PHY 124: Introduction to Physics II Electricity and Magnetism Electric Forces and Fields Kartik Ghosh

Why Electricity, Magnetism, Optics, and Modern Physics? Understand the Nature

The Universe Made of all particles that exist and the space where all events occur

Matters Everything exists in the universe is made from tiny Atoms

Protein Fish Amino Acid Everything exists in the universe is made from tiny Atoms Atom: A very basic unit of matter

The Bohr Model of an Atom Nucleus: Protons with positive charge + Neutrons with no Charge Electrons: Move around the nucleus with negative charge Number of electrons = Number of protons in an Atom. Atom is always neutral

The Bohr Model of Atom and Photon Bohr’s Postulates: Stationary States: Electrons in certain orbit without radiation Atom radiates only when electron makes a transition from one to other state Frequency of the photon is given by hf = E i -E f

Everything is made using these Atoms only

Elementary Particles Electron: Fundamental Subatomic Particle Mass (m e ) = 9.11x kg Charge (e) = x Coulomb or C Spin = 1/2

Proton and Neutron Proton: Fundamental Subatomic Particle Mass (m p ) = 1.673x kg Charge (e) = x Coulomb or C Spin = 1/2 Neutron: Fundamental Subatomic Particle Mass (m n ) = 1.675x kg Charge (e) = 0 Spin =1/2

Photon Charge = 0 Rest Mass = 0 Spin = 1 Speed of light in vacuum (c) = constant c = x10 8 ~ 3 x 10 8 m/s

Electricity, Magnetism, Optics, Modern Physics Electrostatics: Interaction among charges Electricity and Magnetism : Movement of the charge particles Optics and Modern Physics: Interaction among electrons or atoms with photons

Two Important Elementary Particles Electron Photon

Electrostatics (Ch-20 &Ch-21)  Electric Charges and Forces  Charges, Atoms, and Molecules  Coulomb’s Law (Force between Charges)  The Concept of the Electric Field  Applications of the Electric Field  Conductors in Electric Fields  Forces and Torques on Charges in Electric Fields  Electric Potential Energy and the Electric Potential  Using the Electric Potential  Calculating the Electrical Potential  Sources of Electric Potential  Connecting Potential and Field  The Electrocardiogram  Capacitance and Capacitors  Dielectrics and Capacitors  Energy and Capacitors

What Will We Learn From Electrostatics?  Total charge in any system: Q  Electric Field at any point: E  Electric Potential at any point: V  Force between charges: F  Energy in any system: U

Chapter 20: Electric Charges, Forces and Fields Electric Charges and Forces Charges, Atoms, and Molecules Coulomb’s Law (Force between Charges) The Concept of the Electric Field Applications of the Electric Field Conductors in Electric Fields Forces and Torques on Charges in Electric Fields

Electric Charge Electron: Fundamental Subatomic Particle Mass (m e ) = 9.11x kg Charge (e) = x Coulomb or C Spin = 1/2

Discovering Electricity-I Expt-1 Expt-2 Expt-3 Nothing Happens Repel each otherAttract each other

Discovering Electricity-II Expt-4 Two Charged Rods Expt-5 Expt-6 Greater forces with more rubbing Less forces with increasing distance Weakly attracted with wool Weakly repelled with Silk Both rods attract the paper

Charge Model I 1. Charging: Transfer of charge by rubbing or some other way 2.Two kinds of charge: Positive and negative 3.Like charges repel and opposite charges attract 4.Magnitude of force increases with the increase of charges and decreases with the increase of separation 5.Neutral objects have an equal number of positive and negative charges

Discovering Electricity-III Expt-7Expt-8 Expt-9 Same charge as plastic One has charge and other does not Same charge as plastic Both have charges

Visualization of Charge

Charge Model II 6. Two types of materials. Conductors and Insulators In conductors charges move easily and in Insulators charges are remain fixed in place 7. Charges can be transferred from one object to another by contact 8. Total charge in the universe is conserved: it can not be created or destroyed by any physical process

Visualization of charges 6. Two types of materials. Conductors and Insulators In conductors charges move easily and in Insulators charges are remain fixed in place 7. Charges can be transferred from one object to another by contact 8. Total charge in the universe is conserved: it can not be created or destroyed by any physical process

Triboelectric Charging MaterialRelative charging with rubbing Rabbit fur+ + + Glass Human hair+ + Nylon+ + + Silk+ Paper+ Cotton- Wood- Amber- - - Rubber- - PVC Teflon- - -

Electroscope

Charge Polarization

Charging by Induction (Polarization) Polarization Polarization induces opposite charges on the surface

Charging by Induction (Polarization)

Polarization: Applications Pulling water Attracting neutral object

Quantization of Charges Charge is quantized, occurring in “bits” of e, the magnitude of the fundamental charge on the electron or proton

Charges, Atoms, and Molecules Model View of an Atom

Atomic view of charging

Electric Dipoles

Hydrogen Bonding

Hydrogen Bonds in DNA

Forces between Charges ( Coulomb’s Law) Coulomb’s Law

Comparison of Gravitational and Electrical Forces Forces between electron and proton in an atom

Forces on a charge due to other charges

Examples

Forces in One Dimension If q 1 = q 2, Then F net =0

Forces in One Dimension

Where do they collide? (a) Close to A (b) Close to B (c) At C

Location of a Zero net Force x

Forces in Two Dimension

Superposition of Forces Determine a net force on a particular charge by all other charges

How does the net force compare? -q charge is placed at either point A or B The net force at A is (a)Less than at B (b) Greater than at B (c)Equal to at B

How does the net force compare?

Uniform Spherical Charge Distributions Can be treated as total charge of the sphere located at the center of the sphere

The velocity of an electron in Bohr Orbits Velocity of an electron v = ? For the first orbit v = 2.19 x 10 6 m

The Concept of an Electric Field Presence of Charge alters the space around it be creating an electric field.

The Electric Field of a Point Charge

Electric Field

Superposition of Fields Same as Force: Vector sum of the fields due to all charges

Electric Field Lines

Rules for Drawing Electric Field Lines

Electric Field Lines for a Point Charge

Electric Field Lines for Systems of Charges

Which is true?

Electric Field Lines for Systems of Charges

Electric Field Lines in A Parallel-Plate Capacitor

What are the signs of q 1 and q 2 ?

Conductors in Electric Fields Any excess charge placed on a conductor moves to its exterior surface At equilibrium E = 0 within a conductor A conductor shield a cavity within it from external electric fields Ground is a good conductor Grounding: Connect a conductor to the ground

Conductors in Electric Fields

Forces on Charges in Electric Fields

Forces and Torques on a dipole in Electric Fields

Problems Charges A container holds a gas consisting of 1.50 moles of oxygen molecules. One in a million of these molecules has lost a single electron. What is the net charge of the gas? 1 mole of gas has N = x molecules

Problems Force Find the direction and magnitude of the net electrostatic force exerted on the point charge q 2. Let q=+2.4  C and d =33 cm

Problems Determine a net force on a particular charge by all other charges

Problems Let q 2 be at the origin and q 3 be on the positive x-axis.

Problems Electric Fields Consider a system consisting of three charges, q 1 =+5.00  C, q 2 =  C and q 3 =  C, at the vertices of an equilateral triangle of side d = 2.75 cm (a)Find the magnitude of the electric field at a point halfway between the charges q 1 and q 2 (b)Is the magnitude of the electric field halfway between the charges q 2 and q 3 greater than, or less than, or the same as the electric field found in part (a)? Explain. (c)Find the magnitude of the electric field at the point specified in part (b).

Problems Let q1 be at the origin and q3 be on the positive x-axis. At a point halfway between charges q1 and q2 the contributions to the electric field attributed to each of those charges cancel one another. The remaining contribution comes from q3

Problems At this location, the electric fields of q2and q3 add, and the resulting field points toward q3. The field due to q1 will have the same magnitude as found in part (a), and will be perpendicular to the combined fields of q2 and q3. The vector sum of the electric fields from all three charges will have a magnitude greater than that found in part (a).

Problems

The End