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**Chapter 8 – Electricity and Magnetism**

8.1 – Ohm’s Law Ohm’s Law shows the relationship between amps, volts, and ohms. I = V R I = current V= voltage R = resistance

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**Ohm’s law tells us how much current flows for different amounts of voltage.**

If the resistance changes, a device does not obey Ohm’s Law.

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**A current versus voltage graph shows if the resistance changes.**

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**Resistance occurs because the charges bounce into and around atoms as they move through a material.**

Voltage goes up = charges move faster = more current. Materials obey Ohm’s Law because the speed of the moving charges increases with the voltage.

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**Resistance of metals increases with higher temp**

Resistance of metals increases with higher temp. Hot metals = more resistance A resistor is a component that is used to control the current in many circuits. The two basic kinds are fixed and variable.

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Resistor Color Codes

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Many types of controls use variable resistors such as a dimmer switch for a light. A potentiometer is a variable resistor.

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**8.2 – Work, Energy, and Power 3 electrical quantities:**

Amps – measures current Volts – measure potential energy difference Ohms – measures resistance of current to flow

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**Most appliances are usually labeled using watts or kilowatts**

this is how the electric company charges you Watt is a unit of power. Power is the rate at which energy flows.

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P = VI A hair dryer draws a current of 10 amps. It is plugged into a 120 V circuit, what is its power? I = 10 A V = 120 V P=VI = 120V(10A) = 1200 W

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**Another unit of power is the horsepower. 1Hp equals 746 watts.**

Utility companies charge customers for a unit called the kilowatt-hour (kWh). Electric companies charge for kWh over a set period of time.

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**Higher power usually means more current**

Higher power usually means more current. If there is too much power in a wire it can melt and start a fire. Reducing the heat in wires: Smaller resistance = more current with less voltage. Less voltage = less power is lost as heat

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**Thicker wires have lower resistance**

Thicker wires have lower resistance. Wires come in gauges, the bigger the gauge the higher the resistance. To carry a lot of current, you want low resistance, so you need a lower gauge (thicker) wire. The longer the wire is the more resistance it has. Remember that the length and wire thickness are both important.

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