1. Pearson Prentice Hall Physical Science: Concepts in Action
Chapter 20
Electricity

2. 20.1 Electric Charge & Static Electricity
Objectives:
1. Explain what produces a net electric charge
2. Analyze whether an electric force is attractive or repulsive
3. Describe how to determine the strength of an electric field
4. Determine three ways in which charge is transferred
5. Explain how static discharge occurs

3. What Produces Electric Charge
Def: electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other
Recall that protons are positively charges while electrons are negatively charged

4. What Produces Electric Charge
An excess or shortage of electrons produces a net electric charge

5. Attractive and Repulsive
Def: electric force is the force of attraction or repulsion between electrically charged objects
The strength of the electric force depends on how far apart the objects are from each other
The farther apart the objects are, the less strength the force has

6. How to Determine the Strength of an Electric Field
Def: an electric field is the effect an electric charge has on other charges in the space around it
Put another way, an electric field is the area around an object where the EF can be felt

7. How to Determine the Strength of an Electric Field
Field lines are used to show the effect due to a charge
+ lines go out, - lines point inward
The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge
EF ≈ 1/d2 ≈ means proportional to
This means if the distance is doubled, the strength decreases by ¼

8. Three Ways to Transfer Charge
Charge can be transferred by friction, by contact and by induction
The Law of Conservation of Charge states that the total charge in an isolated system is constant
Friction: electrons are rubbed from one item to another (ex: from hair to balloon)

9. Three Ways to Transfer Charge
Contact: a neutral item touches a charged item (ex: Van de Graaf)

10. Three Ways to Transfer Charge
Induction – 3 objects, end with opp. charge

11. Lightning is an example
Static Discharge
Static discharge occurs when a pathway through which charges can move forms suddenly
Lightning is an example

12. 20.1 Short Response 1) How is a net electric charge produced?
2) What determines whether charges attract or repel?
3) Name 2 factors that affect the strength of an electric field.
4) List 3 methods of charge transfer
5) Explain how static discharge occurs
6) How does electric force depend on the amount of charge and the distance between charges?
7) Why does plastic food wrap cling better to some materials than others?
8) When a glass rod is rubbed with neutral silk, the glass becomes positively charged. What charge does the silk now have? Explain.

13. 20.2 Electric Current Objectives: 1. Name two types of current
2. Give examples of conductors and insulators
3. Describe factors that affect electrical resistance
4. Explain what causes and electric current
5. Explain how voltage, current and resistance are related

14. Two Types of Current
Def: electric current is the continuous flow of electric charge
Charge flows in only one direction in direct current (DC)
Alternating current (AC) is a flow of electric charge that regularly reverses direction

15. Conductors and Insulators
Def: an electrical conductor is a material through which charge can flow easily
Def: an electrical insulator is a material through which charge cannot flow easily
Metals such as copper and silver are good electrical conductors
Materials such as wood, plastic, rubber and good electrical insulators

17. Electrical Resistance & Factors that affect Resistance
Def: resistance is opposition to the flow of charges in a material
A material’s thickness, length and temperature affect resistance

18. Electrical Resistance & Factors that affect Resistance
Def: a superconductor is a material that has almost zero resistance when it is cooled to low temperatures

19. What Causes Electric Current
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
Recall that potential energy is the energy of position (energy waiting to happen)
Def: potential difference is the difference in electrical PE between two places in an electric field
Def: voltage is electrical potential difference

20. V = IR
Voltage = Current * Resistance
(V) (Amps) (Ohms)

21. Voltage, Current and Resistance
Def: a battery is a device that converts chemical energy to electrical energy
The unit of resistance is called the ohm
Increasing the voltage increases the current
Keeping the same voltage and increasing the resistance decreases the current

22. P = IV
Power = Current * Voltage
(watts) (amps) (volts)

23. 20.2 Short Response 1) List the 2 types of current and describe.
2) Name 2 good electrical conductors and 3 good insulators.
3) What variables affect the resistance of a material?
4) What causes charge to flow?
5) According to Ohm’s law, how is voltage related to resistance and current?
6) What is a superconductor
7) Suppose you have two wires of equal length made from the same material. How is it possible for the wires to have different resistances?
8) Use Ohm’s law (V=IR) to explain how 2 circuits could have the same current but different resistances.