1 Ch32 Electrostatics Intro. To Electric Charge Physics Spring,2015 Mrs. Kummer

2 First, The Atom The parts of an atom include: -nucleus -home of the proton -home of the neutron -orbits of the electrons -home of the electrons -electrons have very small mass

3 First, The Atom

4 Second, The Periodic Table What do all the symbols and numbers mean? 47 = Atomic Number Atomic # = # protons Atomic # = # electrons Ag = Element Symbol Silver = Element Name = Atomic Mass Atomic Mass = #p + + #n

5 Electrostatics Introduction Electrostatics: study of electricity at rest To study it, we need to build on what we know….baby steps Electrical Forces/Charges Conservation of Charge Coulomb’s Law Conductors & Insulators 3 Ways to Charge an Object

6 Atoms & Electricity All matter is made of atoms, which contain electrons, protons, and neutrons (n). Objects normally contain equal numbers of electrons (e - ) and protons (p + ) : these are neutral. When an object has an uneven amount in the no. of electrons and protons, it is electrically charged. Neutrons are not involved in electric interactions. They are neutral.

7 Electric Charge An object with more electrons than protons is said to carry a negative charge. An object with more protons than electrons is said to carry a positive charge. The imbalance between p + and e – in a charged object is proportionally very small… The difference may be only particles out of a total of ; i.e. an imbalance of 1 per 10 trillion!

Point Charges & Masses* *Add these to your formula sheets e- charge = x C p+ charge = x C n charge = 0 (no charge) Mass of e- = 9.11 X kg Mass of p+ = X kg Mass of n = X kg 8

9 Units for Electric Charge Electric Charge: Symbol = q Metric Unit: Coulomb, C A Coulomb is defined as the number of electrons passing through a current of 1 Amp each second: C = A·s. A Coulomb IS NOT the number of e- a substance has 1 C= 6.25 X e- 1.60x10-19 C/e It is much easier to measure charge flowing in a current than to count protons and electrons individually, don’t you agree?????

Charges can be transferred, but never created nor destroyed If you transfer to another object, you can make an ion (cation for + ions and anions for – ions) 10 Electric Charge is Conserved

11 Electric Charge Charged objects exert electric forces on each other: Opposite (+ & -) charges attract; Like (+ & +) (- & -) charges repel. Like energy and momentum, electric charge is a conserved quantity: Charge cannot be created or destroyed, but it can be transferred, or MOVED between objects.

Charging by Contact Charging by friction can happen three ways: Triboelectric Charging: A table of specific items that are naturally VERY positive (fur) to naturally VERY negative (PVC Pipe) Charging by Collision: Colliding ice crystals in clouds cause charge separation in cloud that can bring Lightning (e.g. cloud would stay +, ground becomes attracted and – comes to surface, then lightning) Electrical Polarization: a + charged rod nearby something neutral, causes the neutral object to send – to surface, as they are attracted to + 12

13 Electric Charge There are three material types related to Electric Charge: 1.Conductors-material where electric charge flows easily 2.Insulators-material where flow of e - charge is reduced or prevented 3.Semiconductors-material where flow can be made to behave like BOTH (e.g. PV cells in solar panels)

14 Electric Charge There are three common ways for a neutral object to become charged. 1.Friction 2.Conduction 3.Induction

15 Charging by Friction Charging by Friction occurs when two different materials rub against each other, causing a transfer of electrons. The material with the greater affinity for electrons becomes negative, and the other material becomes positive. E.g. Clothes in a dryer. E.g. Rub a balloon on your hair.

16 Charging by Friction Go To Triboelectric Series Table

17 Charging by Conduction Charging by Conduction occurs when a neutral object is placed in contact with an already-charged object. If the object is –, electric repulsion will push some of the excess electrons from the charged to the neutral object.

18 Charging by Conduction If the objects are spheres of the same size, and are made out of conductive material, they will have the equal amounts of charge after contact. The charge is located on the surface (not the interior) because the electrons repel each other.

19 Charging by Conduction In solids, protons are usually not in motion; only electrons are able to move. If the object is +, electric attraction will pull some electrons from the neutral object to the charged one.

20 Charging by Induction Charging by Induction uses a similar mechanism to polarization. A charged object (“A”) is brought close to (but not touching) the object (“B”) to be charged. This object is then connected to a more distant object (“C”) such as Earth.

21 Charging by Induction “A” produces electric force, causing electrons to flow between “B” and “C”. When “B” and “C” are separated, “B” has a charge of the opposite sign compared to “A”. “C” has a charge of the same sign as “A”.

22 Coulomb’s Law DO NOT WRITE THIS!!!!! States the force between two charges varies directly as the product of the charges and inversely as the square of the distance between them* *for charged particles that are small compared with the distance between them WRITE THIS!!!!!! Or, in terms of Math equation: F e = k q 1 q 2 d 2 (Eq. 7) Where F e = the Electric Force (N) k = Coulomb’s Constant, = 9.0 X 10 9 (Nm 2 /C 2 ) q 1 = charge of particle #1 (C) q 2 =charge of particle #2 (C) d = distance between two particles (m)

23 Coulomb’s Law Example Two pellets, each with a charge of 10μC, are separated by a distance of 30cm. What is the electric force, F e, between the two pellets? Step1: Given: q 1 =1.0 X C q 2 =1.0 X C k = 9.0 X 10 9 Nm 2 /C 2 d = 0.30m Step 2: Unknown: F e =? Step 3: Write Formula:F e = k q 1 q 2 d 2 Step 4: Plug-In Values: F e = (9.0 X 10 9 ) (1.0 X )(1.0 X ) (0.30) 2 F e = 10 N

Let’s Go Practice!

Coulomb’s Law vs. Newton’s LUG F e = k q 1 q 2 k=9X10 9 Nm 2 d 2 C 2 Fg = G m 1 m 2 G=6.67 X10 9 Nm 2 d 2 kg 2 25

Coulomb’s or Newton’s LUG As F goes ↑, q or m goes _____ As d goes ↑, F goes ________ 26

To find the total force, you need to take 2 charges at a time Keep their individual forces as attractive (+/-) or repelling (-/-, +/+) Add them up by vector addition: F 1 = F 12 +F 13 See diagram on board This is called superposition: using vector addition to make F 1 27 Finding Total Electric Force, Fe