1. Electric Field

2. Force at a Distance  Electric charges act on each other at a distance. Empty space between charges Net force on a charge  Force has to act on another charge. Specific charge for a specific value of force Value needed for force magnitude  

3. Test Charge  Introduce an arbitrary positive test charge of magnitude q.  The force can be calculated from the charges using Coulomb’s law.  The test charge can be moved to other points in space to find the force. q   q

4. Field Strength  The electric field is the electric force per unit test charge. Measured in N/C  The field has a value at every point in space. Independent of force value Parallel to force direction q   q

5. Point Charge  The field of a point charge is just like the vector force that would act at a point near the charge. Magnitude comes from Coulomb’s lawMagnitude comes from Coulomb’s law

6. Superposition  Two point charges of +10. nC are separated by 8.0 mm. Find the electric field at a point 4.0 mm perpendicular to the midpoint of the charges.  The electric fields of point charges add together. Vector addition  The x-components cancel out.  The y-components are equal. k = 8.99 x 10 9 Nm 2 /C 2 q = 10 x 10 -9 C x = y = 4.0 x 10 -3 m  Answer: E = 4.0 x 10 6 N/C

7. Screened Charge  Two oppositely charged point charges of 10. nC are separated by 2.0 mm. Find the electric field at a point 20.0 mm from the midpoint along the line of the charges on the positive side.  Only the x components matter. k = 8.99 x 10 9 Nm 2 /C 2 q = 10 x 10 -9 C r = 20.0 x 10 -3 m d = 1.0 x 10 -3 m  Answer: E = 4.5 x 10 4 N/C

8. Environmental Fields  Electric fields exist from charges in the environment.  Distance affects the field strength from point sources.  Typical field strengths in N/C Radio waves: 0.1 Living room: 3 30 cm from a stereo: 90 Fair weather sky: 150 Shower (water on): 800 Thunderstorm: 10,000 Cell membrane: 10,000,000 next