Electrostatics Physics

What is electrostatics? Electricity at rest Electricity at rest Involves forces and behaviors of electric charges Involves forces and behaviors of electric charges Better known as Better known as static electricity

Electrical Forces Arise from particles in atoms Arise from particles in atoms Protons Protons Positive (+) charge Positive (+) charge Located in nucleus Located in nucleus Electrons Electrons Negative (-) charge Negative (-) charge Located in electron cloud Located in electron cloud Protons and electrons have opposite charges of equal magnitude. Protons and electrons have opposite charges of equal magnitude.

Electrical Forces Golden Rule- Golden Rule- Like charges repel Like charges repel Opposite attract Opposite attract Forces are VERY large Forces are VERY large Much stronger than Much stronger thangravity Typically cancel each other out Typically cancel each other out

Transfer of electrons Neutral atoms have equal # of protons and electrons Neutral atoms have equal # of protons and electrons No net charge No net charge Ions-charged atoms Ions-charged atoms Positive ions- Positive ions- Net + charge; lost one or more electrons Net + charge; lost one or more electrons Negative ions- Negative ions- Net – charge; gained one or more electrons Net – charge; gained one or more electrons Why are electrons lost or gained and not protons? Why are electrons lost or gained and not protons?

Transfer of electrons Protons are held tightly in nucleus and are much more massive than electrons Protons are held tightly in nucleus and are much more massive than electrons Energy required to remove electrons varies for different substances Energy required to remove electrons varies for different substances Conservation of Charge Conservation of Charge Electrons cannot be created or destroyed! Electrons cannot be created or destroyed! Only transferred from one material to another Only transferred from one material to another Cannot be divided into fractions of electrons Cannot be divided into fractions of electrons Charge will always be equal to a whole # multiple of an electron Charge will always be equal to a whole # multiple of an electron

Conductors vs. Insulators Conductors Conductors Electrons are free to roam in the material Electrons are free to roam in the material An introduced charge will spread evenly over the surface and will be easily discharged by contact An introduced charge will spread evenly over the surface and will be easily discharged by contact Ex: metals Ex: metals Insulators Insulators Tightly bound outer electrons An introduced charge will slowly enter the material and is not easily discharged by contact Ex: rubber and glass

More conductors Semiconductors Semiconductors Can behave as conductors or insulators Can behave as conductors or insulators Used in layers to make transistors Used in layers to make transistors Germanium and Silicon Germanium and Silicon Insulators in pure crystalline form Insulators in pure crystalline form Conductivity increases with impurities (addition or removal of electrons) Conductivity increases with impurities (addition or removal of electrons) Superconductors Superconductors Become infinitely conductive at temp. near absolute zero Superconductivity has been found in some nonmetallic compounds at extremely high temp Once electric current is established, electrons will flow indefinitely

Charging by friction Friction between objects can cause a transfer of electrons Friction between objects can cause a transfer of electrons Ex: Rubbing your feet on carpet; rubbing a balloon on your head Ex: Rubbing your feet on carpet; rubbing a balloon on your head Object A loses 3 e -  Object B gains ?? Object A loses 3 e -  Object B gains ??rubber–cotton–skin–silk–wool–glass–fur ELECTRON GRABBERSELECTRON DONORS

Charging by Induction Charging an object w/o touching it Charging an object w/o touching it Conductors Conductors Electrons will move freely upon the surface Electrons will move freely upon the surface Ex: lightning Ex: lightning Grounding an object Grounding an object Allows charges to move on/off a conductor by touching it Allows charges to move on/off a conductor by touching it Charges are free to flow to a practically infinite reservoir for electrical charge—the ground Charges are free to flow to a practically infinite reservoir for electrical charge—the ground

Charged objects DO NOT TOUCH Charging at a distance Charge is temporary Charging by Induction

Polarization Induction in insulators Induction in insulators Because electrons do not move freely, they rearrange themselves w/in the atoms or molecules Because electrons do not move freely, they rearrange themselves w/in the atoms or molecules Ex: a charged balloon sticking to a wall Ex: a charged balloon sticking to a wall

Charging by Contact A charge will transfer from a charged object to a neutral object if they come in contact A charge will transfer from a charged object to a neutral object if they come in contact Conductors Conductors Charge will spread evenly because they repel each other Charge will spread evenly because they repel each other Insulators Insulators Neutral object will need to be touched in several places to distribute an even charge Neutral object will need to be touched in several places to distribute an even charge

Charging by Contact Charging by Contact

Need to Know Coulomb - SI unit for charge (C) 1 C = 6.24E 18 particles q proton = 1.6E -19 C q electron = -1.6E -19 C q neutron = 0 PrefixSymbolMultiplier Mega-M 10 6 kilo-k 10 3 centi-c milli-m micro- nano-n 10 -9

Gravitation vs. Coulomb’s Law Law of Universal Gravitation Law of Universal Gravitation Only attractive force Only attractive force G = 6.7 x Nm 2 /kg 2 G = 6.7 x Nm 2 /kg 2 Coulomb’s Law Coulomb’s Law Attractive and Repulsive Force Attractive and Repulsive Force k = 9.0 x 10 9 Nm 2 /C 2 k = 9.0 x 10 9 Nm 2 /C 2

Coulomb’s Law F - force between two charges (N) F - force between two charges (N) k - Coulomb’s constant k - Coulomb’s constant 9 x 10 9 Nm 2 /C 2 9 x 10 9 Nm 2 /C 2 d = distance between charged particles d = distance between charged particles q = charge of particle q = charge of particle

Coulomb’s Law Follows the Inverse-square Law Follows the Inverse-square Law If you double the distance between 2 charges then the elec. force is ¼ as much If you double the distance between 2 charges then the elec. force is ¼ as much If distance is halved then force increases by 4. If distance is halved then force increases by 4.