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Electric Current and Ohm’s Law Chapter 20.2

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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

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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).

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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).

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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)

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

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

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

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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)

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

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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

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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.

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

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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

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

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

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

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Battery Device that converts chemical energy to electrical energy

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Questions Page 607 Questions 1 - 4

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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

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

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

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

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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

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