Ch. 33 ELECTRIC FIELDS

1. Ch. 33 ELECTRIC FIELDS AND POTENTIAL

2. Fields and forces • A field is used to describe any quantity that has a value for all points in space. • You can think of the field as the way forces are transmitted between objects. • Charge creates an electric fieldthat creates forces on other charges.

3. Fields and forces • Mass creates a gravitational fieldthat exerts forces on other masses.

5. Fields and forces • Gravitational forces are far weaker than electric forces.

6. Electric Field • The electric force is establishing a “field” telling particles which way to move and how fast • Electric “field lines” tell a positive • charge which way to move. • a positive charge itself has field lines • pointing away from it, • this is how a positively-charged “test-particle” would respond if placed in the vicinity • (repulsive force). + Run Away! +

7. LINES OF FORCE For two or more opposite charges, the lines emanate from a positive charge and terminate on a negative Field lines can be used to represent the amount of force http://kingfish.coastal.edu/physics/physlets/EField/EField_lines.html

8. Drawing the electric field

10. ELECTRIC FIELD PROPERTIES • An electric field is avector quantity: • The magnitude is measured by its effect on charges located outside of the field

11. Electric Field Applet http://falstad.com/vector3de/

12. ----> ELECTRIC SHIELDING • When charge is placed in the interior of a conducting sphere, the charge will migrate to the outside of the sphere.  • There will be no electric fields in the interior.

13. ELECTRIC SHIELDING • A closed metallic surface will shield out external electric fields.  • Such a surface works by the electrons adjusting their position so as to shield the external charge.

14. ELECTRIC SHIELDING Would the occupant inside the hummer (below) be safe in a thunderstorm? Hint: The electrons that drench the top of the car are mutually repelled and spread over the surface • Shielding only occurs with charged conducting media, or those whose electric field cancels to zero

15. Analogy between gravitational and electrical potential energy: …

16. Gravitational fields analogy • On the Earth’s surface, the gravitational field creates 9.8 N of force on each kilogram of mass. • With gravity, the strength of the field is in Newtonsper kilogram (N/kg) because the field describes the amount of force per kilogram of mass.

17. ELECTRIC POTENTIAL ENERGY • Objects can have PE, charged objects have • Potential Electric Energy— • Energy a charge possesses by virtue of its location

18. ELECTRIC POTENTIAL ENERGY • Remember Work = ∆ PE • Work is required to push a charged particle against the electric field of a charged body • Electric potential energy is increased when work is done to push it against an electric field

19. Electric Potential example … In order to bring two like charges near each other work must be done In order to separate two opposite charges, work must be done.  Remember that whenever work gets done, energy changes form.

20. Electric Potential example … As the monkey does work on the positive charge, he increases the energy of that charge.  The closer he brings it, the more electrical potential energy it has. When he releases the charge, work gets done on the charge which changes its energy from electrical potential energy to kinetic energy. 

21. Electric fields and electric force • With the electric field, the strength is in Newtonsper coulomb (N/C). • The electric field describes the amount of force per coulomb of charge. • E = Electric Field Intensity or strength • F = the force exerted on the charge • Q = the quantity of charge on the test charge

22. Using Coulomb’s Law to find (E) field intensity Expresses the field strength in terms of the two variables that affect it. The electric field strength is dependent upon the quantity of charge on the source charge (Q) and the distance of separation (d) from the source charge.

23. Electric Field Intensity vs. Gravitational Field Practice Problems

24. ELECTRIC POTENTIAL ENERGY Look!:Potential V is the analog of height/level/altitude/elevation h.

25. ELECTRIC POTENTIAL • A group of nine charges will have nine times the potential energy as one • When working with electricity, electric potential energy per charge is considered

26. Electrostatic Potential Energy and Potential Difference Electric potential is defined as potential energy per unit charge: Electric potential energy per charge is called Electric Potential= electric PE / charge Unit of electric potential: the volt (V). 1 V = I J/C. • 1e = 1.6 x 10-19C

27. The electrical potential (not energy) is the amount of energy per unit of  charge. Electric Potential Formula = 1V … • 1e = 1.6 x 10-19C

28. Relation between Electric Potential and Electric Field Work is charge multiplied by potential: Work is also force multiplied by distance:

29. The Electron Volt, a Unit of Energy One electron volt (eV) is the energy gained by an electron moving through a potential difference of one volt.

30. Volts and Batteries For batteries we refer to the potential difference of the charges within the battery. a "D-cell" has a rating of 1.5 volts which means that for every Coulomb of charge that moves from the negative side of the cell to the positive side will do 1.5 Joules worth of work. …

31. Volts and Batteries The difference between the D-cell and the AA-cell is that the D-cell has more Coulombs worth of charge, so it will last longer. Note: Related to energy and potential difference, this animation is correct.  Related to circuits it has afew flawsthat were necessary to keeping the concept of voltage clear. …

32. VOLTS / VOLTAGE • 1 volt = 1(joule/coulomb) • Since electric potential is measured in volts, it is commonly called voltage • One coulomb is a very large amount of charge • 1e = 1.6 x 10-19C • High voltage requires great energy only if a great amount of charge is involved Rub a balloon on your head, and it becomes negatively charged, perhaps to several thousand volts!

33. ELECTRIC ENERGY STORAGE