1. Chapter 20 Electricity Chapter 21 Magnetism
Prentice Hall 2006

2. Assignments for Chapter 20
Define vocabulary terms in (24)
Write the key concepts in (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:

3. 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

4. Electric Charge and Force
srikant.org/core/node8.html

6. 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.

7. 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 coulombs
>proton charge is x coulombs

8. devices used to detect charges:
Van deGraaf Generators
Electroscopes
Pith balls

9. Van de Graaf Generator

10. physicslearning.colorado.edu/PIRA/PiraSubTOC....
Electroscopes
physicslearning.colorado.edu/PIRA/PiraSubTOC....

11. Pith Balls Charging a Pith Ball On the following diagrams;
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
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.

12. 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.

13. 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)

14. 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.

15. 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

16. 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

17. 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

18. 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,…)

19. Ohm’s Law V V = IR V, potential difference in volts, v
I, current in amperes, a
R, resistance in ohms, W
* 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

20. 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

21. Practice Problems

22. Practice Problems, answers
1. R=V/I = 24v/0.80a = 3.0 x 101 W
2. R=V/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

23. 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

27. Sw.1 4

28. 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

29. 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)

30. 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

31. 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

32. Your assignment for 20.3
611/1-3 math practice
613/5-8

33. 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

34. 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

35. 21.1&2 Magnetism & Electromagnetism
Define vocabulary terms in (15)
Write the key concepts in (10)
651/1-10 (complete sentences), 14, 15,17, 30
Workbook pages:

36. 21.1&2 Magnetism & Electromagnetism
Review:
1. Like magnetic poles _____
2. Unlike magnetic poles ____
3. Magnetic field lines run from ___ to ___

37. answers Like poles repel Unlike poles attract
Magnetic field flows from N to S

38. Earth’s Magnetic Field

39. 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

40. 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.

41. Right hand rule used in designing and understanding of generators
I current flow
B magnetic field direction
F resulting force acting on wire

42. 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.

43. 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.

44. 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