1. Electrostatics
Chapter 20

2. Electrical Forces and Charges
All atoms held together by electrical forces
Electrical forces much stronger than gravitational forces
Two types of electrical charge called positive and negative
Most materials have as much positive as negative charge and so are electrically neutral

3. Electrical Forces and Charges
Negative charge of electron exactly equal in magnitude to positive charge of proton
Like charges repel; opposite charges attract
Can be demonstrated with electroscopes and charged rods

4. Conservation of Charge
If electrons are removed or added to a neutral atom, a net charge results; atom now is an ion
If electron(s) removed, positive ion results
If electron(s) added, negative ion results
Outermost electrons often can be easily removed by contact with certain materials

5. Conservation of Charge
Result is one object with negative charge (extra electrons) and one with positive charge (lost electrons)
No charge created by this process, only rearrangement of charges
Charge is conserved, never created or destroyed

6. Quantization of Charge
No matter how much charge is assembled for any purpose, it’s always made up of electrons and/or protons
Smallest amount of charge possible is that of one electron or proton
Charge is quantized: it comes in certain size small packages or quanta

7. Charles Augustin de Coulomb

8. Coulomb’s Law
A law that relates the electrical force with the amount of charge and the distance between the charges
Fundamentally identical to the law of universal gravitation
F = kq1q2/d 2
k is a constant equal to 9.0 x 109 Nm2/C2

9. Coulomb’s Law
Fundamental unit of electric charge is the coulomb (C), a large amount of charge equal to that of 6.25 billion billion electrons
Like the law of gravitation, this is an inverse square law: changing the distance between the charges changes the force by the square of the reciprocal of the distance change - triple d, get 1/9 F

10. Coulomb’s Law
Gravitational forces predominate with large objects because electrical forces cancel by equal and opposite charges
Electrical forces control chemical bonding and the creation of compounds involving sharing or taking of electrons

11. Conductors and Insulators
Conductor: material through which electric charge can easily move - usually a metal because of loosely held electrons
Insulator: material that inhibits the motion of charge - electrons are tightly bound to atoms
Extremely wide range of conductivity

12. Conductors and Insulators
Between insulators and conductors are semiconductors - materials like Si and Ge that are normally insulators but can be made to conduct by the addition of certain impurities
At very cold temperatures, some materials become superconductors, with no resistance to charge flow

13. Buildup of Static Charge
When contact is made between materials that attract electrons differently, electrons can be transferred
One object becomes negatively charged, the other gets equal positive charge
Often seen in daily life, such as socks in the dryer, a wool sweater, scuffing shoes on carpet

14. Charging by Conduction
When charged object makes contact with a conductor, electrons can flow into or out of the conductor.
Leaves a charge of the same type on the conductor after charged object is removed

15. Charging by Induction
Bringing charged object near a conductor will cause electrons to be rearranged on the conducting surface
Negatively charge rod near metal sphere will cause electrons to move away from rod, inducing positive charge near rod
If second sphere touches first sphere, electrons will run onto it to get away from rod

16. Charging by Induction
If spheres are now separated, one will remain positive other negative
Can also be charge sphere by induction if it is touched while charged rod is nearby, allowing path for charge flow (grounding)
Opposite charge of rod remains on sphere

17. Charging by Induction
Same process occurs during thunderstorm; opposite charge is induced on earth by cloud
Franklin’s kite experiment proved this
Pointed objects allow charge to concentrate and leak off, preventing lightning strikes - lightning rod. invented by Franklin

18. Charging by Induction

19. Charge Polarization
Insulators can also be charged by induction but results in polarization of the molecules since electrons can’t move freely
One side of atom or molecule becomes slightly positive, other side slightly negative
Explains how bits of paper can be picked up by charged comb or balloon can stick to wall

20. Charge Polarization
Since opposite charges are closer together than like charges, attractive force results
Some molecules naturally polarized, like water; called electric dipoles

21. Charge Polarization