Electromagnetism. Magnetism & Electricity

Transcription

1 Magnetism Electricity Electromagnetism 1

2 Magnetism is a force created by magnets. Magnetism is also called magnetic force. Only magnets produce magnetic force. Magnetic force is invisible and can be felt by objects around or near a magnet. Magnetic force attracts or repels other objects or magnets. What is magnetism? 2

3 A magnet is an object that produces magnetic force. The strength of a magnet s magnetic force depends on the size of the magnet and the material the magnet is made of. Most magnets are made of iron, nickel or cobalt. However, not all iron, nickel or cobalt objects produce magnetic force. The strongest magnets are made of rare Earth metals. The strongest and most commonly used rare Earth metal magnet is called the Neodymium magnet. 3

4 A magnet has ends called poles. One pole is called the north pole (N). The other is called the south pole (S). A bar magnet is a straight, rectangular magnet. One end of the bar magnet is the north pole. The other end is the south pole. A horseshoe magnet is a curved, horseshoe-shaped magnet. One end of the curve is the north pole. The other end is the south pole. Magnets can also be shaped like disks, rings and cylinders. 4

5 The north pole of one magnet is attracted to the south pole of a second magnet. The north pole of one magnet is repelled by the north pole of a second magnet. For this reason, we say like poles repel and unlike poles attract. 5

6 You cannot separate the north and south pole of a magnet. If you cut a bar magnet in half, you get two smaller bar magnets. You do not get a north pole magnet and a south pole magnet. 6

7 All matter is made of atoms. Atoms have magnetic fields. Atoms group together when their magnetic fields align. These groups are called domains. In a magnet, domains point in the same direction. This produces an object with a uniform magnetic field. What makes a magnet magnetic? In a non-magnet, the domains point in random directions. 7

8 There are 3 main types of magnets: 1. A permanent magnet is an object made of matter that is permanently magnetized. Some iron, nickel or cobalt objects are permanent magnets. 2. A temporary magnet is an object that becomes a magnet through magnetization. Over time, a temporary magnet loses its magnetism. Iron, nickel and cobalt objects can become temporary magnets. 3. An electromagnet is a temporary magnet produced by an electric current. You will learn more about electromagnets later in this presentation. 8

9 Objects that are attracted or repelled by magnets are called magnetic materials. Most magnetic materials are made of iron, nickel or cobalt. Some alloys (mixture of metals) are magnetic as well. For example, steel is a magnetic alloy made of iron and carbon. Lodestone (magnetite) is a magnetic rock commonly found inside Earth. 9

10 If you run a magnet over a magnetic object, you can convert that object into a temporary magnet. This conversion is called magnetization. When magnetizing an object, you must run the magnet in the same direction over that object a few times. This causes the object s domains to align which makes a magnet. Only magnetic objects can be magnetized. 10

11 Demagnetization is the loss of a magnet s magnetic force. Heating, hammering or dropping a magnet can cause demagnetization. Temporary magnets are easy to demagnetize. Permanent magnets are more difficult to demagnetize. It s important to handle permanent magnets carefully. Jarring a permanent magnet will weaken it over time. Heating a permanent magnet to a very high temperature can completely demagnetize it. 11

12 The area of a magnetic force around a magnet is called the magnetic field. Magnetic objects are influenced by a magnet if in the magnet s magnetic field. The strength of the magnetic field weakens as you move away from the magnet. Stronger magnets have larger and more powerful magnetic fields. 12

13 The magnetic field begins at the magnet s north pole. It travels outward and around a magnet to the south pole and then through the magnet from the south pole to the north pole. Magnetic field lines are lines we draw to help us visualize a magnetic field. Magnetic field lines are drawn with arrows to show the direction of the magnetic field. Magnetic field lines never cross. 13

14 A magnetic field is invisible. However, you can visualize a magnet s magnetic field with iron filings. Iron filings are small, needleshaped pieces of iron that orient themselves along the magnetic field of a magnet. In the photo, you can see the bar magnet s magnetic field because of the iron filings. 14

15 The Earth is made of layers. The outer layer is called the crust. Below the crust is the mantle. The center of Earth is called the core. The core is made of iron. The outer core is made of molten, flowing iron. The inner core is made of solid iron. 15

16 Earth s iron core acts like a giant bar magnet. It creates a magnetic field around Earth. Earth s magnetic field extends outward around Earth and down towards the core of Earth at the poles. 16

17 Earth s magnetic field forms a protective shield around Earth. This shield is called the magnetosphere. The magnetosphere extends 37,000 miles out into space! 17

18 The magnetosphere protects us from solar wind. Solar wind is a dangerous stream of charged particles hurled out from the Sun. These particles deflect off Earth s magnetosphere. 18

19 Most solar wind particles deflect off of the magnetosphere. However, some particles are trapped by Earth s magnetic field at the poles. These particles bump into gas molecules in the Earth s atmosphere at the poles. The bumping excites the gas molecules and causes them to emit bright colors of light. This phenomenon is called an aurora. Aurorae occur at both poles. An aurora at the North pole is called aurora borealis or Northern lights. An aurora at the South pole is called aurora australis or Southern lights. 19

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21 Earth s iron core also creates magnetic poles on Earth. Earth s magnetic poles are not the same at Earth s geographic poles. The magnetic poles are located about 965 km or 10 from Earth s geographic poles. 21

22 The magnetic poles move over time due to changes in the flow of molten iron in Earth s outer core. This movement is called Polar Drift. The poles drift 55 to 60 km each year. The picture shows drift of the magnetic North pole in relation to the geographic North pole. 22

23 A compass is a device that detects Earth s magnetic field and points towards the magnetic poles. A compass contains a small magnetic needle. The needle is suspended so it can spin freely inside the compass casing. The needle spins to align with Earth s magnetic field and points towards the magnetic poles. A compass is a great tool for navigation. It helps a person determine North, South, East and West and orients a person as to his/her location on Earth. 23

24 The North pole of a compass needle points toward the North geographic pole. Remember that the North pole of one magnet is attracted to the South pole of a second magnet. If the North pole of the compass needle points North, technically Earth s magnetic South pole is located at the geographic North end of Earth. 24

25 When far away from the North pole, a compass works well. The difference between the geographic North pole and magnetic pole is not significant. You can assume that the needle points towards the true, geographic North pole. 25 When far away from the poles, a compass works well. The difference between the magnetic and geographic pole is not significant.

26 When you are close to the North pole, a compass does not work. The difference between the geographic North pole and magnetic pole is significant. The direction of the needle will point about 10 off from true North, which is a big deal if trying to navigate around the North pole. When close to the poles, a compass does not work well. The difference between the magnetic and geographic poles is significant. 26

27 The stripe on the back of a credit card is a magnetic stripe. It is called a magstripe. The magstripe is made of tiny iron-based magnetic particles. These particles are selectively magnetized to store data or information. A credit card that is exposed to a strong magnetic field or heat no longer works because the magstripe becomes demagnetized. 27

28 A computer hard drive is made of magnetic material that can be magnetized to store data and demagnetized to erase it. Data is stored in extremely small magnetic domains on platters on the hard drive. The arm stores, reads or erases the information on the platters. The information on a hard drive can be erased if the computer is exposed to extreme heat or strong magnetic fields. 28

29 MRI (magnetic resonance imaging) is a type of medical imaging that produces images of the body. These images are used to diagnosis diseases and anatomy abnormalities. An MRI scanner uses strong magnets and magnetic fields to create images of the body. Unlike X-ray and CT-Scan, MRI does not expose a patient to radiation. 29

30 Electric force is an invisible force created by an electrically charged particle. Only electrically charged particles produce electric force. Particles with an electric charge are attracted to or repelled by other particles with an electric charge. Objects can become electrically charged if electrically charged particles build up on that object. What is electric force? 30

31 An electrically charged particle (simply called a charge) can have a positive (+) or a negative ( ) charge. Particles with opposite charges attract each other. Particles with like charges repel each other. This attraction and repulsion is electric force. Particles without an electric charge are neutral. Neutral particles are not attracted to or repelled by particles with an electric charge. 31

32 The area of electric force around an electrically charged particle is called an electric field. A particle with a more electric charge produce a stronger and larger electric field. The strength of the electric field decreases as you move away from the electrically charged particle/object. 32

33 An electric field is always directed away from a positive charge and towards a negative charge. We draw the electric field around a particle or between particles with lines called electric field lines to help visualize an electric field. Electric field lines are drawn with arrows to show the direction of the electric field. Electric field lines never cross. 33

34 Energy is contained within an electric field. This energy is called electric energy. Electric energy is also called electricity. Electric energy can be transferred or stored. In other words, the energy can move from one place to another or it can build up on an object. Electricity that builds up on an object is called static electricity. Electricity that moves from one place to another is called current electricity. When we use the term electricity, we often are talking about current electricity. However, you should know there are actually two types of electricity. 34

35 All objects have both positive and negative charges. Neutral objects have equal amounts of positive and negative charges. If negative charges build up on an object, the object becomes negatively charged. If positive charges build up on an object, the object becomes positively charged. An electric field is created by the build up of charge on surface the object. The electric field generates static electricity. It s called static electricity because the charges remain on the surface of an object and do not move or flow. 35

36 When two objects rub against each other quickly, charges from one object can transfer to the other object. Specifically, electrons (negative charges) are transferred from one object to the other. This results in two charged objects. The object that gains electrons becomes negative. The object that loses electrons becomes positive. Objects with different charges will attract. Objects with like charges will repel. 36 How does static electricity build up on an object?

37 Example: 1. Rub a neutral balloon with a neutral wool mitten. 2. The balloon will gain negative charges from the mitten and become negative. The mitten becomes positive because it loses negative charges. 37

38 3. Press the balloon against a neutral wall. The balloon will attract positive charges on the wall and stick to it. 4. Over time, the balloon loses excess negative charges and static electricity. The balloon becomes neutral and falls. 38

39 When you go down a slide, the rubbing between you and the slide creates static electricity. You accumulate negative charges. The charges build up in your hair. Because each hair has the same charge, the hairs repel each other and stick straight up! 39

40 The loss of static electricity is called electric discharge. You may see a spark or feel a zap when an object suddenly loses static electricity. When you rub your feet on carpet, charge builds up on you and you gain static electricity. When you touch something, you discharge the excess charges. If you touch a metal object, like a door handle, you may seem a small spark or feel a zap. This is because metal is very conductive and discharges static electricity very quickly. 40

41 Lightning is a powerful form of electric discharge. 41

42 During a thunderstorm, water droplets and small bits of ice bump into each other in a thundercloud. The collision between these droplets creates a build up of positive and negative charges in the thundercloud. The negative charges sink to the bottom of thunderclouds. 42

43 Beneath the thundercloud positive charges rise and build up on structures and ground. This occurs because the negative charges in the thundercloud attract the positive charges on and near Earth s surface. 43

44 The negative charges discharge from the cloud to the ground or objects on the ground. The movement of charges (called electric current) is so strong that it produces a bright flash of light (lightning). Lightning can also occur within a cloud. This is called intracloud lightning. 44

45 Flowing electrical energy is called current electricity. It is called so because a flow of charges is called current. When charges move, they carry electrical energy (electricity) with them. Electricity is used to power electrical devices. 45

46 In order for charges to move, they must have a path to travel. This path is called a circuit. A circuit is a closed loop around which a current flows. A circuit consists of a power source, wire and electrical devices. The power source provides a force or pressure to push the current. The wire connects electrical devices and provides a path for the current to travel. The electrical devices use electrical energy. 46

47 A battery generates electricity by pushing current through a circuit. A battery has three parts: positive terminal (cathode) negative terminal (anode) electrolytes The terminals are found on the ends of the battery. The inside of a battery contains electrolytes (positive and negative particles). 47

48 When the terminals are connected to the wire of a circuit, reactions occur inside the battery. The reactions cause negative charges (electrons) to build up at the negative terminal. The electrons move from the negative terminal through the circuit to the positive terminal to generate current. 48

49 When we study the flow of current, we consider the direction of positive charges. Positive charges would move from the positive terminal to the negative terminal the opposite of the actual flow of current. This is called conventional current. It may seem backwards to study current this way, but it s how scientists around the world describe the flow of current. 49

50 A circuit cannot have breaks. It must be a closed and continuous loop. A circuit without breaks is called a closed circuit. The power source and electrical devices must be attached to the circuit so that the current has a path to travel from the negative terminal to the positive terminal of the battery (or batteries). 50

51 If there is a break in a circuit or the circuit is not a closed, continuous loop, no current will flow. This is called an open circuit. 51

52 If wires are not attached to the power source correctly, the circuit is open. If wires are not attached to appliances correctly, the circuit is open. 52

53 Some circuits have a switch. The switch acts as a circuit breaker. When the switch is open, the circuit is open and current will not flow. When the switch is closed, the circuit is closed and current will flow. 53

54 Electricity travels more easily through certain material. Materials that allow electricity to flow freely are called conductors. Metals are good conductors. Copper, silver and aluminum are commonly used conductors. Electrical wire is usually made of copper because it is inexpensive but a good conductor. 54

55 Insulators are materials that do not allow electricity to flow easily. Rubber and plastic are insulators. Electrical wires are often wrapped with an insulator to keep current flowing through the wire and to prevent electric shock. Electric shock occurs when electric current jumps from a circuit to your body. Depending on the strength of the current, electric shock can be very dangerous or even deadly. 55

56 An electromagnet is a magnet created by an electric current. When the wire connected to a battery (or other power supply) is coiled or wrapped around a magnetic object, the object becomes a magnet. Current must be able to flow through the wire (the circuit must be closed). When the wire is disconnected from the battery, the object loses its magnetism. 56

57 The strength of an electromagnet depends on: 1. The voltage produced by the power source. A stronger power source produces more voltage. More voltage produces a stronger current. A stronger current will produce a stronger electromagnet. 2. The number of times the wire is coiled. Increasing the times a wire is wrapped around a magnetic object will produce a stronger electromagnet. 57

58 An electric current can be created by a magnet. If you place a loop of wire inside a magnet s magnetic field and then move the magnet, you create an electric current in the wire. A current is created because a moving magnet has a moving magnetic field, which pushes charges (electrons) in the wire. An electric generator is a device that uses magnets to create an electric current and thus, electricity. 58

59 How an electric generator works: A loop of wire is placed within a magnet. The loop of wire is connected to a rotating shaft. When the shaft rotates, the loop of wire rotates within the magnet. A rotating loop of wire within a magnet has the same result as a rotating magnet around a loop of wire. A current, and thus electricity, is generated in the wire. 59

60 Virtually all the electricity we use is produced by massive electric generators at power plants across the country. At most power plants, fossil fuels are burned to heat water to make steam. The steam spins a big fan called a turbine. The turbine rotates the shaft in an electric generator. The shaft spins the loop of wire inside the magnet, which generates electricity. 60

61 Some power plants do not burn fossil fuels to turn the turbine. A hydropower plant uses running water to turn the turbine. A windmill farm uses wind to turn the turbine. Because these power plants do not burn fossil fuels, no air pollution is produced. 61

62 Images obtained from commons.wikimedia.org courtesy: Aney, Bios, Evan Amos, Aniali, smithlf, RobertKulhmann, Hborkyb, Cavit Other images obtained from the Public Domain Clipart by: Stephanie Elkowitz Atoms & Reactions 62