Understanding Electrostatics and Charges

1. Physics

2. What does that word even mean?  Electrostatics = electricity at rest  Electrostatics involves electric charges, the forces between them, and their behavior in materials  Reminder:  Protons (+)  Electrons (-)  Neutrons (no charge)

3. Charges  The fundamental rule at the base of all electrical phenomena is that like charges repel and opposite charges attract  An object that has unequal numbers of electrons and protons is electrically charged .

4. Conservation of Charge  The principle that electrons are neither created nor destroyed but are simply transferred from one material to another is known as conservation of charge . When electrons are transferred from the fur to the rod, the rod becomes negatively charged.

5. Coulomb’s Law  For charged particles, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them .  1 C (coulomb) is the charge of 6.24 billion billion electrons  k = 9.00 * 10 9 Nm 2 /C 2

6.  q1 and q2 are charges, measured in [C]  k = proportionality const = 9.00 * 10 9 Nm 2 /C 2  d = distance between the charges [m]

7. Example:  Consider two small charged objects, one with a charge of 5 C and the other of unknown charge. When they are separated by a distance of 1.2 m, each exerts a force of 2.8 N on the other. What is the charge of the second object? F = k * q1 * q2 d 2 2.8 =(9*10 9) * 5 * q2 (1.2) 2 q 2 = 2.8 * (1.2) 2 (9*10 9) * 5

8. Conductors and Insulators  Materials through which electric charge can flow are called conductors  Metals are good conductors  In insulators , electrons are tightly bound and remain with a particular atom (they are not free to wander about)  Rubber and glass are good insulators and poor conductors of electricity  Semiconductors are materials that can be made to behave sometimes as insulators and sometimes as conductors.  Atoms in a semiconductor hold their electrons until given small energy boosts.

9. Plasmas  Plasmas are good conductors  Plasma – the 4 th state of matter (not a solid, liquid or a gas!)  Contains ions or free electrons  Stars and neon signs are both Plasmas

10. Charging by Friction and Contact  Electrons transferred by friction :  We can stroke a cat’s fur and hear the crackle of sparks that are produced  Comb our hair in front of a mirror in a dark room and see as well as hear the sparks of electricity  Scuff our shoes across a rug and feel the tingle as we reach for the doorknob  Electrons transferred by contact :  A charged rod is placed in contact with a neutral object, some charge will transfer to the neutral object

11. Charging by Induction  Induction – charging of an object without direct contact  If a charged object is brought near a conducting surface, even without physical contact, electrons will move in the conducting surface

12. Grounding  When we allow charges to move off (or onto) a conductor by touching it, it is common to say that we are grounding it  When we touch the metal surface with a finger, charges that repel each other have a conducting path to a practically infinite reservoir for electric charge—the ground.