Electromagnetic Induction Essay, Research Paper
The phenomenon called electromagnetic induction was first noticed and investigated by Michael Faraday, in 1831. Electromagnetic induction is the production of an electromotive force (emf) in a conductor as a result of a changing magnetic field about the conductor and is a very important concept. Faraday discovered that, whenever the magnetic field about an electromagnet was made to grow and collapse by closing and opening the electric circuit of which it was a part, an electric current could be detected in a separate conductor nearby. Faraday also investigated the possibility that a current could be produced by a magnetic field being placed near a coiled wire. Just placing the magnet near the wire could not produce a current. Faraday discovered that a current could be produced in this situation only if the magnet had some velocity. The magnet could be moved in either a positive or negative direction but had to be in motion to produce any current in the wire. The current in the coil is called an induced current, because the current is brought about (or ?induced?) by a changing magnetic field (Cutnell and Johnson 705). The induced current is sustained by an emf. Since a source of emf is always needed to produce a current, the coil itself behaves as if it were a source of emf. The emf is known as an induced emf. Thus, a changing magnetic field induces an emf in the coil, and the emf leads to an induced current (705). He also found that moving a conductor near a stationary permanent magnet caused a current to flow in the wire as long as it was moving as in the magnet and coiled wire set-up. Faraday visualized a magnetic field as composed of many lines of induction, along which a small magnetic compass would point. The aggregate of the lines intersecting a given area is called the magnetic flux. Faraday attributed the electrical effects to a changing magnetic flux.
The necessity of motion to produce a current is due to the fact that electromagnetic induction involves a time-varying magnetic field. The same effects can be produced by moving the coil toward and away from a motionless magnetic source. In either case, the key to producing the current is certainly the motion of the magnet or the wire. The magnetic lines of the magnetic field must pass through a loop of the coiled wire. The value of the magnetic flux is proportional to the total number of lines passing through the loop (Serway and Faughn 653). The magnetic flux can be stated in an equation equal to the flux: f = (B)(A) or f = (B)(A) cos q. The value for the magnetic field (B) is multiplied by the area of one loop of the wire coil (A) and the angle at which the magnetic field crosses the plane of the loop. This conclusion lead to the development of other law involving electromagnetic flux.
Sometime after Faraday?s experiments and conclusions, Scottish physicist James Clerk Maxwell proposed that the fundamental effect of changing magnetic flux was the production of an electric field, not only in a conductor, where it could drive an electric charge, but also in space even in the absence of electric charges. Maxwell formulated the mathematical expression relating the change in magnetic flux to the induced electromotive force (emf). This relationship, known as Faraday’s law of induction, states that the magnitude of the emf induced in a circuit is proportional to the rate of change of the magnetic flux that cuts across the circuit. The induced emf along any moving or fixed mathematical path in a constant or changing magnetic field equals the rate at which magnetic flux sweeps across the path (Ohanian 784). The subsequent magnetic field produced in the coil will be in the opposite direction of the magnetic field of the bar magnet. This due to the relationships between the emf, the current, and the magnetic field. If the field were produced in the direction of the magnet?s magnetic field, the system would continue to build in charge due to the effects of an increase in the electromagnetic flux acting on the coil. The system would result in disaster if continued in that manner. The field must, by law, resist the increase of the magnetic flux acting on the coil in order to maintain the balance of the system. The equation for this is: E = – N (qf / qt) where N is the number of loops in the coiled wire and t is the time in which the flux, f, is changed.
This experiment will explore a few of the situations in which a current can be induced by a magnetic field. These have proven useful for the possibilities of producing a current with magnetism. The translation of this is that through construction of generators, the magnetic field passing through the coiled wire produces a useful source of electricity. The induced current and induced emf relate to the amperage and voltage passing through many of our homes today. These discoveries were used to revolutionize the way we lived at the turn of the century by providing the physical laws needed by inventors to produce new technology.
I.Currents Induced in Straight Wires:
1.Connect the single wire apparatus to the power supply as shown. The ammeter should be on high scale. Place one of the small silver compasses on the back ledge. Rotate the ledge and plot the magnetic field. Remember that the magnetic field always runs from north to south. Therefore always put the arrow on your field line in the direction that the north arrow points.
2.Turn the current up to around 5 amperes. Be careful not to touch any wires or you will get a very bad shock. Also hurry in taking your measurements or the circuit breaker will blow. Rotate the ledge again and plot the field. Read your textbook on the theory of magnetic fields for straight wires before doing this. Reverse the leads so the current flows in the opposite direction and repeat.
3.Now connect the wire loop apparatus, the ammeter, and the power supply in a series circuit. Turn the power supply up until the ammeter reads about 2 amperes. This time you will be using one of the larger gold compasses. Hold the compass on the inside of the loop, taking note of the way the needle points. Repeat outside of the loop on all sides. Draw the loop on your paper and plot the magnetic field. Reverse the direction of the current flow through the loop and repeat the measurements.
4.Set the panel voltage to 1.5 volts using the voltmeter. Connect a coil to the supply as shown below. Insert the slotted cardboard
in the hole. Using the compass on the cardboard, map out the magnetic field. Include the directional arrows. Reverse the direction of the current and repeat.
II.Currents Induced By a Bar Magnet:
1.Connect a galvanometer (most sensitive scale) to the terminals of the coil, as shown below. Quickly insert one end of the bar magnet into the coil, wait, then, quickly remove the magnet. What are your observations? Use sketches of the coil to indicate current directions. There are four cases to be considered: (1) north inserted, (2) north withdrawn, (3) south inserted, and (4) south withdrawn. For each case there are four pictures. Therefore, a total of 16 diagrams are required. The direction the galvanometer needle moves is the same direction as the current is flowing. Remember the bar magnet has a field running from N to S. When this is inserted in the coil, a current is set up in order to produce a magnetic field that will cancel out the field of the bar magnet. Is the field produced by the current in the coil in the right direction to cancel the field of the bar magnet?
2.Repeat part 1, but much more slowly than before. Compare results. Does the speed have an effect on the strength of the magnetic field produced?
3.Repeat the procedure with the other end of the magnet.
III. Currents Induced by Current- Carrying Coils:
1.Connect a second coil t the 1.5v power supply oriented as shown. Quickly move coil A up to coil B, maintaining orientation shown above (note effects). Indicate the current in each coil. Quickly move coil A away from B. Indicate the directions of the currents in the coils. Remember current flows from to ? and is set in coil A. Does coil A behave exactly like the bar magnet did?
2.Now disconnect one wire from coil A and move coil A up to coil B. Reconnect the wire to coil A (note effect), disconnect wire (note effect). Indicate direction of currents in coils for each case.
I.1. The magnetic field of a straight wire was found to be:
2.The magnetic field of the same wire with current in opposite direction:
3. The magnetic fields of loops of wire with current in opposite directions:
4. The mapped magnetic field from a loop attached to a voltage:
II.1. The diagrams indicating the insertion of the north pole of a bar magnet into the coil:
The diagram with the south pole of the bar magnet being inserted into the coil:
The diagram of the north pole of the bar magnet being withdrawn form the coil:
c.d. The diagram for the south- pole of the bar magnet being withdrawn from the coil: e.f.
III.1. Diagrams for current- carrying coils being moved together:
2. Diagrams for coils being placed together with a wire detached from coil A and then replaced and removed after being positioned in close proximity:
3. Diagrams for the insertion of a soft iron rod (nail) through the two coils with circuit suddenly closed:
These are for the closing circuit with the current in the powered coil flowing in the opposite direction:
These diagrams are for the situation of breaking the circuit with a soft iron rod inserted through the coils:
These diagrams are for the breaking of the circuit with the current flowing in the opposite direction from the previous circuit:
The results of this laboratory are not represented as calculations. The diagrams in the previous section constitutes a large portion of the answers to the questions and assignments within the procedure. Most of the questions are represented in the previous section and those questions requiring a verbal answer are fulfilled in this section.
The first question from the second part of the Procedure section asks for observations of the swift insertion of the bar magnet into the coil. The galvanometer needle moves into position and then settles back to neutral after the magnet stops.
The next question, also in that section, asks if the field produced by the current in the coil was in the opposite direction of the magnetic field of the bar magnet? The field produced is bound by physical law to be in the opposite direction of the field of the bar magnet. The field acts to cancel the effect of the magnet?s field on the coil.
The next question asks if the speed of the inserted magnet has any effect on the strength of the magnetic field produced. The answer is yes; the field produced in the coil is weaker as the magnet is inserted and withdrawn at a slower pace.
The final question of the laboratory, from the third section of the procedure, asks if the coil attached to the power supply acts like the bar magnet did when moved close to another coil. The answer is yes; the powered coil has a magnetic field due to the current passing through it. When placed near the other coil at some rate of speed, the galvanometer attached to the second coil reacts to the current being produced in the coil.
Percentage Error Difference:
This laboratory does not involve any numerical calculations to be compared to theoretical values. Due to this fact, there is no percentage error difference found in the course of these experiments. That said, any error in the reporting of the results and data of this lab would be the result of human error. Any wrongful interpretations or misappropriation of the experimental situations would be attributed to the student. This is the only source of error in this laboratory.
The laboratory results were very clear. The equipment was used in its proper manner and subsequently produced accurate results. The mapping of the magnetic fields around the current- carrying straight and looped wires were found to be consistent with the instruction provided in the textbook. The change in direction of the current produced to appropriate resulting magnetic field as compared to the text.
The bar magnet and coil section of the laboratory allowed for a close comparison with the theory behind electromagnetic induction. As the magnet was inserted into the coil, the galvanometer needle registered a current. The direction of the resulting field could then be produced with guidance from the theory and a little deductive reasoning.
The final section of the procedure also held to the predictions of theory. The idea that a powered coil would act like a bar magnet when moved into close proximity with a coil attached to a galvanometer also proved to be true. The galvanometer reacted in the same way as if the magnet were being inserted into the coil. The powered coil has a magnetic field of its own due to the current in the wire. As a result of this field, the galvanometer detects a current in the coil attached to it exactly like the situations involving a bar magnet.
The results of this laboratory indicates a successful representation of the basic theoretical guidelines. The equipment was satisfactory for the tasks described in the procedure and the results were equally as satisfying. With this success, the student sees the theory in a tangible form and this would help to cement the concepts of electromagnetic induction in their memory.
1.Cutnell, John D. and Johnson, Kenneth W. Physics. 3rd ed. John Wiley & Sons, inc., New York, 1995
2. Ohanian, Hans C. Physics. 2nd ed. W. W. Norton & Company, New York, 1989.
3. Serway, Raymond A and Faughn, Jerry S. College Physics. 5th ed. Saunders College Publishing, Orlando, 1999.