Electricity – Chapter 16 There are 2 types of Electricity 1. Static Electricity (Electrostatics) - a build up of electric charges 2. Current Electricity - electrons flow in a determined path

Electrostatics - Transfer of charged particles All atoms are made up of smaller particles: protons (+) and electrons (-) Protons are very secure in the atom. However, electrons will tend to move about freely. Electricity is a result of the movement of electrons, NOT Protons. + -

Insulators and Conductors Conductors Materials which have electrons in their atoms which are free to move about. Conductors allow for easy electron flow. Insulators Materials which have their electrons tightly bound to the nucleus and cannot easily move. Insulators impede electron flow. Metal is a good conductor. Plastic is a good insulator. Note:Semiconductors are a “third category” having both properties of conductors and insulators depending on external physical properties.

Conductors vs. Insulators

Static Electricity (ElectroStatics) What’s the net charge of a material? Electrically Negative: more total electrons than protons Electrically Neutral: equal number of electrons and protons Electrically Positive: fewer electrons than protons.

Objects become charged (positive or negative) when electrons transfer from one material to another. If the atoms of a material have 1 or 2 electrons in their outer shell, they will give them away to other materials. If the atoms of a material have 6 or 7 electrons in their outer shell, they will steal electrons from other materials. Electrons are transferred between materials when they are rubbed against each other. - when there is FRICTION.

Triboelectric series Human Hands (if very dry) Leather Rabbit Fur Glass Human Hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel (neutral) Wood Amber Hard Rubber Nickel, Copper Brass, Silver Gold, Platinum Polyester Styrene (Styrofoam) Saran Wrap Polyurethane Polyethylene (scotch tape) Polypropylene Vinyl (PVC) Silicon Teflon Gives up e-’s and becomes MORE POSITIVE Accepts donor e-’s and becomes MORE NEGATIVE If we did a study of many materials and put them in order from those with the least desire for electrons to those with a very strong desire for electrons we would have created a Triboelectric series. If two items from the list are rubbed together, then the item that is higher on the list will end up more positive and the lower one will end up more negatively charged. For example, if leather were rubbed with wool, the leather becomes positive and the wool negative. Yet if rubber is rubbed with wool, the rubber becomes negative and the wool positive.

Transfer of charged particles If you are wearing rubber soled shoes and you walk across a new carpet, the carpet can easily give up its electrons and you will readily accept them. If you are wearing leather soled shoes and you walk across a new carpet, you will give some of your electrons to the carpet and it will accept them. When you rub one object against another, electrons (NOT protons) will be transferred from one object to the other.

If the two objects are physically close together, and if there is a large enough charge difference, both objects will try to return to their most stable condition. They do this by the “jump” of electrons from the more negatively charged object to the less negatively charged one. In this way, they will both be equally charged and there will no longer be a charge separation. This is the ZAP that you see and feel. Charge difference: This is when there exists a difference in the net charges of two different objects.

Grounding You don’t have to be “zapped” if you don’t want to... If you are always in contact with the ground, then any electrons that you may possibly gain will immediately travel through your body and into the earth. You remain neutral and NO electron discharge “spark” can occur. This is called grounding. You can “ground” an object by placing a wire from the object and into the ground. This is called a grounding strap.

What is the charge on an object? It was discovered that the charge on an electron is a known and constant number. 1 e - = x Coulomb So, if a material has one more electron than protons, it has a charge of: (1) x (-1.6 x ) = -1.6 x Coulomb If a material has two more electrons than protons, it has a charge of: (2) x (-1.6 x ) = -3.2 x Coulomb In general... If a material has x more electrons than protons, it has a charge of: (x) x (-1.6 x ) Coulomb Unit of Charge

What is the charge on an object? The charge on each electron is 1.6 x Coulomb So, if a material has one less electron than protons, it has a charge of: (1) x (1.6 x ) = +1.6 x Coulomb If a material has two less electrons than protons, it has a charge of: (2) x (1.6 x ) = +3.2 x Coulomb In general... If a material has x less electrons than protons, it has a charge of: (x) x (1.6 x ) Coulomb

Charging by Friction: The actual amount of charge on each object can be calculated if you know the number of electrons that have transferred.

Particle charges Net negative charge : When an object has more negative charges (electrons) than positive charges (protons) Net positive charge : When an object has fewer negative charges (electron) than positive charges (protons) Question: What is the net charge on an object that has gained 2,000 electrons (2 x 10 3 electrons) via transfer by friction? What is the net charge on an object that has lost 10,000 electrons (1 x 10 4 electrons) via transfer by friction?

If two charged objects are put near each other, they will interact: If the charges on the objects are similar, they will repel each other. If the charges on the objects are opposite, they will attract each other Like charges repel + - Unlike charges attract

Benjamin Franklin: Physicist and Statesman Franklin conducted many experiments on static electricity and was a leader in the field. Most of his theories were correct, but some were backward.

Induction – inducing a net charge on an object Franklin noticed that you do not have to touch two objects together to create this attraction/repulsion force. He also noticed that you can create a force between a charged object and a neutral object. This is caused by inducing a net charge on an otherwise neutral object. Here’s how it works: A negatively charged object will have an over abundance of electrons. If it is placed NEAR a neutral object, the electrons in the neutral object will migrate to the far side of the object causing a temporary positive net force closest to the first object Before After

Lightning In large rain clouds, the movement of water particles past each other causes friction. The top of a storm cloud becomes positively charged while the bottom becomes negatively charged. This large area of negative charges induces a temporary positive charge on the ground surface. If the cloud is close enough to the surface, electrons from the cloud will travel from the cloud downward in small sections called “leaders”. This is why Lightning looks jaggedy.

Coulomb’s Law

Given: Q 1 = 1.00 C Q 2 = 1.00 C d = 1.00 m Find: F elect = ??? F elect = k Q 1 Q 2 / d 2 F elect = (9.0 x 10 9 Nm 2 /C 2 ) (1.00 C ) (1.00 C ) / (1.00 m ) 2 F elect = 9.0 x 10 9 N Example: