Electric Fields and Potential

1. Electric Fields and Potential • Force Fields • gravitational field • electric field • magnetic field • Action at a distance (In contrast to force fields, mechanical forces require contact.)

2. Electric Field • The force between two charged particles is the interaction between one charge and the electric field set up by another. • Similar to the attraction of a satellite to the gravitational field of the planet it orbits. • Where the force is greatest, the field is strongest. The magnitude of force falls off with the square of distance from the source of charge

3. Electric Fields • Coulomb’s Law defines the amount of force on a charge in an electric field • F = k q1q2 / d2 • The strength of an electric field is defined as the force placed on a “test charge” divided by the size of the charge. • E = Fon q’ / q’

4. + Vector Representation of Electric Forces Vectors have magnitude and direction

5. Electric Field Lines • Vector representation of fields are complicated to draw. • Electric field lines are another model to represent the strength of a field. • Arrows go from positive to negative charge

6. Electric Field Representation of Electric Forces • Direction of force is in line with the source charge • positive source charge: pointing away • negative source charge: pointing toward source • Magnitude related to spacing of lines • higher magnitude: closer spacing • lower magnitude: lines farther apart • inverse square law

7. 2+ 1- More complicated electric fields http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

8. Electric Potential Energy • Potential Energy is energy due to the location (stored energy). • You have electrical potential energy when: • positive and negative charges are separated. • like charges are pushed together. • This is a total amount of energy for a group of charges.

9. Electrical Potential Energy + + + + + + d F + Work is done to move the positively charged particle towards the positively charged source. This results in electrical potential energy.

10. Electric Potential V = Ed • Electric potential energy per charge • Often called voltage • This is different from electrical potential energy. The EPE is a total for a group, EP is per unit of charge.

11. Electric potential is measured in volts. electric potential energy electric potential = -------------------------------- charge Joule 1 Volt = -------------------------------- Coulomb

12. Capacitor C = q/V • Stores electrical energy • Two plates separated by an insulating space • Positive charge on one plate • Negative charge on the other plate • Potential energy is in the form of the electric field between the plates • It takes work to separate the charges

13. Van de Graaff Generator • Large metal sphere supported by an insulating tube • Charge moved from base to sphere by rubber belt • Charge generated by electric potential • Charge transferred by metal combs

14. Van de Graaff Generator • Sphere can be raised to high potential • high voltage, about 3,000,000 volts • However charge is small so electrical potential energy is small • electrical potential is high but electrical potential energy is low • High voltage but low charge