Electric Charge. Electric Charge ( q ) unbalanced charges positive and negative charges. n Units Coulombs (C)

Electric Charge Electric Charge ( q ) unbalanced charges positive and negative charges n Units Coulombs (C)

Electric Charge How do objects become charged?

Types of materials Conductors materials in which electric charges move freely

Types of materials Conductors materials in which electric charges move freely Insulators materials in which electric charges do not move freely

Types of materials Conductors materials in which electric charges move freely Insulators materials in which electric charges do not move freely Semiconductors materials with electrical properties between those of conductors and insulators

Polarization (Induced charge separation) Only electrons move so that the object is still neutral but polar. Any charged object will attract a neutral object.

Polarization (Induced charge separation)

Friction Transfer of electrons from one object to another due to rubbing

Conduction 1. Contact between two objects 2. Electric charge (electrons) are transferred 3. Object acquires same charge 4. Excess charge will distribute over surface of a conductor.

Conduction

Induction 1. No contact between two objects 2. Object is charged during grounding 3. Object acquires opposite charge

Properties of Atomic particles Particle Mass Charge Electron Proton Neutron me = 9.11 x 10-31 kg mp = 1.673 x 10-27 kg mn = 1.675 x 10-27 kg q = -e q = -1.60 x 10-19 C q = +e q = +1.60 x 10-19 C q = 0 q = 0 C

Conservation of electric charge The total electric charge of an isolated system remains constant. A balloon has gained 2500 electrons after being rubbed with wool. What is the charge on the balloon? What is the charge on the wool?

Conservation of electric charge A rubber rod acquires a charge of -4.5 μc. How many excess electrons does this represent? Which of the following charges are NOT possible for an object to have? (A) -3.2 x 10-19 C (B) 4.8 x 10-19 C (C) 5.6 x 10-19 C (D) 1.6 x 10-20 C (E) 5.6 C

Conservation of electric charge A rubber rod acquires a charge of -4.5 μc. How many excess electrons does this represent? 4.5 x 10-6 C 1 e 1.60 x 10-19 C = 2.8125 x 1013 e Which of the following charges are NOT possible for an object to have? (A) -3.2 x 10-19 C (B) 4.8 x 10-19 C (C) 5.6 x 10-19 C (D) 1.6 x 10-20 C (E) 5.6 C 2 e 3 e 3.5 x 10 19 e - 3.5 e.1 e

Electric Force (Electrostatic Force, Coulomb Force)! F E = k q q 1 2 E r 2 Charles-Augustin de Coulomb

Electric Force (Electrostatic Force, Coulomb Force) Use the electric force to estimate the speed of the electron in a hydrogen atom. r = 5.29 x 10-11 m

Electric Force (Electrostatic Force, Coulomb Force) What is the magnitude of charge on each sphere? (Assume both spheres have same charge) 5

Electric Force (Electrostatic Force, Coulomb Force) What is the magnitude of charge on each sphere? (Assume both spheres have same charge)

Superposition of Electric Forces and Electric Fields Determine the net electrostatic force on sphere q 1.

Superposition of Electric Forces and Electric Fields

Superposition of Electric Forces and Electric Fields Two small charges are placed as shown below. Where can a third charge of +q be placed so that the net force acting on it is zero?

Superposition of Electric Forces and Electric Fields Two small charges are placed as shown below. Where can a third charge of +q be placed so that the net force acting on it is zero? F 13 = F 23 k q q 1 3 = k q q 2 3 r 2 r 2 k (q 1 )( q 3 ) (x) 2 = k (q 2 )( q 3 ) (1.0 m - x) 2 (q 2 ) (q 1 ) (1.0 m - x)2 = = +1q (x) 2 +4q (+1/4)x 2 = (1.0 m -x) 2 = take square root = (1/2)x = 1.0 m - x x = 2/3 (.67 m from q 1 ) x = distance of q 3 from q 1 Note the point of equilibrium does not depend on the charge of q 3, but the other two charges

Superposition of Electric Forces and Electric Fields Two small charges are placed as shown below. Where can a third charge of +q be placed so that the net force acting on it is zero?

Superposition of Electric Forces and Electric Fields Two small charges are placed as shown below. Where can a third charge of +q be placed so that the net force acting on it is zero? F 13 = F 23 k q q 1 3 = k q q 2 3 r 2 r 2 k (q 1 )( q 3 ) (x) 2 = k (q 2 )( q 3 ) (1.0 m - x) 2 (q 2 ) (q 1 ) (1.0 m - x)2 = = +1q (x) 2-4q -(1/4)x 2 = (1.0 m -x) 2 = take square root = -(1/2)x = 1.0 m - x x = 2.0 m from q 1 (1.0 m to the right of q 2 )

Superposition of Electric Forces and Electric Fields The charges on three identical metal spheres are -6.0 μc. They are placed on the x and y axes as shown. Calculate the net electric force acting on q 1.

Applications of Electrostatic Force

Model of Current A parallel plate capacitor is charged. What would happen if the two plates were connected with a wire?

Model of Current Further study shows that while the discharge is taking place, the wire gets warm, a light bulb can be made to glow, and a compass needle can be deflected. These are indicators of current flow in the wire.

Model of Current The current model of a metal is that it consists of a three-dimensional lattice of positively charged ions (lattice ions) surrounded by a sea of delocalized electrons (free/conduction electrons).

Model of Current

Electric Circuit

Electric Circuit Emf (ε ) is the maximum potential difference of the battery. The positive terminal is 9 volts higher than the negative terminal.

Electric Circuit

Cells and Batteries

Electric Current Electric Current the rate of flow of electric charge Units A (Ampere) = C/s Δq I= t

Current in a Resistor

Conventional Current vs. Electron Flow

Electric Circuits Closed circuit complete pathway for current

Electric Circuits Open circuit incomplete pathway for current break in circuit infinite resistance

Electric Circuits Short circuit circuit with little to no resistance extremely high current overheating

Resistance Resistance ratio of potential difference applied across a piece of material to the current through the material n Units Ω (Ohms) I = ΔV R

Ohm s Law http://www.micro.magnet.fsu.edu/electromag/java/ohmslaw/

Conductivity

Resistivity and Conductivity For a wire conductor: A short fat cold wire is the best conductor A long hot skinny wire has the most resistance R = ρl A

Power Power energy per unit time (rate of energy use or production) Units W (Watts) = J/s

Power Kilowatt-hour (kwh) amount of energy not power

Circuit Schematics

Ammeter: measures current Ammeter Placement: circuit must be broken to insert ammeter in series Ideal ammeter: has zero resistance so it will not affect current flowing through it

Voltmeter Voltmeter: measures potential difference Placement: must be placed in parallel to measure potential difference between two points Ideal voltmeter: has infinite resistance so it will not allow any current to flow through it and disrupt circuit

Meters in a Circuit Which is an ammeter and which is a voltmeter?

Voltmeter and Ammeter

Galvanometer

Series & Parallel Circuits

Series & Parallel Analogies

Series Circuits

Parallel Circuits R eq = R 1 i R 2 R 1 + R 2

Kirchoff s Junction Law

Equivalent Resistance Series Circuit

Equivalent Resistance n Parallel Circuit R eq = R 1 i R 2 R 1 + R 2

Equivalent Resistance

Equivalent Resistance 1 R T = 1 R 1 + 1 R 2 = 1 7.00 Ω + 1 10.0 Ω = 4.12 Ω R T = R 1 + R 2 + R 3 = 4.00 Ω + 4.12 Ω + 9.00 Ω = 17.12 Ω

Equivalent Resistance

Equivalent Resistance R T = R 1 + R 2 = 5.0 Ω + 5.0 Ω = 10.0 Ω 1 R T = 1 R 1 + 1 R 2 = 1 5.0 Ω + 1 10.0 Ω = 3.3 Ω R T = R 1 + R 2 + R 3 = 5.0 Ω + 3.3 Ω + 1.5 Ω = 9.8 Ω

Equivalent Resistance Find the voltage drop across and the current through each resistor.

Equivalent Resistance I T = V T R T = 24 V 240 Ω =.10 A V 1 = I 1 i R 1 V 1 = (.10 A)(110 Ω) V 1 = 11 V V 2 = V T - V 1 = 24 V - 11 V = 13 V I 2 = V 2 R 2 = 13 V 180 Ω =.072 A V 2 = V 3 = 13 V I 3 = I 3A = I 3B = I T - I 2 =.10 A -.072 A =.028 A V 3A = I 3A i R 3A = (.028 A)(220 Ω) = 6.16 V V 3B = I 3B i R 3B = (.028 A)(250 Ω) = 7 V

Light bulbs in Compound Circuits Compare the brightness of the light bulbs in each case.

Light bulbs in Compound Circuits Compare the brightness of the light bulbs in each case. Bulbs are expected brightness - same V - I T high since R T low A brightest (full current & greater V) B = C (expected brightness) Both bulbs are very dim - split V - I T low since R T high

Light bulbs in Compound Circuits

Switches in Circuits 1. How will the ammeter reading change when the switch is opened?

Switches in Circuits 1. How will the ammeter reading change when the switch is opened? Reading (current) will decrease since total resistance increases.

Switches in Circuits 1. How will the ammeter reading change when the switch is closed?

Switches in Circuits 1. How will the ammeter reading change when the switch is closed? Reading (current) will increase since total resistance decreases.

Internal Resistance of Batteries The battery has some internal resistance. As the external resistance decreases, more and more of the energy supplied by the battery is used up inside the battery. In a battery, internal resistance is due to the chemicals within. In a generator, the internal resistance is the resistance of the wires and other components within.

Internal Resistance in Batteries Electromotive force (emf or ε or V T ) total energy per unit charge supplied by the battery Units V or J/C Terminal Voltage (V term ) potential difference across the terminals of the battery Ideal Behavior: V term always equals emf since no internal resistance

Internal Resistance in Batteries Real Behavior: Think of battery as internal E and tiny internal resistor r V term only equals the emf when no current is flowing E is split between R and r When R>>r, V term emf As R decreases, V r increases and V R decreases

Internal Resistance in Batteries

Internal Resistance in Batteries A resistor is connected to a 12 V source and a switch. With the switch open, a voltmeter reads the potential difference across the battery as 12 V yet with the switch closed, the voltmeter reads only 9.6 V and an ammeter reads 0.40 A for the current through the resistor. Sketch an appropriate circuit diagram and calculate the internal resistance of the source.