Electric Charge and Static Electricity

Electric Charge All matter is made up of atoms Atoms contain Protons (+) Neutrons (0) Electrons (-)

Law of Electric Charges The law of electric charges states that like charges repel, and opposite charges attract. Protons are positively charged and electrons are negatively charged, so they are attracted to each other. Without this attraction, electrons would not be held in atoms.

Law of Electric Charges

Electric Force The force between the charged objects is an The size of the electric force depends on 2 things: The amount of charge (the greater the charge, the greater the force) The distance between charges (the further the distance, the less the force)

Electric Field An electric field is the region around a charged object where electric forces can be exerted on another charged object. (Repelled or attracted)

Charged Objects Atoms do not have a charge because the number of electrons and protons cancel each other out. Ex. 3 protons (+) & 3 electrons (-) = 0

Charged Objects How do objects get charged? They either gain or lose electrons. Why not protons? Ex. 3 protons (+) & 5 electrons (-) = 7 protons (+) & 2 electrons (-) =

How Can You Charge Objects? There are 3 ways objects can be charged: Friction Conduction Induction **In each of these, only the electrons move. The protons stay in the nucleus**

Friction Charging by friction occurs when electrons are “wiped” from one object onto another. Ex: If you use a cloth to rub a plastic ruler, electrons move from the cloth to the ruler. The ruler gains electrons and the cloth loses electrons.

Conduction Charging by conduction happens when electrons move from one object to another through direct contact (touching). Ex: Suppose you touch an uncharged piece of metal with a positively charged glass rod. Electrons from the metal will move to the glass rod. The metal loses electrons and becomes positively charged. 

Induction Charging by induction happens when charges in an uncharged object are rearranged without direct contact with a charged object. Ex: If you charge up a balloon through friction and place the balloon near pieces of paper, the charges of the paper will be rearranged and the paper will be attracted to the balloon.

Conservation of Charge When you charge something by any method, no charges are created or destroyed. The numbers of electrons and protons stay the same. Electrons simply move from one atom to another, which makes areas that have different charges.

Conductors and Insulators An electrical conductor is a material in which charges can move easily. Most metals are good conductors because some of their electrons are free to move. Conductors are used to make wires. For example, a lamp cord has metal wire and metal prongs. Copper, aluminum, and mercury are good conductors.

Conductors and Insulators An electrical insulator is a material in which charges cannot move easily. Insulators do not conduct charges very well because their electrons cannot flow freely. The electrons are tightly held in the atoms of the insulator. The insulating material in a lamp cord stops charges from leaving the wire and protects you from electric shock. Plastic, rubber, glass, wood, and air are good insulators.

Static Electricity Static electricity is the electric charge at rest on an object. When something is static, it is not moving. The charges of static electricity do not move away from the object that they are in. So, the object keeps its charge. Ex. Clothes taken out of a dryer

Electric Discharge The loss of static electricity as charges move off an object is called electric discharge. Sometimes, electric discharge happens quickly. Ex. wearing rubber-soled shoes on carpet, lightning Sometimes, electric discharge happens slowly. Ex: static on clothes

How Lightning Forms

Lightning Lightning usually strikes the highest point in a charged area because that point provides the shortest path for the charges to reach the ground. Anything that sticks up or out in an area can provide a path for lightning. A lightning rod is a pointed rod connected to the ground by a wire. Objects, such as a lightning rod, that are joined to Earth by a conductor, such as a wire, are “grounded.” Any object that is grounded provides a path for electric charges to move to Earth. Because Earth is so large, it can give up or absorb charges without being damaged. When lightning strikes a lightning rod, the electric charges are carried safely to Earth through the rod’s wire. By directing the charge to Earth, the rods prevent lightning from damaging buildings.

Section 2 Electric Current Flow of Electric Charges When electric charges are made to flow through a wire or similar material, they produce an electric current. The amount of charge that passes through the wire in a unit of time is the rate of electric current. The unit for the rate of current is the ampere. The name of the unit is often shortened to amp or A. The number of amps describes the amount of charge flowing past a given point each second. To produce electric current, charges must flow continuously from one place to another. Current requires an electric circuit, unbroken path through which electric charges can flow.

Conductors and Insulators A conductor transfers electric charge well. An insulator does not transfer electric charge well. Metals, such as silver, copper, aluminum, and iron, are good conductors. A conductor is a material through which charge can flow easily. In a conductor, atoms contain electrons that are bound loosely. These electrons, called conduction electrons, are able to move throughout the conductor. As these electrons flow through a conductor, they form an electric current. A material through which charges cannot flow easily is called an insulator. The electrons in an insulator are bound tightly to their atoms and do not move easily. Rubber, glass, sand, plastic, and wood are good insulators. Insulators are used to stop the flow of charges.

Voltage Potential energy is the energy an object has as a result of its position, or height. Higher Potential Energy Lower Potential Energy Charges in an electric circuit flow because of a difference in electrical potential energy. The difference in electrical potential energy between two places in a circuit is called voltage, or potential difference. Voltage causes a current in an electric circuit. You can think of voltage as the amount of force pushing an electric current.

An electric circuit also requires a source of energy, such as a battery, to maintain a voltage. A voltage source is a device that creates a potential difference in an electric circuit. Batteries and generators are examples of voltage sources. A voltage source has two terminals. The voltage between the terminals causes charges to move around the circuit.

Current Depends on Resistance The amount of current that exists in a circuit depends on more than just the voltage. Current also depends on the resistance of the material. Resistance is the measure of how difficult it is for charges to flow through a material. The greater the resistance, the less current there is for a given voltage. The unit of measure of resistance is the ohm (Ω). The ohm is named for Georg Ohm, a German physicist who investigated resistance

Factors That Determine Resistance The first factor is the material from which the wire is made. The second factor is length. Long wires have more resistance than short wires. Diameter is the third factor. Wider pipe or wire can carry more electricity. The fourth factor that determines the resistance of a wire is the temperature of the wire. The electrical resistance of most materials increases as temperature increases.

Path of Least Resistance If an electric charge can flow through either of two paths, more of the charge will flow through the path with lower resistance. The bird doesn’t get hurt because charges flow through the path of least resistance. Since the bird’s body offers more resistance than the wire, charges flow directly through the wire without harming the bird.