Electricity 7-1, 7-2 Electric Charge and Electric Current

Static Electricity Static Electricity- accumulation of excess electric charges on an obj. pg. 194 Protons = + electrical charge Electrons= - electrical charge Example of electron transfer: Walk on carpet, the electrons are transferred from carpet to your shoes Shoes have a more - charge and carpet is more + = static electricity

Conservation of Charge When an object becomes charged the charge is neither created nor destroyed (Law of Conservation of Charge) Electrons move from one object to another

Opposites Attract **Opposite charges attract- like charges repel (+) charges are attracted to (–) charges.

Electric Field Surrounds every electric charge- force that causes electrons to move

Conductors and Insulators Conductors- material in which electrons move freely ex. Metal, skin Insulators- material that doesn’t allow electrons to flow smoothly. Ex. Plastics, wood, glass, rubber

Transfer of Charge Charging by contact- process of transferring charge by touching or rubbing (rubbing balloon on head) Fig. 7 pg Charging by induction- the rearrangement of electrons on a neutral object (static cling) Lightning- large amount of static discharge Fig. 8

Detecting Electric Charge Electroscope Fig. 10 pg. 199 No charge- leaves hang straight down Object is charged- leaves repel (move apart)

Electric Current 7-2 Notes 813 I. Voltage Difference- the push that causes charges to move and is measured in volts (V). (move from areas of high voltage to low voltage) An object may lose voltage as it flows through a circuit. II. Circuit- a CLOSED, conducting path (must be closed to have a circuit) III. Electric Current- a flow of charges through a wire or any conductor. Current is measured in Amperes (A) Current is the flow of electrons. Most wall sockets are 120 V in our homes.

Batteries Batteries keep current flowing in a circuit. Dry Cell (D, C batteries)- electrons flow from positive to negative terminal. Positive terminal is where carbon rod is located. Wet Cell- car battery is a wet cell battery.

Resistance The tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. All materials have SOME electrical resistance. Resistance is measured in ohms pg. 205 Metals (conductors) have low resistance. Insulators have high resistance. Resistance as temperature Thick v. Thin wire= Thin wire has a greater resistance.

Ohm’s Law Increasing resistance causes the current to decrease. Increasing the pressure causes the current to increase. Ohm’s Law- the current in a circuit equals the voltage difference divided by the resistance. I = V / R current = volts/ resistance or V = IR volts= current x resistance Practice Problems from Ohm’s Law 1. What is the voltage difference across a 70 ohm resistor if a.9A current is flowing through it? 2. A current of.24 A flows through a wire. What’s the resistance if the voltage is 70V?