How To Understand Electricity

Transcription

1 Introduction to Electricity Table of Contents Bellringer Write a definition for electric charge in your own words in your science journal. When do you experience electric charges most, in winter or in summer? Explain your reasoning. Objectives Describe how charged objects interact by using the law of electric charges. Describe three ways in which an object can become charged. Compare conductors with insulators. Give two examples of static electricity and electric discharge. 2,400 volts will kill you every time. Conductors will transfer electricity. A lemon can be used like a battery. A light bulb resists the flow of electricity through a wire. When you walk across the carpet and grab a door knob, you could get shocked by over 1000 volts. The inside of a light bulb will heat up to over 1000 degrees Fahrenheit. A circuit breaker will keep too much electricity from flowing through a wire. A short circuit is a wire in your house with only one switch or receptacle. Sparrows can sit on high voltage electrical lines because their feet are insulated. Static is not really a form of electricity. For a light bulb to light up, it has to be screwed into the socket. When you slide across a fabric couch to give your friend a peck on the cheek you will sometimes get a shocking surprise. This happens because as you slid across the fabric you stole electrons from the couch. This leaves you with a negative charge while your friend has a neutral or no charge. When you get close enough the electrons will jump from you to your friend and give you both a jolt Vocabulary Negative charge Electric Field Conductor Insulator Voltage Current Circuit breaker Fuse

2 Electric Charge Charges Exert Force -- like charges repel and opposite charges attract. An object will have a negative charge when it has more electrons (-) than protons (+). electric force The force between charged objects electric field is the region around a charged object in which an electric force is exerted 3 ways to transfer a charge Friction electrons are wiped from one object onto another. Conduction electrons move from one object to another by direct contact. Induction charges in an uncharged object are rearranged without direct contact X Charge It!, continued Charge It!, continued 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.

3 Charging by Contact Van de Graaff generator electrical conductor is a material in which charges can move easily. **metals copper, aluminum, mercury Visual Concept electrical insulator is a material in which charges cannot move easily. **plastic, wood, glass Static electricity is the electric charge at rest on an object. Electric Discharge The loss of static electricity as charges move off an object

4 X, continued Lightning Dangers It is particularly dangerous to be at the beach or on a golf course during a lightning storm. Even standing under a tree during a storm is dangerous. Lightning Rods A lightning rod is a pointed rod connected to the ground by a wire. Objects that are joined to Earth by a conductor, such as a wire, are grounded. Objectives Describe electric current. Describe voltage and its relationship to electric current. Electric Current Electric current is the rate at which charges pass through a given point. amperes, or amps. Describe resistance and its relationship to electric current. Electric Current, continued Electric Current, continued AC and DC There are two kinds of electric current direct current (DC) and alternating current (AC). Commanding Electrons to Move This electric field is created so quickly that all electrons start moving through the wire at the same instant. Think of the electric field as a command to the electrons to charge ahead.

5 Voltage Voltage is the potential difference between two points in a circuit. (size of electricity) volts (V). Voltage and Energy Voltage is a measure of how much work is needed to move a charge between two points.you can think of voltage as the amount of energy released as a charge moves between two points in the path of a current. Voltage, continued Voltage and Electric Current As long as there is a voltage between two points on a wire, charges will flow in the wire. The size of the current depends on the voltage. Resistance Resistance is the opposition to the flow of electric charge. Good conductors, have low resistance. Poor conductors, have higher resistance. Factors That Affect Resistance Click below to watch the Visual Concept. Visual Concept You may stop the video at any time by pressing the Esc key. Resistance and Temperature In general, the resistance of metals increases as temperature rises.

6 Generating Parts of a Cell A cell contains a mixture of chemicals called an electrolyte. Every cell also has a pair of electrodes made from conducting materials. Kinds of Cells Two kinds of cells are wet cells and dry cells. The electrolytes in dry cells are solid or pastelike. Generating, continued Thermocouples Thermal energy can be converted into electrical energy by a thermocouple. Photocells A photocell converts light energy into electrical energy. Bellringer How fast is a nanosecond? A nanosecond (ns) is onebillionth of a second. Electrical signals travel at 30 cm/ns. Calculate how far electrical signals travel in 1 second. Record your answers in your science journal. Objectives Use Ohm s law to calculate voltage, current, and resistance. Calculate electric power. Determine the electrical energy used by a device. Compare the power ratings of different electrical appliances. Connecting Current, Voltage, and Resistance Georg Ohm ( ) studied the resistances of materials. He measured the current that resulted from different voltages applied to a piece of metal wire. Ohm s Law Ohm found that the ratio of voltage to current is a constant for each material. This ratio is the resistance of the material. R V, or V I R I

7 Connecting Current, Voltage, and Resistance, continued Electric Power Electric Power The rate at which electrical energy is changed into other forms of energy power voltage current, or P V I Watt (W) : unit for power the kilowatt (kw). One kilowatt is equal to 1,000 W. Electric Power, continued Power Ratings When you read the wattage label on a light bulb, you are reading the bulb s power rating. The power rating describes the rate at which an electric device uses electrical energy. Measuring The amount of electrical energy used in a home depends the power of the electrical devices in the house and the length of the time that those devices are on. The equation for electrical energy is as follows: electrical energy power time, or E P t

8 Measuring, continued Measuring Household Energy Use Different amounts of electrical energy are used each day in a home. Electric companies usually calculate electrical energy by multiplying the power in kilowatts by the time in hours. Measuring, continued It s All About the Bulb All of the lighting in a home could make up as much as 25% of a home s energy consumption. So, buying and using energy efficient light bulbs can make a big difference in the amount of energy used by a household. How to Save Energy Every appliance uses energy. Replacing items that have high power ratings with items that have lower ratings is a way to save energy. Bellringer Objectives What happens when you turn the lights on? What allows lights to be turned on and off? Write your answer or draw an explanatory picture for later reference in your science journal. What happens beyond the confines of the room when you flip the light switch? Name the three essential parts of a circuit. Compare series circuits with parallel circuits. Explain how fuses and circuit breakers protect your home against short circuits and circuit overloads. Parts of an Electric Circuit Forming a Loop An electric circuit is a complete, closed path through which electric charges flow. All circuits need three basic parts: an energy source, wires, and a load.

9 Types of Circuits Series Circuits all parts are connected in a single loop. Series Circuit Only one pathway for the electricity Uses for Series Circuits Series circuits are useful in wiring burglar alarms. If any part of the circuit fails, there will be no current in the system and the alarm will sound. Parallel Circuit Multiple pathways for the electricity Short Circuit Parallel Circuits a circuit in which loads are connected side by side. Uses for Parallel Circuits Almost all appliances are built with parallel circuits so that they will keep working if part of the system fails. Parts of an Electric Circuit, continued A Switch to Control a Circuit Sometimes, a circuit also contains a switch. A switch is used to open and close a circuit.

10 Household Circuit Safety Circuit Failure Broken wires or water can cause a short circuit. In a short circuit, charges do not go through one or more loads in the circuit. Fuses a thin strip of metal that burns in half if the current is too high. Circuit Breakers a switch that automatically opens if the current is too high. Household Circuit Safety, continued Circuit Breakers A circuit breaker is a switch that automatically opens if the current is too high. Charges stop flowing. Electrical Safety Tips Do not overload circuits by plugging in too many electrical devices. Do not use electrical devices near water. Electromagnetism Table of Contents Section 2 Magnetism from Electricity Section 3 Electricity from Magnetism Magnetism Any object that exerts a force towards iron is called a magnet. It was first discovered in the iron ore called lodestone The area around a magnet, that the magnetic force can be felt is called the Magnetic Field Area of a magnet where the magnetic force is strongest is called a Magnetic Pole Properties of Magnets, continued Magnetic Fields A magnetic field exists in the region around a magnet in which magnetic forces can act.

11 The Cause of Magnetism Atoms and Domains In materials such as iron, nickel, and cobalt, groups of atoms are in tiny areas called domains. The north and south poles of the atoms in a domain line up and make a strong magnetic field. The Cause of Magnetism, continued Losing Alignment When domains move, the magnet is demagnetized, or loses its magnetic properties. Making Magnets You can make a magnet from demagnetized material if you line up its domains with another magnet. The Cause of Magnetism, continued Cutting a Magnet When you cut a magnet in half, you end up with two magnets. Kinds of Magnets Temporary and Permanent Magnets Temporary magnets are made from materials that are easy to magnetize. But they tend to lose their magnetization easily. Permanent magnets are difficult to magnetize, but tend to keep their magnetic properties longer. Atoms and Domains Some magnets, called ferromagnets, are made of iron, nickel, cobalt, or mixtures of those metals. Another kind of magnet is the electromagnet. This is a magnet made by an electric current.

12 Earth as a Magnet One Giant Magnet Earth behaves as if it has a bar magnet running through its center. Poles of a Compass Needle The point of a compass needle is attracted to the south pole of a magnet. Opposite poles of magnets attract each other. South Magnetic Pole near North Geographic Pole A compass needle points north because the magnetic pole of Earth that is closest to the geographic North Pole is a magnetic south pole. Electromagnetism Earth s Magnetic Field Click below to watch the Visual Concept. Visual Concept Moving charges create a magnetic field A moving magnetic field can create an electric current in a wire (electromagnetic induction) electric motor is a device that converts electrical energy to mechanical energy You may stop the video at any time by pressing the Esc key. generator is just the opposite of the motor; it converts mechanical energy to electrical energy Section 3 Electricity from Magnetism