a 10 C charge were placed in an electric field of strength 10 N/C, what force would it experience?
E = F/q 10 C x 10 N/C = 100 N

5. Example
An electric field is to be measured using a positive test charge of 4.0 x 10-5 C. This test charge experiences a force of 0.60 N acting at an angle of 10o. What is the magnitude and direction of the electric field at the location of the test charge?
Known: Unknown
q = +4.0 x 10-5 C E = ??? At 10o
F = 0.60 N at 10o

6. E = F / q
0.60 N / 4.0 x 10-5 C
E = 1.5 x 104 N/C at 10o

7. Electric Field Lines Strength of field is shown by spacing of lines
Closer together  strong
Far apart  weak
As previously shown, positive outward, negative inward

8. Electric Fields: 2 or more charges
When there are two or more, the field is the vector sum from individual charges
Lines become more curved
Lines will leave a positive charge and enter a negative charge

9. Electric Field Lines Also called lines of force.
Lines are vector quantity with longer vectors from stronger fields.
Lines are spaced closer together where the field is stronger.
Lines go to infinity.
With two or more opposite charges, the lines start at the (+) and go to the (-).

10. Van de Graff machine
Transfers large amounts of charge from one part of the machine to the top meal terminal
Person touches it becomes charged electrically and the charges repel *stands hair up*