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Chapter 20 ElectricityChapter 21 Magnetism

Prentice Hall 2006

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Assignments for Chapter 20

• Define vocabulary terms in 20.1-3 (24) • Write the key concepts in 20.1-3 (14) • 611/1-3 Math Practice re Power• 613/5-8• 625/1-10(complete sentences), 14,15,17,18,27,29,30• Workbook pages: 239-248

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20.1 Electric Charge and Static Electricity

A. Electric charge – an electrical property of matter that creates a force between objects.

*based on number of electrons and protons in the objects

*excess number of electrons creates a negative charge - atom gains electrons

*excess number of protons creates a positive charge - atom loses electrons

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Electric Charge and Force

srikant.org/core/node8.html

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Electric Force Fields

www.stkate.edu/physics/phys112/index.html

The strength of an electric field depends on the a)Amount of charge that produced the field andb)The distance from the charge.Potential difference is the difference in electrical potential difference between 2 places in an electric field.

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20.1 Electric Charge and Static Electricity, continued

*equal number of electrons and protons creates a neutral charge

*unit of charge is a coulomb

>electron charge is –1.6 x 10-19 coulombs

>proton charge is +1.6 x 10-19 coulombs

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devices used to detect charges:

Van deGraaf Generators

Electroscopes

Pith balls

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Van de Graaf Generator

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physicslearning.colorado.edu/PIRA/PiraSubTOC....

• www.columbia.edu/.../rce/main/demo/eandm.html

Electroscopes

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Pith Balls

                     

  Discussion  1) The neutral pith ball has equal number of (+) and (-) charges  2) The negative (-) charges on the pith ball are repelled away form the rod. The positive (+) charges on the pith ball are closer to the negative (-) charges on the rod than the pith balls positive (+) charges. The pith balls positive charges are therefore attracted more strongly than the pith balls negative (-) charges are repelled. Hence there is an overall attraction 3) When rod makes contact, surface negative (-) charges are able to move onto the pith ball, giving it an overall negative charge. 4) The pith ball is then repelled away from the rod.

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Discussion of prior slide  1) The neutral pith ball has equal number of (+) and (-) charges

 2) The negative (-) charges on the pith ball are repelled away form the

rod. The positive (+) charges on the pith ball are closer to the negative (-) charges on the rod than the pith balls positive (+) charges. The pith balls positive charges are therefore attracted more strongly than the pith balls negative (-) charges are repelled. Hence there is an overall attraction

 3) When rod makes contact, surface negative (-) charges are able to

move onto the pith ball, giving it an overall negative charge. 4) The pith ball is then repelled away from the rod.

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Static Electricity

• Study of the behavior of electric charges (at rest) including how charge is transferred between objects by– Friction (like walking across a carpet)– By conduction (contact)– By induction (bringing a charged object

near a neutral object)

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Electrical Potential EnergyPotential difference

(measured in volts) between two points is what

causes electricity to move.

Lightning is a natural result of this.

Batteries provide a potential difference this across their terminals and produce direct current.

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DC circuit is produced by a battery.

Potential Difference (voltage drop) is

maintained across the + and - terminals

of a battery.What is the difference between the circuit at left and the one above?

The circuit at left is in series

The circuit above is in parallel

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20.2 For current to flow:

• You must have – Source of voltage– Complete path or circuit– Conductor with low resistance– Device to use the energy (i.e., light bulb)

• Current flows from positive to negative– This is considered conventional current– Direction of positive charge movement that is

equivalent to actual motion of charge in the material

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Types of Current

• Direct current (DC) - flows in one direction only– Flashlight and car batteries produce DC

• Alternating current (AC) - flows back and forth– This is produced by an electrical generating

power plant that sends electricity to places like businesses, schools, and homes

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Resistances

• Conductors have low resistances and allow charges to flow easily– Samples: metals

• Insulators have high resistances because electrons are tightly bound to its atoms– Samples: plastics, dry wood

• Superconductors have little or no resistance below their critical temperature– Samples: some metals (Nb, Sn, Hg,…)

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Ohm’s Law

• V = IR

• V, potential difference in volts, v

• I, current in amperes, a

• R, resistance in ohms, • *

Resistance is affected by a material’s thickness, length, and temperature. *Increasing the thickness of a metal wire

will reduce its resistance.

V

I R

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Problem samples

Find the resistance in a circuit with an 8.0 volt battery and 0.2 amp flowing when the current is on.

R = V

I

R = 8.0 v

0.2 a

R = 40.

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• Practice Problems

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Practice Problems, answers

• 1. R=V/I = 24v/0.80a = 3.0 x 101 • 2. R=V/I = 120v/0.50a = 240 • 3. V = IR = (0.50a)(12) = 6.0 v

• 4. I = V/R = 1.5 v/ 3.5 = 0.43a

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20.3 Circuits• Two main types

– Series where there is one path for current flow– Parallel where there is more than one path for

current to flow. Most circuits in your home are of this type - parallel.

– Series Parallel

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Sw.1

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Answers to the previous 4 slides

• Frame 25: no, no, yes

• Frame 26: 1-abc; 2-ab; 3-a; 4-abc; 5-abc

• Frame 27: 1-a; 2-c; 3-a

• Frame 28: 4-d; 5-d; 6-c

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Electrical Safety

• These devices are used make electricity safer:– Circuit breaker (if too much current flows,

this opens the circuit - page 609)– Fuse (if too much current flows, the wire in

this melts and opens the circuit - page 612)– Ground-fault circuit interrupter (this

automatically opens the circuit if it senses unequal currents - see page 613)

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Schematic Diagrams

• The figures on the prior slide are schematic diagrams where symbols represent the parts of a circuit. See page 374 in book.

lamp

Wires, Connected, Crossing

           

           

Wires, Not Connected, Crossing

           

           

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Power = Current x Voltage

P = IV, unit: watt = amp volt

An electric space heater requires 29 amp of 120 v current to adequately warm a room. What is the power rating of the heater?

P = IV

P = 29a (120v)

P = 3.5 x 103 watt

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Your assignment for 20.3

• 611/1-3 math practice

• 613/5-8

20.4 Electronic Devices

• Electronic Signal - information sent as patterns in controlled flow of electrons through a circuit.

• Analog signals are produced by continuously changing voltage or current (page 619)

• Vacuum tubes can be used to change AC into DC, increase signal strength, turn current on or off.

• Semiconductors are made from crystalline solids and conduct current under certain circumstances

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Other electrical information

• Transistors are solid state components with 3 layers of semiconductors

• Transistors amplify a mobile phone’s incoming signal

• A diode maintains proper voltage levels in the circuits in a mobile phone

• An Integrated Circuit is a thin slice of silicon that contains many solid-state components

21.1&2 Magnetism & Electromagnetism

• Define vocabulary terms in 21.1-3 (15)

• Write the key concepts in 21.1-3 (10)

• 651/1-10 (complete sentences), 14, 15,17, 30

• Workbook pages: 251-258

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21.1&2 Magnetism & Electromagnetism

Review:

1. Like magnetic poles _____

2. Unlike magnetic poles ____

3. Magnetic field lines run from ___ to ___

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answers

• Like poles repel

• Unlike poles attract

• Magnetic field flows from N to S

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Earth’s Magnetic Field

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Electricity can produce magnetism and …

• Magnetism can produce electricity!!!

• When electricity flows through a conductor, a magnetic field is produced

• Right hand rule for a straight conductor

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21.3 Electrical Energy Generation and Transmission

• When a coil of wire moves in a magnetic field, electricity will flow in the coil (aka: electromagnetic induction)

• This is how our electricity is generated

• Faraday’s Law - an electric current can be produced in a circuit by a changing magnetic field.

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Right hand rule used in designing and understanding of generators

I current flow

B magnetic field direction

F resulting force acting on wire

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Faraday’s Law

• Just by moving a magnet in a coil of wire or [as in fig. (c), the switch is turned on or off] electricity is produced. This is how electricity is generated.

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Transformers

These change the voltage and current of electricity by having different numbers of turns on the primary and secondary coils

Before electricity

gets to your house

it is stepped down

by a passing through

a step-down

transformer.

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More ElectroMagnetic Information

• The stronger the magnetic force of a magnet, the more effect it has on another magnetic or on moving charges

• Magnetic fields are produced by moving electric charges

• AC and DC generators are similar except DC generators use commutators to produce direct current

• Heat from fossil fuels spins a turbine (coil of wire that spins in a magnetic field) that generates electrical energy