1. Chapter 20 Electricity

2. 20.1: Electric Charge and Static Electricity

3. Electric Charge
Question: What do static cling and lightening have in common?
Answer: They both have a similar cause – the movement of charged particles
Electric charge: a property that causes subatomic particles such as protons and electrons to attract or repel each other. 

4. Types of Electric Charge
There are two types of electric charge:
Positive (protons)
Negative (electrons) 
Remember that in an atom, the charges are balanced… 
An excess or shortage of electrons produces a net electric charge. 
The SI unit of electric charge is the coulomb (C) 

5. Coulomb It takes about 6.2 x 1018 electrons to produce one coulomb
A lightning bolt is about 20 coulombs of charge.
A flash camera uses the energy from coulombs of charge for each flash. 

6. Electric Charges
Why do two balloons repel off each other after being rubbed on your hair? 
Like charges repel, opposite charges attract. 
Electric force- the attraction or repulsion between electrically charged objects

7. Electric Force Electric force depends on two main factors:
Charge
Distance
How does changing the size of the charge or the distance between charges affect the electric force? (Refer to figure 3, p. 601)
Doubling the net charge on one object doubles the electric force
However, doubling the distance between the objects will reduce the electric force to one fourth as strong! 

8. Reviewing the Atomic Structure
Why are negative charges (electrons) more mobile than positive charges (protons)?
Think about where the subatomic particles are located…
Protons concentrated in the nucleus
Electrons found outside the nucleus

9. Electric Fields
Electric field: the effect an electric charge has on other charges in the space around it  
The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

10. Note the direction of the field of a positive versus negative charge.
The direction of each field line shows the direction of the force on a positive charge (repels or attracts)
The lines representing the field are close near the charge, where the field is strongest!
The more net charge an object has, the greater the force is on it 

11. Electric Fields

12. Static Electricity
Static electricity: the study of the behavior of electric charges, including how charge is transferred between objects 
What are some ways that charges can build up on an object or move from one object to another? 

13. Transferring Charge Charge can be transferred three ways: Friction
Contact
Induction

15. Conserving Charge
Guess what happens to the total amount of charge before and after the transfer of charge occurs (regardless of the way the charge is transferred)? 
Law of Conservation of Charge: the total charge in an isolated system is constant. 

16. Charging by Friction
A charge builds up when one material has a greater attraction for electrons than the other and a build up of charge is created when they are rubbed together (friction).  

17. Charging by Friction
Because electrons transfer from one material to the other, an overall positive charge is produced on the material that loses electrons.  
An electric charge is produced when the opposite charges come close together! 

18. Transfer of Charge (friction)

19. Charging by Contact
A Van de Graaff generator is an example of how an electric charge can be obtained by contact. 
When someone touches the metal sphere, enough charge is acquired to make hair stand on end. 

21. Charging by Induction
Induction- the transfer of charge without contact between materials 

22. Why do I get shocked by the doorknob?

23. An Example of Charging by Induction
This is what happens when you walk across carpet and pick up extra electrons (so your feet/hands are negatively charged).  
The net negative charge in your hand repels the electrons in the doorknob.  
The electrons in the doorknob move to the base and leave a net positive charge on the part closest to your hand.  
The attraction between positive and negative charges creates a spark! 
The name for this spark is called static discharge

24. Static Discharge
Static discharge occurs when a pathway through which charges can move forms suddenly. 
Lightning is a dramatic example of static discharge

25. Lightening & Static Discharge
Friction between moving air masses causes a charge to build up in the clouds
The lower part of the cloud has an overall negative charge, which induces an overall positive charge on the ground below it.
As the amount of charge on the cloud increases, the force of attraction increases between the charges in the ground and the cloud
Eventually, the air becomes charged, forming a pathway for the electrons to travel to the ground. 

26. Lightening  Static Discharge

28. 20.2: Electric Current and Ohm’s Law

29. Electric Current
For a flashlight to work properly, it must form a continuous path through which charges can flow, in other words, make an electric current. 
Electric current- a continuous flow of electric charge
The SI unit of electric current is the ampere (A), or amp
1 amp = 1 coulomb/second 

30. Types of Current There are two types of current: Direct Current (DC)
Alternating Current (AC)

31. Direct Current (DC) Direct current: charge flows in ONE direction (DC)
Example: battery-operated devices like a flashlight

32. Alternating Current (AC)
Alternating current- a flow of electric charge that regularly reverses its direction (AC)
Example: most homes and schools use this type of current

34. Important to Notice Notice Figure 7 on p. 604—
In direct currents, like a flashlight, the electrons flow from the negative terminal of one battery to the positive terminal of another battery

35. Important to Notice
However, look at the direction of the flow of the current…
Why does the current seem to be moving in the opposite direction?
Current moves in the opposite direction because scientists define current as the direction in which positive charges would flow. 

36. Conductors & Insulators
What is the material required for running electrical currents usually made of and why? 
Metals such as copper and silver are good electrical conductors.  
Wood, plastic, rubber, and air are good electrical insulators. 

38. Resistance What do you think of when you hear the term resistance?
Resistance: is opposition to the flow of charges in a material
The SI unit of resistance is the ohm. 

39. Resistance
As electrons move through the wire, they bump into each other and convert some kinetic energy into thermal energy. 
Because of this, less energy is available to move the electrons through the wire, so the current is reduced. 

40. Various Ways to Increase Resistance
There are various ways to increase resistance in a current… 
A material’s thickness, length, and temperature affect its resistance. 

41. Thickness & Resistance
Using a thicker wire is like drinking a milkshake through a thicker straw, it becomes easier!

42. Length & Resistance
If you have a longer wire, the resistance is greater because it has to travel further—so length increases it! 

43. Temperature & Resistance
As temperature increases, collisions increase. When there are more collisions, there is more energy therefore more resistance

44. Superconductors
Could the temperature ever be lowered enough to achieve a resistance of zero??? 
This is the idea behind superconductors. 

45. Voltage
In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy 
What would the electrical energy source be in a flashlight? 

47. Ohm’s Law
German scientist Georg Ohm found a mathematical formula that relates voltage, resistance, and current. 
He said that as resistance increases, voltage decreases. 

48. Ohm’s Law
Ohm’s law is stated in the following formula: V = I x R or I = V/R
V = voltage
R = resistance
I = current
Increasing the voltage increases the current.
Keeping the same voltage and increasing the resistance decreases the current.   

49. 20.3: Electric Circuits

50. Electric Circuits
Electric circuit: a complete path through which charges can flow 
There are circuit diagrams designed to read the path of a current. 
REMEMBER!!! The direction of current is defined as the direction in which positive charges will flow. Electrons flow in the opposite direction. 

51. Types of Circuits There are two types of circuits: Series Circuit
Parallel Circuit

52. Series Circuit
Series circuits: charge has ONE path through which it can flow. 
There are some problems to this system that keeps people from wiring series circuits in their homes… 
If one element stops functioning in a series circuit, none of the elements can operate! 
Basically, if a light bulb were to burn out, the TV would shut off! 

53. Series Circuit
Also, adding bulbs to a series circuit increases the resistance, therefore, decreasing the current and brightness of the bulb!

54. Parallel Circuit
Parallel circuit: an electrical circuit with two or more paths through which charges can flow. 
If one bulb burns out, the charge can still flow so the others stay lit.  
If one element stops functioning in a parallel circuit, the rest of the elements still can operate.

56. Electrical Power
To find the electrical power of the system, use the following equation: 
P (watts) = I (amps) x V (volts)
Electric power can be calculated by multiplying voltage by current. 

57. Power Ratings
An appliance’s power rating tells how much power it uses under normal conditions. 
To find the electrical energy used by an appliance, use the following equation: 
E = P x T
E = energy
P = power
T = time

58. Safety Measures for Circuits
Fuses:
Prevents current overload- wire melts if too much current and stops flow  “Blowing a fuse”

59. Safety Measures for Circuits
Circuit breakers
A switch that opens when current in the circuit is too high
Must be reset before using again

61. Safety Measures for Circuits
Insulation  Don’t touch bare wires!

63. Safety Measures for Circuits
Grounded plugs (Ground Fault Circuit Interrupter  GFCI)
Transfers excess charge through a conductor to the Earth
Example: the third prong on electrical plugs