Electricity Positive and Negative Charge 2 Electric Charges

Electricity

Positive and Negative Charge • 2 Electric Charges: • Proton = Positive charge • Electron = Negative Charge • The amount of positive charge on a proton equals the amount of negative charge on an electron.

Transferring Charge • Some materials hold onto their electrons more tightly or loosely. • When you walk on the carpet, electrons are transferred from the carpet to the soles of your shoes.

Transferring Charge • The soles of your shoes have an excess of electrons and become negatively charged. • • The carpet has lost electrons and has an excess of positive charge. • The accumulation of excess electric charge on an object is called static electricity.

Conservation of Charge • According to the law of conservation of charge, charge can be transferred from object to object, but it cannot be created or destroyed. • Whenever an object becomes charged, electric charges have moved from one place to another.

Behavior of Charges Opposite charges attract Like charges repel • Unlike charges attract each other, and like charges repel each other. • The force between any two objects that are electrically charged decreases as the objects get farther apart.

Electric Fields • An electric field surrounds every electric charge and exerts the force that causes other electric charges to be attracted or repelled. • Any charge that is placed in an electric field will be pushed or pulled by the field.

Conductors and Insulators • If you reach for a metal doorknob after walking across a carpet, you might see a spark. • The spark is caused by electrons moving from your hand to the doorknob.

Conductors and Insulators • A material in which electrons are able to move easily is a conductor. *Metals • Electrolytes • Human body • Earth’s Crust • A material in which electrons are not able to move easily is an insulator. • Plastics • Styrofoam • Wood • Rubber • Paper

Charging Objects • Rubbing two materials together can result in a transfer of electrons. • Then one material is left with a positive charge and the other with an equal amount of negative charge. • The process of transferring charge by touching or rubbing is called charging by contact.

Charging Objects • Because electrical forces act at a distance, charged objects brought near a neutral object will cause electrons to rearrange their positions on the neutral object. • The rearrangement of electrons on a neutral object caused by a nearby charged object is called charging by induction.

Induction of an Electric Field (Balloon and Board Drawing)

Series Circuit • The current only has 1 loop to flow through. • Used in holiday lights and flashlights • Disadvantage: one break in the circuit disrupts entire loop.

Parallel Circuits • Contain 2 or more pathways for current to move through. • Ex: Homes, cars and airplanes are wired with parallel circuits. • Advantages: Some branches can be turned off without affecting the others.

Fuses • 1 useful device that prevents electric circuits from overheating. • Fuses are small pieces of metal that melt if the current becomes too high. The melting causes a break in the circuit and stops the flow of current. Fuses must be replaced once “blown”

Circuit Breaker • Device used to prevent electric circuits from overheating. • Contains a piece of metal that heats up and expands when the current is too large. • They can be reset by switching back to “on” position

Electric Power • Depends on the current and voltage used in an appliance. • It is measured in Watts or Kilowatts • P = IV • Power = Current (amps) x voltage (volts)

Current and Voltage Difference • Ohm’s Law: The relationship between voltage, resistance, and current. • Electrons move through conductors from an area of high voltage to an area of low voltage. • EX: from a battery to a machine. • Batteries usually provide the voltage difference, which causes electrons to move.

Voltage Difference • In some ways, the electric force that causes charges to flow is similar to the force acting on the water in a pipe. • Water flows from higher pressure to lower pressure.

Voltage Difference • A voltage difference is related to the force that causes electric charges to flow. Voltage difference is measured in volts.

3 Variables in a Circuit 1. Voltage: the measure of volts or the potential difference. • **Electric charges have potential energy that depends on the position in electric fields. • Just as a ball rolls down a hill, a negative charge will move away from another negative charge. (Repulsive forces) This is known as Potential Difference ----- -----

3 Variables in a Circuit 2. Electrical Current: is the flow of charges through a wire. (I)

3 Variables in a Circuit 3. Resistance: The tendency for a material to oppose/resist the flow of electrons. • Resistance is measured in ohms ( ).

2 Ways to change resistance 1. Materials: can have hi or low resistance • Ex: copper has low resistance and can carry Electrons quickly. This wire doesn’t lose much energy or heat. • Ex: Tungsten (W) has a high resistance and doesn’t let electrons flow fast. Therefore it heats up quickly. It loses heat and glows. Used in light bulbs

2 Ways to change resistance 2. Thickness of wire: – Thick wire allows electrons to move fast and has low resistance. – – Thin wire doesn’t allow electrons to move fast and has high resistance.

Ohm’s Law • • Ohm’s Law states that I = V/R V =IR R = V/I