Chapter 20 Electricity Chapter 21 Magnetism

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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

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

www.stkate.edu/physics/phys112/index.html
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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
• gtelectron charge is 1.6 x 10-19 coulombs
• gtproton 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
Charging a Pith Ball     On the following
diagrams i) Show how the negative and positive
charges are arranged and ii) Explain why the
pith ball is first attracted then repelled from
the rod  
                                                 
                                              Disc
ussion   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 Energy

• Potential 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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Ohms Law

• V IR
• V, potential difference in volts, v
• I, current in amperes, a
• R, resistance in ohms, W
• 
  Resistance is affected by a materials
  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. W

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• Practice Problems

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Practice Problems, answers

• 1. RV/I 24v/0.80a 3.0 x 101 W
• 2. RV/I 120v/0.50a 240 W
• 3. V IR (0.50a)(12W) 6.0 v
• 4. I V/R 1.5 v/ 3.5 W 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
4
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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

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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 phones 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