Lecture One

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Lecture One

  1. 1. M agnetic R esonance I maging “MRI” BME 551 Dr. Rami J. Oweis
  2. 2. Electricity and Magnetism <ul><li>Static Electricity </li></ul><ul><ul><li>Electrical Charges </li></ul></ul><ul><ul><li>Basics of Static Electricity </li></ul></ul><ul><ul><li>Explaining Static Electricity </li></ul></ul><ul><ul><li>Materials and static electricity </li></ul></ul><ul><ul><li>Causing Sparks and Lightning </li></ul></ul><ul><ul><li>Controlling Static Electricity </li></ul></ul><ul><ul><li>Generating Static Electricity </li></ul></ul><ul><ul><li>Uses for Static Electricity </li></ul></ul>
  3. 3. Electricity and Magnetism <ul><li>Magnetism </li></ul><ul><ul><li>Magnetism and Lorentz Force </li></ul></ul><ul><ul><li>Magnets </li></ul></ul><ul><ul><li>Detection of a Magnetic Field </li></ul></ul><ul><ul><li>Factors Determining Magnetic Response </li></ul></ul><ul><ul><li>Magnetic Materials </li></ul></ul><ul><ul><li>Electromagnetism </li></ul></ul><ul><ul><li>Electromagnetic Devices and Inventions </li></ul></ul>
  4. 4. Electricity and Magnetism <ul><li>Electrical Current and Circuits </li></ul><ul><ul><li>Direct Current (DC) Electricity </li></ul></ul><ul><ul><li>Ohm's Law for Electrical Circuits </li></ul></ul><ul><ul><li>Series and Parallel DC Circuits </li></ul></ul><ul><ul><li>Alternating Current (AC) Electricity </li></ul></ul><ul><ul><li>Alternating Current (AC) Transformers </li></ul></ul><ul><ul><li>Worldwide AC Voltages and Frequencies </li></ul></ul><ul><ul><li>AC Home Wiring </li></ul></ul><ul><ul><li>Generating Electrical Current </li></ul></ul><ul><ul><li>Electric Power </li></ul></ul>
  5. 5. Electricity and Magnetism <ul><li>Electromagnetic Radiation </li></ul>
  6. 6. Electricity and Magnetism <ul><li>Why do we first need to have a look at electric, magnetic fields, and electromagnetic radiation? </li></ul><ul><li>Understanding of the fundamental physical concepts of electricity and magnetism is necessary for an understanding of MRI </li></ul>
  7. 7. Electricity and Magnetism <ul><li>Electricity is used to produce the primary static magnetic field (Bo) and the secondary gradient magnetic fields (B X , B Y , B Z ). </li></ul><ul><li>These magnetic fields interact with the intrinsic nuclear magnetism of tissue, the proton dipoles. </li></ul><ul><li>The tissue is excited with a radio frequency (RF) pulse produced by an electrically stimulated coil, which usually also receives the magnetic resonance (MR) signal emitted from the body. </li></ul>
  8. 8. Electricity and Magnetism <ul><li>We will explain the following concepts: </li></ul><ul><ul><li>Electrostatics and electrodynamics to develop an understanding of electricity </li></ul></ul><ul><ul><li>The phenomenon of magnetism, which leads into electromagnetism and electromagnetic radiation </li></ul></ul>
  9. 9. Electrical Charges <ul><li>One property of matter is the electric charge . Most sub-atomic particles have either a positive (+) or negative (-) electrical charge. Those that don't are considered neutral. </li></ul><ul><li>The most common charged particles are the electron and proton. </li></ul><ul><li>Atoms with an excess of electrons are called negative ions. Those with missing electrons are called positive ions. </li></ul><ul><li>There is an electrical field that flows between opposite charges, causing an electrical force. This results in an attractive force between the opposite charges and a repelling force between like charges. </li></ul>
  10. 10. Questions to answer <ul><li>Which particles have charges? </li></ul><ul><li>What does the electric field look like? </li></ul><ul><li>What does the electrical force do? </li></ul>
  11. 11. Charged particles <ul><li>An atom is comprised of a nucleus consisting of protons and neutrons and a collection of electrons in orbits or shells around the nucleus. Protons, neutrons and electrons are the most common sub-atomic particles. </li></ul><ul><li>Sub-atomic particles have a positive (+) electrical charge, a negative (-) electrical charge, or no electrical charge at all. For example, a proton has a positive electric charge, an electron has a negative electric charge, and a neutron is neutral and has no electrical charge. </li></ul>
  12. 12. Ions <ul><li>An atom typically has the same number of negative charged electrons as positive charged protons, so its total charge is neutral. But if the atom loses some electrons, it will have more positive charges than negative charges and is called a positive ion. Likewise, if the atom gains an excess of electrons, it is called a negative ion. </li></ul><ul><li>Ions are charged particles. They are often involved in static electricity and electrical current on electrolyte solutions such as salt water. </li></ul>
  13. 13. Unipoles <ul><li>Electrically charged particles are called unipoles, in that they can exist by themselves (&quot;uni&quot; means one). This is different than the case of magnetic poles, where for every N pole, there must be an S pole. Magnets are called dipoles, meaning they must have two poles. </li></ul>
  14. 14. Anti-matter <ul><li>There is duality in the Universe. That means if there is a left hand, there will be a right hand. That also works with particles. </li></ul><ul><li>Since there is a negative charged electron, there is also a version with a positive charge. That anti-electron particle is called the positron. It is the same size and weight as an electron, except it has an opposite charge. </li></ul><ul><li>For the positive charged proton, there is the anti-proton that has a negative charge. These oppositely charged particles are called anti-matter. </li></ul><ul><li>There is even an anti-neutron. It is still neutral in electrical charge, but it spins in the opposite direction. </li></ul>
  15. 15. Electrical field <ul><li>An electrical field surrounds every particle that has an electrical charge. By convention, the lines of the electric field are said to radiate from a (+) particle and move towards a (-) particle. It is not certain if there is any direction of radiation, and there is no real good explanation of what the electric field is made of. It's just there. </li></ul><ul><li>When a positive charged particle (+) like a proton is near a negative charged particle (-) like an electron, the electrical field goes from one to the other. </li></ul>
  16. 16. Electrostatics Out from positive charge Toward a negative charge Like charges repel one another Unlike charge attract each other uncharged fields no electric field The electric field is most easily visualized as imaginary lines radiating from an electric charge
  17. 17. Electric field lines shown moving from a positive particle
  18. 18. Electrical field points from (+) to (-)
  19. 19. Forces acting on charged particles <ul><li>The electrical field acts like a force at a distance and the lines are considered lines of force </li></ul><ul><li>Opposite charges attract: When a positive charged particle is near a negative charged particle, they are attracted to each other by the lines of force. </li></ul>
  20. 20. Static electricity <ul><li>Static electricity is a good example of opposite charges attracting. If electrons are collected on the surface of one material and positive ions are collected on another surface, the negative and positive charges attract. Either the materials are pulled together or a stream of electrons jumps the gap as a spark. </li></ul><ul><li>Note that since protons are in the nucleus, they never collect on a surface in static electricity. Rather, they contribute to the charge of the ions that have lost electrons. </li></ul>
  21. 21. Colliding with anti-matter <ul><li>If an electron would come near its anti-matter twin, the positron, they would be attracted to each other until they collide. Then they would each be annihilated with a large amount of energy given off in the form of electromagnetic radiation and other smaller sub-atomic particles moving at a high speed. Likewise, if a proton comes into contact with an anti-proton, they will annihilate, giving off a large amount of energy. </li></ul>
  22. 22. Not colliding with other matter <ul><li>Although they have opposite charges and are attracted to each other, an electron will never combine with a proton. The reason has to do with other forces being involved that keep them apart, as explained in Quantum Theory. This is the same reason that electrons don't go crashing into the positive charged atomic nucleus. </li></ul>
  23. 23. Like charges repel <ul><li>When particles have the same charge, they repel each other </li></ul><ul><li>This can be seen in a static electricity experiment. Attach strings to two balloons and rub them both on a wool sweater. Then when you hang the balloons next to each other, you can see the electrical forces push them apart </li></ul>Like charges push away from each other
  24. 24. Basics of Static Electricity <ul><li>Static electricity is the situation where electrical charges build up on the surface of a material. It is called “static” because there is no current flowing as in AC or DC electricity. </li></ul><ul><li>Static electricity is usually caused when materials are rubbed together. The result is that objects may be attracted to each other or may even cause a spark to jump from one object to the other. Common examples of static electricity in action are static cling, flyaway hair and the sparks that can occur when you touch something. </li></ul>
  25. 25. Cause of static electricity <ul><li>Static electricity is usually caused when certain materials are rubbed against each other, like wool on plastic or the soles of your shoes on the carpet. The process causes electrons to be pulled from the surface of one material and relocated on the surface of the other material. </li></ul><ul><li>The material that loses electrons ends up with an excess of positive (+) charges. The material that gains electrons ends up an excess of negative (-) charges on its surface. </li></ul>
  26. 26. <ul><li>Rubbing a balloon on a wool sweater  creates charges on the surfaces </li></ul>
  27. 27. Electrons pulled from orbit <ul><li>The gain or loss of electrons can be explained by recalling that atoms consist of a nucleus of neutrons and positively charged protons, surrounded by negatively charged electrons. Normally, there is the same number of electrons as protons in each atom. </li></ul><ul><li>But if some object pulls away electrons from their orbit or shell around the nucleus, that causes the atom to have a positive charge because it has more protons than electrons. Likewise, the other material will have extra electrons in its shell, giving the atoms a negative charge. </li></ul>
  28. 28. Charges on surface <ul><li>Note that the charged atoms are on the surface of the material. Static electricity is different than regular electricity that flows through metal wires. Most of the time the materials involved in static electricity are nonconductors of electricity. </li></ul><ul><li>If electrical charges build up on the outside of a metal, most of them will dissipate into the metal, similar to an electrical current. </li></ul>
  29. 29. Prefers dry air <ul><li>When the air is humid, water molecules can collect on the surface of various materials. This can prevent the buildup of electrical charges. The reason has to do with the shape of the water molecule and its own electrical forces. </li></ul><ul><li>Thus, static electricity is formed much better when the air is dry or the humidity is low. </li></ul>
  30. 30. Force field causes attraction <ul><li>An object that has static electricity charges built up on its surface has an electrical force field coming from the surface. This field will mildly attract neutral objects or those with no charge. The field will strongly attract an object that has an opposite charge on its surface. From this we get the expression: &quot;Opposites attract.&quot; </li></ul><ul><li>If two objects have the same charge, the electrical force field will cause those objects to push away from each other or repel. </li></ul>
  31. 31. Attraction <ul><li>Rub a balloon on a wool sweater. The balloon collects negative electrical charges on its surface and the wool collects positive charges. You can then stick the balloon to the wall, which does not have an excess of either charge. The balloon will also stick to the wool, although the charges may jump back to the original material in a short time. </li></ul><ul><li>You can also run a comb through your hair to charge the comb with static electricity. The comb can then be used to attract neutral pieces of tissue. </li></ul>
  32. 32. <ul><li>Picking up tissue with a comb </li></ul>
  33. 33. Repulsion <ul><li>Comb your hair on a dry day or after using a hair drier. The plastic comb collects negative charges from the hair, causing the hair to have an excess of positive charges. Since like charges repel, the hair strand will tend to push away from each other, causing the &quot;flyaway hair&quot; effect. </li></ul>
  34. 34. Why sparks fly <ul><li>When two objects that have opposite charges get near each other, the electrical field pulls them together. </li></ul><ul><li>What actually happens is that the negatively charged (-) electrons are attracted to the atoms in the other material that have an excess positive (+) charge. Things are much more stable if all the atoms have an equal number of (+) and (-) charges. </li></ul>
  35. 35. Strong forces hold electrons <ul><li>The reason the electrons can't leave their present material is because of strong molecular forces that keep them where they are. If there are enough positive (+) charges attracting them, and the distance is not too great, some of the electrons will break loose and fly across the gap to the (+) side. </li></ul>
  36. 36. Once it starts <ul><li>Once a few electrons start to move across the gap, they heat up the air, such that more and more will jump across the gap. This heats the air even more. It all happens very fast, and the air gets so hot that it glows for a short time. That is a spark. </li></ul><ul><li>The same thing happens with lightning, except on a much larger scale, with higher voltages and current. </li></ul>
  37. 37. Explaining Static Electricity <ul><li>Static electricity occurs when there are an excess of positive (+) or negative (-) charges on an object's surface. This condition is caused from rubbing certain materials together. Static electricity is not caused by friction, as is popularly thought. The position of the material in the Triboelectric Series determines how effectively the charges will be exchanged. </li></ul>
  38. 38. Causing excess charges <ul><li>Putting certain materials together and then pulling them apart causes excess electrical charges to be created on their surfaces. This can be done by pushing them together and pulling them apart or by rubbing the materials together, which is the main way static electricity is created. </li></ul>
  39. 39. Excess of charges <ul><li>Most matter is electrically neutral. That means its atoms and molecules have the same number of electrons as protons. If a material somehow obtains extra electrons and attaches them to the atom's outer orbits or shells, that material has a negative ( - ) charge. Likewise, if a material loses electrons, it has an excess of positive (+) charges. The electric field from the excess of charges then causes the static electric effects of attraction, repulsion or a spark. </li></ul>
  40. 40. Stealing electrons <ul><li>According to Solar System Model (or Bohr Model) of the atom, electrons are in orbits or shells around the nucleus. A maximum number of electrons are allowed in each orbit. Forces in each atom seek to reach that maximum number, such that if an element is just one electron short of the maximum amount in its outer orbit, it would try to &quot;steal&quot; an electron from another element that may be just starting its outer orbit. This is the basis of chemical reactions. </li></ul>
  41. 41. Adhesive force takes electrons <ul><li>That force will also tend to hold two different materials together. In that situation, the force is called the adhesive molecular force. When different materials are pressed together and then pulled apart, the adhesive molecular force pulls electrons from material unto the other. This creates the static electricity. </li></ul><ul><li>You can see this effect with a piece of Scotch tape or similar tape. First verify that it is not attracted to your finger. Then stick it to some surface and then pull it off. Put you finger near the tape and it will now be attracted to your finger, showing that there is an excess of charges on the tape. </li></ul>
  42. 42. Not friction <ul><li>Although your can create static electricity by pressing materials together and pulling them apart, rubbing them together works even better, except in the case of something sticky like tape. </li></ul><ul><li>One unfortunate result from saying that rubbing materials creates static electricity is that most people think that friction causes the charges to build up. It is not friction that causes static electricity, rather it is the adhesive forces that pull off electrons. </li></ul>
  43. 43. Triboelectric Series <ul><li>The Triboelectric Series lists materials according to how likely they are to let go of electrons or to take on electrons from other materials. Most of the materials in the Triboelectric Series are complex compounds and the release or attraction of electrons has to do with their molecular structure or geometry. </li></ul><ul><li>Dry human skin and rabbit fur have the greatest tendency to give up electrons when rubbed on something and become positively ( + ) charged. Teflon and vinyl have the greatest tendency to become negatively charged ( - ) when rubbed. If you want to create static electricity, rubbing fur on Teflon should give the best results. </li></ul>
  44. 44. Static Electric Sparks and Lightning <ul><li>A spark is a stream of electrons jumping across an air gap, heating the air until it glows and expands. Certain conditions can cause enough static electricity buildup to cause a spark or lightning. A spark often requires both a conductor and non-conductor. Lightning is an extreme example of a spark </li></ul>
  45. 45. Conditions for sparks <ul><li>Sparks do not happen easily. They are violent occurrences that require special conditions. They need both non-conductors and conductors to occur. The way this happens can get complex. These conditions include walking on a carpet on a very dry day or the rapid movement of tiny water particles in a summer storm. </li></ul>
  46. 46. In the home <ul><li>The Triboelectric Series shows that when certain materials are in contact, they can cause a great increase in electrical charges on the surfaces of those materials. This is typically the case in for sparks that people personally experience. </li></ul>
  47. 47. In the clouds <ul><li>Normally water inhibits static electricity, but in the case of thunderstorms, there is so much movement of air and water droplets within the clouds that charges collect on the surface of the droplets. Enormous amount of charges can collect in the clouds, some positive ( + ) and some negative ( - ). </li></ul>
  48. 48. Sparks require conductors <ul><li>You know that static electricity collects on the surface of non-conductors. But you seldom—if ever—see a spark fly from one non-conductor to another. The reason is that sparks need conductors, so that the electrons can freely move about and gather enough charges together to be able to jump from one material to another. </li></ul><ul><li>If you took a charged piece of plastic and put it next to some metal, there would be no spark. The charges are held on the surface of the plastic, so that they won't jump the air gap </li></ul><ul><li>Another good example of this concerns how you can get shocked with a spark. You are a conductor of electricity—although not as good as a piece of metal. The reason you conduct electricity is because the of salt in your blood and your cells. Now if you notice, you usually see sparks when you start to touch something metal—like a doorknob—or another person or animal. </li></ul><ul><li>So static electricity is formed and gathered on the surface of a non-conductor, but it must be then transferred to a conductor to cause a spark. </li></ul>
  49. 49. Charges move in conductor <ul><li>When a conductor—like a metal rod—is brought near a charged non-conductor, the free electrons in the conductor will move to one end or the other of the rod, depending on whether the non-conductor surface is positive or negative. </li></ul>
  50. 50. <ul><li>Opposite charges in conductor move toward non-conductor </li></ul>
  51. 51. Charges move in conductor <ul><li>When the conductor is brought into contact with the non-conductor, the electrical charges on the surface of the non-conductor are &quot;sucked&quot; into the conductor. In other words, if negative charges are on the surface of the non-conductor, these electrons will move into the conductor. If positively charged atoms are on the surface, electrons from the metal or conductor will neutralize those atoms, resulting in an excess of positive charges in the conductor. </li></ul><ul><li>Now, if another conductor is brought near the first conductor, the same thing will happen. Since electrons can move so freely in a conductor, many may collect near the surface and actually jump across the air gap as a spark. </li></ul>
  52. 52. Anatomy of a spark <ul><li>Air is a non-conductor of electricity and resists the movement of electrons through it. When the attraction or electrical pressure is great enough between objects with positive ( + ) and negative ( - ) charges--or even between a charged object and a neutral one--some electrons are able to overcome the resistance and jump the air gap. This electrical pressure is also called potential difference or voltage difference. </li></ul>
  53. 53. Heats up the air <ul><li>Since air is a non-conductor of electricity, it does not readily let electrons pass through it. But if the attraction is great enough, some electrons will leave their material and fly to the other object. While they move through the air, may smash into and bounce off molecules or atoms that are in their way. This heats up the air </li></ul>
  54. 54. <ul><li>Spark glows white-hot  </li></ul>
  55. 55. Lower resistance <ul><li>Now, the hotter the air is, the less resistance it gives the electrons. So as the air gets heated, more and more electrons start jumping over to the other side. This only heats the air even more, until it actually gets white-hot. That is the spark or bolt of lightning that you see and feel. </li></ul>
  56. 56. Electrons stop jumping <ul><li>Once enough electrons have made the jump, the attraction is reduced and the flow stops. The spark quickly cools down and the air stops glowing. It is all over in a fraction of a second. Since this happens for such a short time, so you may only feel a slight discomfort from the heat of the spark. But if the spark is a bolt of lightning, it can cause an enormous amount of damage. </li></ul>
  57. 57. Lightning is a real big spark <ul><li>Lightning works the same way as a little spark, except that it happens on a massive scale. Some lightning bolts are several miles long. Compare that to the tiny 1/4 inch or 1 centimeter length of the spark that comes off your finger. </li></ul><ul><li>Lightning is created when water drops are churning around in a thunder cloud. They gather either positive or negative electrical charges, so that soon one cloud may be positive and another cloud may be negative. Or perhaps some object on the earth may have an excess of opposite charge. </li></ul>
  58. 58. <ul><li>Lightning can be quite dramatic </li></ul>
  59. 59. Has high electrical pressure <ul><li>The electrical pressure builds up, the same way as it does for a spark. Since the distances are so much greater between the clouds, the electrical pressure must be extremely high for lightning to start. But once it does and a lightning bolt jumps from one cloud to another, it is a tremendous spectacle. </li></ul>
  60. 60. Most go from cloud to cloud <ul><li>Most lightning bolts are from cloud to cloud, but sometimes there are no positive charged clouds nearby, so the negative cloud, sends its electrons to the ground or any object that may have a slight positive charge. </li></ul>
  61. 61. Thunder <ul><li>Air expands when it is heated and contract when it cools. Since the spark happens so fast, the air expands and contracts very rapidly. When it contracts, the air slaps together, just like when you clap your hands or pop a balloon. The noise you hear from a spark is just a snap, because it is so small. </li></ul><ul><li>On the other hand, the noise of thunder is a tremendous crash, because the size of lightning is so large. The snap of a spark and the crash of thunder are caused by the same effect. The only difference is in the size of the spark. </li></ul>
  62. 62. Controlling Static Electricity <ul><li>One of the biggest complaints that people have about static electricity is that it causes sparks or gives them mild shocks when they touch things or even other people. Most people experience this problem in the winter, but there are others who are are constantly getting shocks and are actually plagued by the problem. </li></ul><ul><li>Certain materials--including dry human skin--can especially build up charges </li></ul>
  63. 63. Increase humidity <ul><li>Static electricity is more active when the air and materials are dry. The humidity is normally lower in the winter, and heating the house further reduces the humidity. Also, locations with a desert climate usually have very low relative humidity. </li></ul><ul><li>One thing you can do is to use a humidifier to raise the humidity in the house. That may help a little </li></ul>
  64. 64. Moisturize skin <ul><li>Some people have very dry skin that may cause the buildup of static charges, especially in the winter. One thing to try is to use moisturizers or lotions on your skin. The only problem with that, of course, is that you might have to put it all over your body. </li></ul><ul><li>You can experiment with different types of moisturizers and in different locations. Perhaps just putting lotion on you hands may be sufficient, since shocks and sparks usually come from touching objects with your hands. </li></ul>
  65. 65. Ground yourself <ul><li>Another idea is to use a metal object like a key and touch other metal things first with key. This will cause the spark to fly from the key and not your finger. That is much more comfortable. You can also use a ring or even a thimble to move the shock from your finger to the metal object. </li></ul><ul><li>One more thing to do is to try to ground yourself before touching another person or something metal. You can touch a wall or wooden table or something. Another way is to use a ring or a key and touch something metal. Let the spark fly that way instead of off your finger. </li></ul>
  66. 66. <ul><li>Using a thimble to protect finger from shock before touching doorknob </li></ul>

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