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.