Electromagnetic Fields

Transcription

1 Electromagnetic Fields Electromagnetic fields are created by items that are charged either positively or negatively. When we say charged, we don t mean that something is only positively charged or only negatively charged. It just means that the object has more positive charges than negative, or more negative charges than positive. You may be asking yourself, where does one get all of the positive and negative charges? Good question. You probably already know the answer. Think back to chemistry, what little things are positive and negative? Protons are positive and neutrons are negative. So when something is positively charged, that means it has more protons than electrons and negative would be just the opposite. Just to be clear, atoms won t lose protons, because they are safely stuck in the nucleus, but they gain and lose electrons all the time. Now, back to fields. As we said, they are created by an object that is positive or negative. You ve heard the term opposites attract? This is where it comes from. It means that opposite charges will attract each other, and similar charges will repel each other. Now, when it come down to it, we can t really say which object is doing the pushing and pulling, so to make it easier, we always think in terms of what a charged object will do to a positively charged object, which we call a test or point charge. We know that similarly charged object will repel, so a positive object will push away a test charge, so we would draw a field coming out of the positive object like this For a negatively charged object, we know it will pull a positive charge towards it, so we draw a field going into it like this: Some fields are stronger than other, and some fields are weaker. It s not as confusing as you might think. In fact, what determines how strong a field is is how positive or negatively charged the object is. And we call this simply put, the object charge. The units we use for this are Coulombs (C). You can calculate how strong a field is by seeing what effect it has on a test charge. To do this, you would take the force it is either pushing or pulling the test charge, and divide it by the amount of the test charge itself. The more intense a field is, the more lines we would draw coming out of it, or going into it, depending on the sign of the charge. Since these line will always spread out the further you draw 2013 Supercharged Science Page 1 of 13

2 them, you can easily tell that the field gets weaker the further away you get from the charged object. Think about it like a magnet. If you bring two magnets together, the pull doesn t get stronger until they are closer together, and when you separate them, the pull gets weaker. The last pertinent information about electric fields is the Electric potential a charge has. You may be thinking, That sounds a lot like potential difference, and isn t that an energy? Well, you would be completely right. The easiest way to remember electric potential is it is the opposite of the field lines. The further away your test charge is from the direction the object is trying to move it, the more potential it has to be moved, so the electric potential increases Supercharged Science Page 2 of 13

3 Geomagnetic Field Fluctuation Activity First a little history, for a long time it was known that magnets would generate fields that could either attract or repulse certain objects, depending on their magnetic ability. It was also known that electricity could generate a field that would attract or repulse objects, depending on their charge. While it is fairly common knowledge now, it was not know that these types of fields are directly related and are referred to as Electromagnetism. In 1820, Hans Christian Ørsted noticed that the magnetic needle in his compass would be deflected when an electric current was passed through a wire that happened to be close by. It was further investigated by other prominent Physicists of the day, but not unified mathematically until James Clerk Maxwell did so in 1873 when he published the Treatise on Electricity and Magnetism. Now, it is recognized as one of the four fundamental forces of nature, meaning it is one of the naturally occurring, most basic forces found throughout the universe with the other three being gravity, and strong and weak nuclear forces. As it is naturally occurring, one of the most readily observable instances is our own home planet. Earth has a naturally occurring magnetic field around it which does many things such as deflecting the Earth from dangerous solar flares and other potentially dangerous radiation in space. One of the most useful applications to people is the use of the magnetic field to help with navigation. If you always know a certain direction (North), then you can orient yourself which direction you wish to go. It is a common misconception that the magnetic fields surrounding the Earth are always stationary. The magnetic fields are constantly changing, and frequently they completely reverse direction completely. Frequently is a geological term, they can occur between periods of 20,000 to 50,000,000 years. We have had the same orientation for the past 780,000 years, and it is predicted we will have another reversal very soon (geologically speaking). While the general orientation is relatively constant, there are constant minor changes and oscillations that can occur throughout the day. This activity will demonstrate minor fluctuations that occur Supercharged Science Page 3 of 13

4 Objective: Observe the minor fluctuations in the Geomagnetic (Earth s magnetic) Field that surround and protect the Earth. Materials: Index Card or Cardborad (some stiff product that can still be shaped) 2 small mirrors (mosaic tile sized) 2 rare earth (neodymium) magnets 30 cm of nylon filament (or thin nylon thread, dental floss for example) 4 6 donut or small bar magnets Laser pointer (any type will work) Glass beaker (water glass, or empty glass jar is fine, as long as the walls are smooth and transparent) wooden clothespin (has to have spring action closing) hot glue gun scissors masking tape thin wooden stick (chopstick, or wooden dowel) aluminum foil large piece of paper Procedure: 1) Place the nylon string between the 2 neodymium magnets so the thread is pinched snugly between them. They are very strong magnets, so they can hold the string fairly easily. 2) Using the hot glue gun, glue the small mirrors to either side of the neodymium magnets that are stuck together. 3) Mount the magnet/mirror apparatus inside the glass beaker (or jar/cylinder). The trick is to mount it so that it doesn t touch the sides or the bottom, and can hang/spin freely. The easiest way is to place the chopstick/dowel across the opening at the top, and tie the string to it so it hangs above the bottom of the beaker in the middle of the beaker. Then cover the top with aluminum foil or paper that will keep any potential airflow out of the beaker. 4) Using your index card and hot glue gun, glue the small donut magnets to a corner of the index card. The trick being to keep all of the magnets with the same orientation (i.e. all North/South poles point up) After you have glued the first magnet, the others will try to pull the paper/magnet up, so use the masking tape to hold the card still. Use about 4 donut magents, or six small bar magnets. Essentially cover ¼ of the index card. This creates a magnetic array. 5) While the card dries, use the glass/magnet apparatus. Notice that no which way you turn the glass, the hanging magnet/mirror always points the same direction. You have created a simple compass that will always point North/South. Feel free to walk around the room to test it. 6) Fold the index cards with the magnet in half, so you can easily stand it up. You will want to place it about cm away from the nagging magnet to essentially cancel out the geomagnetic field, so the small fluctuations will be easily observable. Start with the array close to the glass, and you can see it pulls the hanging magnets towards the array. Slowly pull the array away from the glass until the hanging magnets are hanging normally. Wait a few 2013 Supercharged Science Page 4 of 13

5 minutes for the magnets/mirror to stop swinging. You have now zeroed out your compass, so that any slight magnetic fluctuations will be easily observable. 7) Take the laser pointer and place the clothespin on the pointer over the button, so the clothespin keeps the laser on. 8) Position the laser so it reflects off the mirror and onto a large piece of paper about 3 feet away. After it is in position, you should no longer touch the setup, and it should be in a place that will not get bumped or moved accidentally 9) Circle the point on the paper where the laser is. Continue to check on the paper throughout the day and observe where the laser point is. Look for anytime the laser point is moving or bouncing for a period of seconds or minutes. Analysis: 1) Did the laser point move throughout the day? 2) If the setup was not disturbed, what had to be the cause of the moving laser? 3) What does this tell you about the fluctuations in the geomagnetic fields? 4) If you observed any bouncing or oscillating, what does this tell you about the geomagnetic fields? 2013 Supercharged Science Page 5 of 13

6 Electromagnetic Field calculations An electric field will exert a force on a test charge, depending on the distance away from source of the field. To calculate the intensity or strength of an electric field you would use the following equation: E = F/q E = Electric Field intensity (N/C) F = Force experienced by the test charge (N) q = magnitude of the test charge (C) Here are some practice problems using the GUESS method. 1. A positive charge of C experiences a force of 2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 2. What charge exists on a test charge that experiences a force of 140 N at a point where the electric field intensity is 0.5 N/C? Givens: Equation: Solve: 2013 Supercharged Science Page 6 of 13

7 3. A negative test charge of C is placed in an electric field of 500 N/C intensity. What is the strength of the force exerted on the test charge? Givens: Equation: Solve: 4. A test charge has a force of 200 N on it when it is placed in an electric field of 4500 N/C. What is the magnitude of the charge? Givens: Equation: Solve: 5. A negative charge of C experiences a force of 0.2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 2013 Supercharged Science Page 7 of 13

8 Solutions: 1) Did the laser point move throughout the day? yes 2) If the setup was not disturbed, what had to be the cause of the moving laser? Changes in Earth s electromagnetic fields 3) What does this tell you about the fluctuations in the geomagnetic fields? There are always slight fluctuations 4) If you observed any bouncing or oscillating, what does this tell you about the geomagnetic fields? There have to be some periods where it isn t stationary Electromagnetic Field calculations An electric field will exert a force on a test charge, depending on the distance away from source of the field. To calculate the intensity or strength of an electric field you would use the following equation: E = F/q E = Electric Field intensity (N/C) F = Force experienced by the test charge (N) q = magnitude of the test charge (C) Here are some practice problems using the GUESS method. 1. A positive charge of C experiences a force of 2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 1000 N/C 2013 Supercharged Science Page 8 of 13

9 2. What charge exists on a test charge that experiences a force of 140 N at a point where the electric field intensity is 0.5 N/C? Givens: Equation: Solve: 280 C 3. A negative test charge of C is placed in an electric field of 500 N/C intensity. What is the strength of the force exerted on the test charge? Givens: Equation: Solve: 0.4 N 4. A test charge has a force of 200 N on it when it is placed in an electric field of 4500 N/C. What is the magnitude of the charge? Givens: Equation: Solve: C 2013 Supercharged Science Page 9 of 13

10 5. A negative charge of C experiences a force of 0.2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 2000 N/C 2013 Supercharged Science Page 10 of 13

11 Solutions: 1) Did the laser point move throughout the day? yes 2) If the setup was not disturbed, what had to be the cause of the moving laser? Changes in Earth s electromagnetic fields 3) What does this tell you about the fluctuations in the geomagnetic fields? There are always slight fluctuations 4) If you observed any bouncing or oscillating, what does this tell you about the geomagnetic fields? There have to be some periods where it isn t stationary 2013 Supercharged Science Page 11 of 13

12 Electromagnetic Field calculations 1. A positive charge of C experiences a force of 2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 1000 N/C 2. What charge exists on a test charge that experiences a force of 140 N at a point where the electric field intensity is 0.5 N/C? Givens: Equation: Solve: 280 C 3. A negative test charge of C is placed in an electric field of 500 N/C intensity. What is the strength of the force exerted on the test charge? Givens: Equation: Solve: 0.4 N 2013 Supercharged Science Page 12 of 13

13 4. A test charge has a force of 200 N on it when it is placed in an electric field of 4500 N/C. What is the magnitude of the charge? Givens: Equation: Solve: C 5. A negative charge of C experiences a force of 0.2 N when located at a certain point. What is the electric field intensity at that point? Givens: Equation: Solve: 2000 N/C 2013 Supercharged Science Page 13 of 13