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 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 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 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.