Presentation on theme: "Chapter 8 – Electricity and Magnetism"— Presentation transcript:
1 Chapter 8 – Electricity and Magnetism 8.1 – Ohm’s LawOhm’s Law shows the relationship between amps, volts, and ohms.I = VRI = currentV= voltageR = resistance
2 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.
3 A current versus voltage graph shows if the resistance changes.
4 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.
5 Resistance of metals increases with higher temp Resistance of metals increases with higher temp. Hot metals = more resistanceA resistor is a component that is used to control the current in many circuits. The two basic kinds are fixed and variable.
7 Many types of controls use variable resistors such as a dimmer switch for a light. A potentiometer is a variable resistor.
8 8.2 – Work, Energy, and Power 3 electrical quantities: Amps – measures currentVolts – measure potential energy differenceOhms – measures resistance of current to flow
9 Most appliances are usually labeled using watts or kilowatts this is how the electric company charges youWatt is a unit of power.Power is the rate at which energy flows.
10 P = VIA 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 VP=VI = 120V(10A) = 1200 W
11 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.
12 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
13 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.