Chapter 18 Electricity

What is an electric charge? Atoms contain charged particles called electrons and protons. If two electrons come close together, they push each other apart. They repel each other! When a proton and an electron come close together they are attracted to each other.

Electric Force Charges that are the same repel each other. Charges that are different attract each other. The interaction between electric charges is called electricity. The force between charged objects is called electric force.

Electric Field Electric field extends around a charged object. An electric field is a region around a charged object where the object’s electric force is exerted on other charged objects. Electric fields and forces get weaker the farther away they are from the charge. Electrical fields are invisible.

Static Electricity An uncharged object can become charged by gaining or losing electrons. If an object loses an electron it has more protons and becomes positively charged. If an object gains electrons it has more electrons than protons so it becomes negatively charged. The buildup of charges on an object is called static electricity.

Static Electricity

Static Electricity In static electricity charges build up on an object, but they do not flow continuously. What happens when you rub a balloon to your head? The balloon gains electrons and becomes negatively charged.

Charging by Friction When two uncharged objects are rubbed together some electrons from one object can move onto the other object. The object that gains the electrons becomes negatively charged. The object that loses electrons becomes positively charged. Why? Charging by friction is the transfer of electrons from one uncharged object to another by rubbing the objects together.

Charging by Conduction When a charged object touches another object, electrons can be transferred. Charging by conduction is the transfer of electrons from one object to another by direct contact. Electrons transfer from the object that has more negative charge to the object that has more positive charge.

Static Discharge The loss of static electricity as electric charges transfers from one object to another is called static discharge. When you pull a sweater off and your hair stands up, its because your hair is positively charged. However; it does not stay that way forever. Your hair will continue to gain electrons from the air until your hair becomes neutral or uncharged. Lighting bolts are an example if static discharge.

Static Discharge

Electric Current Electric charges can made to be moved in a confined path. This is called Electric Current. Electric devices such as your TV need electric charges that flow continuously. They require an electric current.

Electric Current When electric charges are made to flow through a material, they produce an electric current. Electric current is the continuous flow of electric charges through a material.

Current in a Circuit The electric currents that power your computer and music player need very specific paths to work. In order to maintain an electric current, charges must be able to flow continuously in a loop. A complete, unbroken path that charges can flow through is called an electric circuit.

Current in a Circuit

Conductors and Insulators Charges can flow more easily through some materials than others. A conductor is a material through which charge can flow easily. Metals such as copper, are good conductors. Wires are surrounded by insulators. Insulators are materials, such as rubber, that do not allow charges to flow.

Conductors and Insulators

Semiconductors Semiconductors are materials that behave sometimes as conductors and sometimes as insulators. Pure silicon acts like an insulator, but when other elements are added, it behaves like a conductor. Silicon is a semiconductor found in most electronic devices.

Resistance The V on a battery stands for volts, which is the unit of voltage. Voltage is the difference in electric potential energy per charge between two points in a circuit. The amount of current in a circuit depends on more than voltage. Current also depends on the resistance of the circuit. Resistance is the measure of how difficult it is for charges to flow through an object. The greater the resistance, the less current there is for a given voltage.

Resistors

4 factors that determine resistance Diameter-more drink through larger straw Length-more milk through short straw Material-conductors allow charge to flow easy Temperature-heat increases resistance If electric current has a choice of two paths, it will move through the path of least resistance.

What is a Circuit Made of? All electric circuits have these basic features: Devices that run on electrical circuits Devices that run on electrical energy Sources of electrical energy Conducting Wires

Circuit Part 1 Batteries and power plants are examples of energy sources. They supply the voltage that causes current flow. When the energy source is a battery, current flows from the positive end to the negative end of the battery.

Circuit Part 2 Energy is always conserved in a circuit. Electrical energy does not get used up. It gets transferred into other forms of energy such as heat, light, mechanical, and sound energy. These devices resist current, so they are represented in a circuit as resistors.

Circuit Part 3 Electric Circuits are connected by conducting wires. The conducting wires complete the path of the current. The allow charge to flow from the energy source to the device that runs on electric current and back to the energy source.

Circuit Part 4 A switch is often included to control the current. Opening a switch breaks the circuit, which shuts off the device.