Magnetic Attraction and Electromagnetism. Spring 2011

Transcription

1 Magnetic Attraction and Electromagnetism Spring 2011

2 The Nature of Magnetism Magnets are found everywhere doorbells, TV s, computers Magnets were discovered in a region in Greece called.you guessed it Magnesia!!! They discovered the unusual rock about 2,000 years ago.

3 The Nature of Magnetism The rock had strange properties It attracted anything that contained iron. This rock is the mineral we call magnetite. Magnetism is defined as the attraction of a magnet to another object. About 1,000 years ago people discovered that magnets have another unusual property

4 The Nature of Magnetism If you swing a rock freely from a string one part of the rock always points in the same direction toward a specific star northern star (also called the lodestar). For this reason magnetic rocks are sometimes called lodestones.

5 Magnetic Poles Any magnet has 2 ends each is a magnetic pole. This is the area where the magnetic effect is the strongest just like one end of a magnetite rock always points on direction One pole will always point north and therefore it is called the north pole. The other is, of course, the south pole. The north and south pole are direct opposites.

6 Attraction... What happens when you bring 2 magnets together? If you bring the 2 north poles together they repel or push away from each other. The same is true for 2 south poles. If you bring one north and one south pole together they are attracted to each other. So alike poles repel unlike poles attract. Any material that exerts a magnetic attraction is considered a magnet.

7 So What s the Big Deal?? The Maglev Train magnetically levitating train Runs solely on magnetism Fast, efficient, and does NOT harm the environment

8 What happens if??? What happens if you break a magnet? Do you get one south pole and one north pole magnet? No actually when a magnet is broken in will develop another pole on that end so it will always have a north and a south end. If you break them again the same process would happen.

9 MAGNETO The character Magneto from the X-Men comic series has the ability to create magnetic fields and use magnetic force to control objects.

10 So, What are Magnetic Fields??? Magnetic forces are exerted all around a magnet, but is strongest at the poles. The region of magnetic force around an object is called the magnetic field. A magnetic field allows magnets to interact without actually touching.

11 Magnetic Fields Magnetic field lines map out the magnetic field around an object. These lines spread out from pole to pole and curve around the magnet to return to the other pole. See the diagram at left.

12 Magnetic Properties Depend on the structure of the atoms making up the object. A spinning electron (negatively charged) creates a magnetic field. A bar magnet has what is called a magnetic domain. It has a strong representation and therefore a stronger attraction. Materials that have a very strong attraction to a magnet are said to be ferromagnetic material. These are items made from iron, nickel, cobalt, samarium, etc.

13 Creating Magnets You can make a magnet by taking ferromagnetic material and place it in a magnetic field or rub a magnet against it. The field has to be relatively strong for this to occur. Once the domains line up your magnet is created. This is why you can magnetize a paperclip to pick up other paperclips. Unfortunately, most of theses are only temporary. Permanent magnets are much more difficult to make and hold their magnetism indefinitely

14 Destroying Magnets You can destroy a magnet field by striking the magnet very hard or dropping it as well. This knocks the domains out of alignment. Heating a magnet will also destroy the magnetism. Above a certain temperature, materials lose their ferromagnetism properties altogether.

15 Compass Defined as a device that has a magnetized needle than can spin freely. The needle usually points north. This occurs because the Earth acts like a giant magnet. Earth has an enormous magnetic field surrounding it just like a huge bar magnet. A scientist named Gilbert believed the center of the Earth contained magnetic rocks unfortunately, he was wrong, but it was a great start.

16 More on Compasses Scientists now know that the magnetism is partially caused by the circulation of molten material made from iron and nickel. The simple fact that Earth has a magnetic field explains why a compass actually works. The magnetized needle aligns its domain with that of the Earth therefore pointing North.

17 Magnetosphere Earth s magnetic field extends into space. Electrically charged particles affect Earth s magnetic field. The Van Allen Belts are located about 1,000-25,000 km above the Earth s surface. In this particular region, electrons and protons move at very high speeds.

18 Solar Winds from the sun also carry electrically charged particles to Earth. The region where Solar Winds shape Earth s magnetic fields is called the magnetosphere. This is a continual process as Earth rotates on it s axis. When the particles move too close to Earth s surface, we see the Aurora Borealis or Northern Lights in the sky. (called the Southern Lights in the Southern hemisphere ) Magnetosphere

19 Earth s Magnetic Field Earth s magnetic directionality has changed throughout history. From time to time the poles will switch due to the magnetic field created by the Earth. The last time the poles switched was about 780,000 years ago. Scientists are perplexed as to why this occurs, but believe it has something to do with the change in flow of the magnetic molten material at Earth s core.

20 Electric Charge Charges Exert Force Atoms are composed of particles with electric charge. The law of electric charges states that like charges repel and opposite charges attract.

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22 Electric Charge, continued The Force Between Protons and Electrons Because protons and electrons have opposite charges, they are attracted to each other. The Electric Force and the Electric Field The force between charged objects is an electric force. An electric field is the region around a charged object in which an electric force is exerted on another charged object.

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24 Charge It! Friction Charging by friction happens when electrons are wiped from one object onto another. Conduction Charging by conduction happens when electrons move from one object to another by direct contact. Induction Charging by induction happens when charges in an uncharged metal object are rearranged without direct contact with a charged object.

25 Charge It!, continued

26 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. Detecting Charge You can use a device called an electroscope to see if something is charged.

27 Static Electricity Static electricity is the electric charge at rest on an object. Electric Discharge The loss of static electricity as charges move off an object is called electric discharge. One of the most dramatic examples of electric discharge is lightning. The next slide shows how lightning is formed.

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29 Static Electricity, 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.

30 Electric Current AC and DC There are two kinds of electric current direct current (DC) and alternating current (AC).

31 Voltage Voltage is the potential difference between two points in a circuit. Voltage is expressed in 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.

32 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. Varying Nature of Voltage Different devices need different levels of voltage.

33 Resistance Resistance is the opposition to the flow of electric charge. Resistance and Material Good conductors, such as copper, have low resistance. Poor conductors, such as iron, have higher resistance. Resistance, Thickness, and Length Thick, short wires have less resistance than thin, long wires.

34 Resistance, continued Resistance and Temperature In general, the resistance of metals increases as temperature rises.

35 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 I, or V = I R

36 Electric Power What Is Electric Power? The rate at which electrical energy is changed into other forms of energy is electric power. Electric power is calculated using the following equation: power = voltage current, or P = V I Watt: The Unit of Power The unit for power is the watt (W). Another common unit of power is the kilowatt (kw). One kilowatt is equal to 1,000 W.

37 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.

38 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.

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40 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.

41 Types of Circuits Series Circuits A series circuit is a circuit in which all parts are connected in a single loop. 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.

42 Types of Circuits, continued Parallel Circuits A parallel circuit is 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.

43 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 fuse has a thin strip of metal. Fuses keep charges from flowing if the current is too high.

44 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.

45 It s Electric! Electrons carry a negative charge. Protons carry a positive charge. When these charges flow through a wire or other object the create an electric current. Electric currents are defined as the flow of a charge through a material. The amount of charge that passes through the wire in a unit of time is measured in amperes (an amp).

46 It s Electric! An electric current produces a magnetic field. The direction of the current determines the direction of the magnetic field. If the current reverses, so does the field.

47 Electric currents do not automatically flow through wires. Current only flows through electric circuits. An electric circuit is a complete path through which electric charges can flow. All electrical devices contain electric currents. Electric Circuits

48 Conductors Electric currents do not flow through all objects. Electric currents move freely through objects called conductors. Examples of conductors: copper, silver, iron, and aluminum are all good conductors. In a conductor, some of the electrons are only loosely bound to their atoms. These electrons are able to move around and generate an electric current.

49 Light switch??? Did you ever wonder why a light comes on when you flip a switch? Electrons are not created every time you flip a switch they are always there in the conductors of the circuit.

50 Insulators Insulators are a different kind of material where charges are not allowed to move freely. The electrons are bound tightly to their atoms and do not flow easily. Examples of good insulators: rubber, glass, sand, plastic, and wood.

51 Electrical Resistance A resistor uses electrical energy as it interferes with, or resists, the flow of charge. Resistance is the opposition to the movement of charges flowing through a material. Resistance depends on an objects atomic structure. (Like trying to cross a crowded room the more collisions the more energy is converted.)

52 Light Bulb Edison used resistance when developing his light bulb. He needed a material that conducted electrical currents, but that would offer enough resistance to make the material heat up and glow. He tried a variety of items: copper wires, silk fibers, shredded corn husks to name a few. He eventually used the metal tungsten for the purpose.

53 SUPERCONDUCTORS!!! A superconductor is a material that has no electrical resistance. This normally occurs at very low temperatures. (That s the problem!) There is no loss of energy and therefore the electrical device is much more efficient. They also strongly repel magnets. A strong magnetic field would destroy a superconductor.

54 Electric Motors Device that changes electrical energy into mechanical energy. All electric motors have an armature a loop or coil of wire that can rotate. Armatures are mounted between the poled of an electromagnet. A Commutator reverses the current.

55 Electricity from Magnetsim Galvanometers - device that measures current Runs by using an electromagnet Hans Christian Oersted discovered that an electric current could make a magnetic field. Michael Faraday first to publish results in great detail. Electromagnetic induction electric current is made by changing a magnetic field.

56 Electric Current continued An electric current is created when a magnet moves in a coil of wire or when a wire moves between the poles of a magnet. Electric Generators uses electromagnetic induction to change mechanical energy into electrical energy. Electric generators generate alternating currents. Transformers increases or decreases voltage of an alternating current.

57 Electromagnets Solenoids is a current carrying wire with many loops. The solenoid creates a magnetic field at the center of the coil. The 2 ends act like poles and can be turned off or on by a switch. If you place a ferromagnetic material inside a solenoid the magnetic field is increased. A solenoid with a ferromagnetic core is called an Electromagnet!

58 Electromagnets are strong magnets that can be turned off or on. These are ideal for lifting large pieces of scrap metal like at a junk yard. When the switch is on the magnet is activated. When the switch is turned off the magnet is no longer magnetized. You can increase and decrease the power of the electromagnet by increasing or decreasing the number of coils of the solenoid or by using a stronger ferromagnetic material. Electromagnets

59 FINITO!! That s All Folks!!