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

• 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.

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

• 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.

• 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.

Resistor Color Codes

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

8.2 – Work, Energy, and Power

• 3 electrical quantities:– Amps – measures current– Volts – measure potential energy difference– Ohms – measures resistance of current to flow

• 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.

• 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

• 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.

• 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

• 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.