Chapter 19 It’s Electric!

Going Back to Basics A charge is not something you can see, weigh, or define, but you can observe how charge affects the behavior of particles.

Charge & Force The difference between 2 charges has a lot do with the forces they exert Recall – a force is a push or pull on an object Charges particles exert pushes & pulls

Opposites Attract A force of attraction exists between oppositely charged particles This is the force responsible for holding the negatively charged electrons in the outer orbital shell that surrounds the positively charged nucleus

Like Charges Repel A force of repulsion exists between particles of the same charge This is the force responsible pushing objects apart

Electric Field Extends outward through space from every charged particle

Electric Field When a charged particle moves into the electric field of another charged particle, it is either pushed or pulled

Developing a Charge A stable atom has an equal # of protons & electrons, so the overall charge is zero because the positives cancel out the negatives For an object to become charged, it needs to either gain or lose electrons creating an ion positive ions = a loss of electrons negative ions = a gain of electrons

Law of Conservation of Charge A neutral object acquires an electric charge when it gains or loses electrons The charge is neither created nor destroyed Charge is only being transferred from one object to another

Methods of Charging 1. Friction – rubbing two objects together, one gains electrons while the other loses electrons (This is how a balloon acquires a charge that allows it to stick to a wall.)

Methods of Charging 2.Conduction – electrons flow from one object to the next by direct contact Conductors, like most metals, allow electric charges to flow freely Insulators, do not allow charges to flow freely (think about plastic handles on power tools!)

Conductio n

Methods of Charging 3.Induction – rearrangement of electric charges Think back to the balloon sticking to the wall The positive charges move closer to the surface of the wall & the negatively charged balloon is attracted to the surface

Induction

Static Electricity the transfer of electrons from one object to another without further movement static means stationary (not moving) it is the build up of electric charges on an object once built up though, the charges do not flow

Electric Discharge static electricity will eventually leave the object this loss of electric charge is called electric discharge can occur quickly or slowly usually, extra electrons will escape onto nearby water molecules in the air

Static Shocks on dry days there are fewer water molecules for the electrons to escape objects are more easily charged & tend to lose their charge quickly this creates a shock, a spark of light, or a crackle of noise lightning is a dramatic example of the discharge of static electricity

Lightning is an Example of Static Electricity It cannot be used as a source of electricity because the current does not flow, it is just rapidly discharged

Lightning Lightning occurs when electrons jump through the air creating intense light & heat. Thunder, the loud boom, happens because the rapidly heated air expands

Lightning It occurs between two areas of different charge from cloud to cloud from one part of a cloud to another part of the same cloud from cloud to ground

How Lightning Works In your typical storm cloud, the negative charge at the bottom of the cloud enhances the positive charge at the ground. The cloud wants to "complete the circuit" and sends out a stepped leader toward the ground. That further enhances the positive charge near the ground, and objects may form a streamer of positive charge that reaches up, trying to make the connection.

He tied a metal key onto the string and, as he suspected it would, electricity from the storm clouds flowed down the string, which was wet, and he received an electrical shock. In Benjamin Franklin’s famous experiment with a kite in 1752, he tried to prove that lightning was electrical & flew a kite during a thunderstorm.

Franklin Discovers... that electricity moves quickly through certain materials, and that a pointed surface attracts better than a flat surface He suggested placing pointed metal rods above the roofs of buildings & running to the ground as protection from lightning The Earth is a good conductor & can easily accept or give up electrons so it acts as a grounding device

Lightning Rods

The Electroscope instrument used to detect an electric charge the thin metal leaves hang straight down from the metal rod when uncharged when charged, the leaves spread apart to indicate the presence of like charges (remember like charges repel)

Is there a charge present? How can you tell?

Does this show charging by friction, conduction or induction?