1. Electric Current and Ohm’s Law Chapter 20.2

2. Key Concepts: – What are two types of current? – What are some examples of conductors and insulators? – What factors affect electrical resistance? – What causes an electric current? – How are voltage, current, and resistance related? Vocabulary: – Electric Current – Resistance – Direct Current – Superconductor – Alternating Current – Potential Difference – Electrical Conductor – Voltage – Battery – Ohm’s Law

3. Electricity: The Mouse and Cheese Analogy All matter is made up of positive charges and negative charges. – The positives have mass and are not usually free to move. – The negatives have no mass and are free to move through some materials (conductors).

4. Mouse and Cheese Analogy Negative charges are attracted to positive charges the same way mice are attracted to cheese. – Any time there is a natural attraction between two things we can use it to make the objects do work. – The negative charges (mice) will gladly do work in order to get to the positive charges (cheese).

5. Current ( I ) Continuous flow of electric charge. AMPERE – SI unit of electric current is the AMPERE (A) or amp. – 1 amp = 1 coulomb per second. Two types of electric current: – Direct Current (DC) – Alternating Current (AC)

6. Mouse and Cheese Analogy Current: – The number of charges (mice) passing a point per second. The rate of flow of charges.

7. Direct Current DC Charges flow in one direction. – Most battery operated items use DC. http://www.geocities.com/capecanaveral/hall/6645/electmag/electricmotor.htm

8. Alternating Current Flow of electric charge that regularly reverses its direction. Most electric current at home and school.

9. Conductor and Insulator Electrical Conductor Electrical Conductor – Material through which charge can flow easily. Copper and Silver Electrical Insulator Electrical Insulator – Coating around a wire that controls the current and keeps it where its needed. Wood, plastic, air Rubber Insulating Material (plastic) Conducting Wire (copper)

10. Superconductor Material that has almost zero resistance when cooled to low temperatures. – Best superconductors must be cooled to 138K.

11. Resistance Opposition to the flow of charges in a material. – SI unit of resistance is an ohm (Ω). Four factors affect the resistance in a wire: – Material type – Thickness – Length – Temperature

12. Mouse and Cheese Analogy Resistance: The opposition to the flow of charge. Any appliance that asks the charge (mouse) to do work will slow it down.

13. Voltage The difference of electrical potential between two points of an electrical circuit. Also called the potential difference.

14. Voltage: – The amount of work that each charge (mouse) will do as it goes through the circuit. – Can also be thought of as the amount of push on the charges or how hungry the mice are. Mouse and Cheese Analogy

15. Voltage VOLT (V) SI unit for voltage is a VOLT (V) One volt = one joule per coulomb V = J/C

16. Voltage and Current Video Link: – Click on the batter below to view video. – http://www.youtube.com/watch?v=1xPjES- sHwg&feature=related.

17. Voltage Sources Three common voltage sources: – Battery – Solar Cell – Generator

18. Battery Device that converts chemical energy to electrical energy

19. Questions Page 607 Questions 1 - 4

20. Ohm’s Law Georg Ohm (1789-1854) – Found a relationship between voltage, current, and resistance. Ohm’s Law: – The voltage (V) in a circuit equals the product of the current (I) and the resistance (R). – V = I x RI = VR = V R I

21. Ohm’s Law Calculate the potential difference (voltage) when the current is 4.0 amps and the resistance is 3.0 ohms.

22. Ohm’s Law Calculate the current when the resistance is 6.0 Ω and the potential difference is 12 V.

23. Ohm’s Law What is the resistance of a wire if the potential difference is 24 V and the current is 4 A?

24. Key Concepts: – What are two types of current? – What are some examples of conductors and insulators? – What factors affect electrical resistance? – What causes an electric current? – How are voltage, current, and resistance related? Vocabulary: – Electric Current – Resistance – Direct Current – Superconductor – Alternating Current – Potential Difference – Electrical Conductor – Voltage – Battery – Ohm’s Law